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<h3><a href="../contents.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">os – Portable access to operating system specific features.</a><ul>
<li><a class="reference internal" href="#process-owner">Process Owner</a></li>
<li><a class="reference internal" href="#process-environment">Process Environment</a></li>
<li><a class="reference internal" href="#process-working-directory">Process Working Directory</a></li>
<li><a class="reference internal" href="#pipes">Pipes</a></li>
<li><a class="reference internal" href="#file-descriptors">File Descriptors</a></li>
<li><a class="reference internal" href="#filesystem-permissions">Filesystem Permissions</a></li>
<li><a class="reference internal" href="#directories">Directories</a></li>
<li><a class="reference internal" href="#symbolic-links">Symbolic Links</a></li>
<li><a class="reference internal" href="#walking-a-directory-tree">Walking a Directory Tree</a></li>
<li><a class="reference internal" href="#running-external-commands">Running External Commands</a></li>
<li><a class="reference internal" href="#creating-processes-with-os-fork">Creating Processes with os.fork()</a></li>
<li><a class="reference internal" href="#waiting-for-a-child">Waiting for a Child</a></li>
<li><a class="reference internal" href="#spawn">Spawn</a></li>
</ul>
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<div class="section" id="module-os">
<span id="os-portable-access-to-operating-system-specific-features"></span><h1>os – Portable access to operating system specific features.<a class="headerlink" href="#module-os" title="Permalink to this headline">¶</a></h1>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Purpose:</th><td class="field-body">Portable access to operating system specific features.</td>
</tr>
<tr class="field"><th class="field-name">Python Version:</th><td class="field-body">1.4 (or earlier)</td>
</tr>
</tbody>
</table>
<p>The <a class="reference internal" href="#module-os" title="os: Portable access to operating system specific features."><tt class="xref py py-mod docutils literal"><span class="pre">os</span></tt></a> module provides a wrapper for platform specific modules
such as <tt class="xref py py-mod docutils literal"><span class="pre">posix</span></tt>, <tt class="xref py py-mod docutils literal"><span class="pre">nt</span></tt>, and <tt class="xref py py-mod docutils literal"><span class="pre">mac</span></tt>. The API for functions
available on all platform should be the same, so using the <a class="reference internal" href="#module-os" title="os: Portable access to operating system specific features."><tt class="xref py py-mod docutils literal"><span class="pre">os</span></tt></a>
module offers some measure of portability. Not all functions are
available on all platforms, however. Many of the process management
functions described in this summary are not available for Windows.</p>
<p>The Python documentation for the <a class="reference internal" href="#module-os" title="os: Portable access to operating system specific features."><tt class="xref py py-mod docutils literal"><span class="pre">os</span></tt></a> module is subtitled
“Miscellaneous operating system interfaces”. The module consists
mostly of functions for creating and managing running processes or
filesystem content (files and directories), with a few other bits of
functionality thrown in besides.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">Some of the example code below will only work on Unix-like
operating systems.</p>
</div>
<div class="section" id="process-owner">
<h2>Process Owner<a class="headerlink" href="#process-owner" title="Permalink to this headline">¶</a></h2>
<p>The first set of functions to cover are used for determining and
changing the process owner ids. These are mostly useful to authors of
daemons or special system programs which need to change permission
level rather than running as <tt class="docutils literal"><span class="pre">root</span></tt>. This section does not try to
explain all of the intricate details of Unix security, process owners,
etc. See the References list below for more details.</p>
<p>This first script shows the real and effective user and group
information for a process, and then changes the effective values. This
is similar to what a daemon would need to do when it starts as root
during a system boot, to lower the privilege level and run as a
different user.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">Before running the example, change the <tt class="xref py py-data docutils literal"><span class="pre">TEST_GID</span></tt> and
<tt class="xref py py-data docutils literal"><span class="pre">TEST_UID</span></tt> values to match a real user.</p>
</div>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">os</span>
<span class="n">TEST_GID</span><span class="o">=</span><span class="mi">501</span>
<span class="n">TEST_UID</span><span class="o">=</span><span class="mi">527</span>
<span class="k">def</span> <span class="nf">show_user_info</span><span class="p">():</span>
<span class="k">print</span> <span class="s">'Effective User :'</span><span class="p">,</span> <span class="n">os</span><span class="o">.</span><span class="n">geteuid</span><span class="p">()</span>
<span class="k">print</span> <span class="s">'Effective Group :'</span><span class="p">,</span> <span class="n">os</span><span class="o">.</span><span class="n">getegid</span><span class="p">()</span>
<span class="k">print</span> <span class="s">'Actual User :'</span><span class="p">,</span> <span class="n">os</span><span class="o">.</span><span class="n">getuid</span><span class="p">(),</span> <span class="n">os</span><span class="o">.</span><span class="n">getlogin</span><span class="p">()</span>
<span class="k">print</span> <span class="s">'Actual Group :'</span><span class="p">,</span> <span class="n">os</span><span class="o">.</span><span class="n">getgid</span><span class="p">()</span>
<span class="k">print</span> <span class="s">'Actual Groups :'</span><span class="p">,</span> <span class="n">os</span><span class="o">.</span><span class="n">getgroups</span><span class="p">()</span>
<span class="k">return</span>
<span class="k">print</span> <span class="s">'BEFORE CHANGE:'</span>
<span class="n">show_user_info</span><span class="p">()</span>
<span class="k">print</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">os</span><span class="o">.</span><span class="n">setegid</span><span class="p">(</span><span class="n">TEST_GID</span><span class="p">)</span>
<span class="k">except</span> <span class="ne">OSError</span><span class="p">:</span>
<span class="k">print</span> <span class="s">'ERROR: Could not change effective group. Re-run as root.'</span>
<span class="k">else</span><span class="p">:</span>
<span class="k">print</span> <span class="s">'CHANGED GROUP:'</span>
<span class="n">show_user_info</span><span class="p">()</span>
<span class="k">print</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">os</span><span class="o">.</span><span class="n">seteuid</span><span class="p">(</span><span class="n">TEST_UID</span><span class="p">)</span>
<span class="k">except</span> <span class="ne">OSError</span><span class="p">:</span>
<span class="k">print</span> <span class="s">'ERROR: Could not change effective user. Re-run as root.'</span>
<span class="k">else</span><span class="p">:</span>
<span class="k">print</span> <span class="s">'CHANGE USER:'</span>
<span class="n">show_user_info</span><span class="p">()</span>
<span class="k">print</span>
</pre></div>
</div>
<p>When run as user with id of 527 and group 501 on OS X, this output is
produced:</p>
<div class="highlight-python"><pre>$ python os_process_user_example.py
BEFORE CHANGE:
Effective User : 527
Effective Group : 501
Actual User : 527 dhellmann
Actual Group : 501
Actual Groups : [501, 102, 204, 100, 98, 80, 61, 12, 500, 101]
CHANGED GROUP:
Effective User : 527
Effective Group : 501
Actual User : 527 dhellmann
Actual Group : 501
Actual Groups : [501, 102, 204, 100, 98, 80, 61, 12, 500, 101]
CHANGE USER:
Effective User : 527
Effective Group : 501
Actual User : 527 dhellmann
Actual Group : 501
Actual Groups : [501, 102, 204, 100, 98, 80, 61, 12, 500, 101]</pre>
</div>
<p>Notice that the values do not change. When not running as root,
processes cannot change their effective owner values. Any attempt to
set the effective user id or group id to anything other than that of
the current user causes an <em class="xref std std-ref">OSError</em>.</p>
<p>Running the same script using <strong class="command">sudo</strong> so that it starts out
with root privileges is a different story.</p>
<div class="highlight-python"><pre>$ sudo python os_process_user_example.py
BEFORE CHANGE:
Effective User : 0
Effective Group : 0
Actual User : 0 dhellmann
Actual Group : 0
Actual Groups : [0, 1, 2, 8, 29, 3, 9, 4, 5, 80, 20]
CHANGED GROUP:
Effective User : 0
Effective Group : 501
Actual User : 0 dhellmann
Actual Group : 0
Actual Groups : [501, 1, 2, 8, 29, 3, 9, 4, 5, 80, 20]
CHANGE USER:
Effective User : 527
Effective Group : 501
Actual User : 0 dhellmann
Actual Group : 0
Actual Groups : [501, 1, 2, 8, 29, 3, 9, 4, 5, 80, 20]</pre>
</div>
<p>In this case, since it starts as root, it can change the effective
user and group for the process. Once the effective UID is changed, the
process is limited to the permissions of that user. Since non-root
users cannot change their effective group, the program needs to change
the group before changing the user.</p>
<p>Besides finding and changing the process owner, there are functions for
determining the current and parent process id, finding and changing the
process group and session ids, as well as finding the controlling terminal id.
These can be useful for sending signals between processes or for complex
applications such as writing a command line shell.</p>
</div>
<div class="section" id="process-environment">
<h2>Process Environment<a class="headerlink" href="#process-environment" title="Permalink to this headline">¶</a></h2>
<p>Another feature of the operating system exposed to a program though
the <a class="reference internal" href="#module-os" title="os: Portable access to operating system specific features."><tt class="xref py py-mod docutils literal"><span class="pre">os</span></tt></a> module is the environment. Variables set in the
environment are visible as strings that can be read through
<tt class="xref py py-data docutils literal"><span class="pre">os.environ</span></tt> or <tt class="xref py py-func docutils literal"><span class="pre">getenv()</span></tt>. Environment variables are
commonly used for configuration values such as search paths, file
locations, and debug flags. This example shows how to retrieve an
environment variable, and pass a value through to a child process.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">os</span>
<span class="k">print</span> <span class="s">'Initial value:'</span><span class="p">,</span> <span class="n">os</span><span class="o">.</span><span class="n">environ</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="s">'TESTVAR'</span><span class="p">,</span> <span class="bp">None</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'Child process:'</span>
<span class="n">os</span><span class="o">.</span><span class="n">system</span><span class="p">(</span><span class="s">'echo $TESTVAR'</span><span class="p">)</span>
<span class="n">os</span><span class="o">.</span><span class="n">environ</span><span class="p">[</span><span class="s">'TESTVAR'</span><span class="p">]</span> <span class="o">=</span> <span class="s">'THIS VALUE WAS CHANGED'</span>
<span class="k">print</span>
<span class="k">print</span> <span class="s">'Changed value:'</span><span class="p">,</span> <span class="n">os</span><span class="o">.</span><span class="n">environ</span><span class="p">[</span><span class="s">'TESTVAR'</span><span class="p">]</span>
<span class="k">print</span> <span class="s">'Child process:'</span>
<span class="n">os</span><span class="o">.</span><span class="n">system</span><span class="p">(</span><span class="s">'echo $TESTVAR'</span><span class="p">)</span>
<span class="k">del</span> <span class="n">os</span><span class="o">.</span><span class="n">environ</span><span class="p">[</span><span class="s">'TESTVAR'</span><span class="p">]</span>
<span class="k">print</span>
<span class="k">print</span> <span class="s">'Removed value:'</span><span class="p">,</span> <span class="n">os</span><span class="o">.</span><span class="n">environ</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="s">'TESTVAR'</span><span class="p">,</span> <span class="bp">None</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'Child process:'</span>
<span class="n">os</span><span class="o">.</span><span class="n">system</span><span class="p">(</span><span class="s">'echo $TESTVAR'</span><span class="p">)</span>
</pre></div>
</div>
<p>The <tt class="xref py py-data docutils literal"><span class="pre">os.environ</span></tt> object follows the standard Python mapping API
for retrieving and setting values. Changes to <tt class="xref py py-data docutils literal"><span class="pre">os.environ</span></tt> are
exported for child processes.</p>
<div class="highlight-python"><pre>$ python -u os_environ_example.py
Initial value: None
Child process:
Changed value: THIS VALUE WAS CHANGED
Child process:
THIS VALUE WAS CHANGED
Removed value: None
Child process:</pre>
</div>
</div>
<div class="section" id="process-working-directory">
<h2>Process Working Directory<a class="headerlink" href="#process-working-directory" title="Permalink to this headline">¶</a></h2>
<p>Operating systems with hierarchical filesystems have a concept of the
<em>current working directory</em> – the directory on the filesystem the
process uses as the starting location when files are accessed with
relative paths. The current working directory can be retrieved with
<tt class="xref py py-func docutils literal"><span class="pre">getcwd()</span></tt> and changed with <tt class="xref py py-func docutils literal"><span class="pre">chdir()</span></tt>.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">os</span>
<span class="k">print</span> <span class="s">'Starting:'</span><span class="p">,</span> <span class="n">os</span><span class="o">.</span><span class="n">getcwd</span><span class="p">()</span>
<span class="k">print</span> <span class="s">'Moving up one:'</span><span class="p">,</span> <span class="n">os</span><span class="o">.</span><span class="n">pardir</span>
<span class="n">os</span><span class="o">.</span><span class="n">chdir</span><span class="p">(</span><span class="n">os</span><span class="o">.</span><span class="n">pardir</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'After move:'</span><span class="p">,</span> <span class="n">os</span><span class="o">.</span><span class="n">getcwd</span><span class="p">()</span>
</pre></div>
</div>
<p><tt class="xref py py-const docutils literal"><span class="pre">os.curdir</span></tt> and <tt class="xref py py-const docutils literal"><span class="pre">os.pardir</span></tt> are used to refer to the
current and parent directories in a portable manner. The output should
not be surprising:</p>
<div class="highlight-python"><pre>$ python os_cwd_example.py
Starting: /Users/dhellmann/Documents/PyMOTW/src/PyMOTW/os
Moving up one: ..
After move: /Users/dhellmann/Documents/PyMOTW/src/PyMOTW</pre>
</div>
</div>
<div class="section" id="pipes">
<h2>Pipes<a class="headerlink" href="#pipes" title="Permalink to this headline">¶</a></h2>
<p>The <a class="reference internal" href="#module-os" title="os: Portable access to operating system specific features."><tt class="xref py py-mod docutils literal"><span class="pre">os</span></tt></a> module provides several functions for managing the I/O
of child processes using <em>pipes</em>. The functions all work essentially
the same way, but return different file handles depending on the type
of input or output desired. For the most part, these functions are
made obsolete by the <a class="reference internal" href="../subprocess/index.html#module-subprocess" title="subprocess: Work with additional processes"><tt class="xref py py-mod docutils literal"><span class="pre">subprocess</span></tt></a> module (added in Python 2.4),
but there is a good chance legacy code uses them.</p>
<p>The most commonly used pipe function is <tt class="xref py py-func docutils literal"><span class="pre">popen()</span></tt>. It creates a
new process running the command given and attaches a single stream to
the input or output of that process, depending on the <em>mode</em>
argument. While <tt class="xref py py-func docutils literal"><span class="pre">popen()</span></tt> functions work on Windows, some of these
examples assume a Unix-like shell.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">os</span>
<span class="k">print</span> <span class="s">'popen, read:'</span>
<span class="n">pipe_stdout</span> <span class="o">=</span> <span class="n">os</span><span class="o">.</span><span class="n">popen</span><span class="p">(</span><span class="s">'echo "to stdout"'</span><span class="p">,</span> <span class="s">'r'</span><span class="p">)</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">stdout_value</span> <span class="o">=</span> <span class="n">pipe_stdout</span><span class="o">.</span><span class="n">read</span><span class="p">()</span>
<span class="k">finally</span><span class="p">:</span>
<span class="n">pipe_stdout</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
<span class="k">print</span> <span class="s">'</span><span class="se">\t</span><span class="s">stdout:'</span><span class="p">,</span> <span class="nb">repr</span><span class="p">(</span><span class="n">stdout_value</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'</span><span class="se">\n</span><span class="s">popen, write:'</span>
<span class="n">pipe_stdin</span> <span class="o">=</span> <span class="n">os</span><span class="o">.</span><span class="n">popen</span><span class="p">(</span><span class="s">'cat -'</span><span class="p">,</span> <span class="s">'w'</span><span class="p">)</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">pipe_stdin</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="s">'</span><span class="se">\t</span><span class="s">stdin: to stdin</span><span class="se">\n</span><span class="s">'</span><span class="p">)</span>
<span class="k">finally</span><span class="p">:</span>
<span class="n">pipe_stdin</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
</pre></div>
</div>
<p>The descriptions of the streams also assume Unix-like terminology:</p>
<ul class="simple">
<li>stdin - The “standard input” stream for a process (file descriptor 0) is
readable by the process. This is usually where terminal input goes.</li>
<li>stdout - The “standard output” stream for a process (file descriptor
1) is writable by the process, and is used for displaying regular
output to the user.</li>
<li>stderr - The “standard error” stream for a process (file descriptor 2) is
writable by the process, and is used for conveying error messages.</li>
</ul>
<div class="highlight-python"><pre>$ python -u os_popen.py
popen, read:
stdout: 'to stdout\n'
popen, write:
stdin: to stdin</pre>
</div>
<p>The caller can only read from or write to the streams associated with
the child process, which limits the usefulness. The other
<tt class="xref py py-func docutils literal"><span class="pre">popen()</span></tt> variants provide additional streams so it is possible to
work with stdin, stdout, and stderr as needed.</p>
<p>For example, <tt class="xref py py-func docutils literal"><span class="pre">popen2()</span></tt> returns a write-only stream attached to
stdin of the child process, and a read-only stream attached to its
stdout.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">os</span>
<span class="k">print</span> <span class="s">'popen2:'</span>
<span class="n">pipe_stdin</span><span class="p">,</span> <span class="n">pipe_stdout</span> <span class="o">=</span> <span class="n">os</span><span class="o">.</span><span class="n">popen2</span><span class="p">(</span><span class="s">'cat -'</span><span class="p">)</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">pipe_stdin</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="s">'through stdin to stdout'</span><span class="p">)</span>
<span class="k">finally</span><span class="p">:</span>
<span class="n">pipe_stdin</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">stdout_value</span> <span class="o">=</span> <span class="n">pipe_stdout</span><span class="o">.</span><span class="n">read</span><span class="p">()</span>
<span class="k">finally</span><span class="p">:</span>
<span class="n">pipe_stdout</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
<span class="k">print</span> <span class="s">'</span><span class="se">\t</span><span class="s">pass through:'</span><span class="p">,</span> <span class="nb">repr</span><span class="p">(</span><span class="n">stdout_value</span><span class="p">)</span>
</pre></div>
</div>
<p>This simplistic example illustrates bi-directional communication. The
value written to stdin is read by <tt class="docutils literal"><span class="pre">cat</span></tt> (because of the <tt class="docutils literal"><span class="pre">'-'</span></tt>
argument), then written back to stdout. A more complicated process
could pass other types of messages back and forth through the pipe;
even serialized objects.</p>
<div class="highlight-python"><pre>$ python -u os_popen2.py
popen2:
pass through: 'through stdin to stdout'</pre>
</div>
<p>In most cases, it is desirable to have access to both stdout and
stderr. The stdout stream is used for message passing and the stderr
stream is used for errors, so reading from it separately reduces the
complexity for parsing any error messages. The <tt class="xref py py-func docutils literal"><span class="pre">popen3()</span></tt>
function returns three open streams tied to stdin, stdout, and stderr
of the new process.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">os</span>
<span class="k">print</span> <span class="s">'popen3:'</span>
<span class="n">pipe_stdin</span><span class="p">,</span> <span class="n">pipe_stdout</span><span class="p">,</span> <span class="n">pipe_stderr</span> <span class="o">=</span> <span class="n">os</span><span class="o">.</span><span class="n">popen3</span><span class="p">(</span><span class="s">'cat -; echo ";to stderr" 1>&2'</span><span class="p">)</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">pipe_stdin</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="s">'through stdin to stdout'</span><span class="p">)</span>
<span class="k">finally</span><span class="p">:</span>
<span class="n">pipe_stdin</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">stdout_value</span> <span class="o">=</span> <span class="n">pipe_stdout</span><span class="o">.</span><span class="n">read</span><span class="p">()</span>
<span class="k">finally</span><span class="p">:</span>
<span class="n">pipe_stdout</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
<span class="k">print</span> <span class="s">'</span><span class="se">\t</span><span class="s">pass through:'</span><span class="p">,</span> <span class="nb">repr</span><span class="p">(</span><span class="n">stdout_value</span><span class="p">)</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">stderr_value</span> <span class="o">=</span> <span class="n">pipe_stderr</span><span class="o">.</span><span class="n">read</span><span class="p">()</span>
<span class="k">finally</span><span class="p">:</span>
<span class="n">pipe_stderr</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
<span class="k">print</span> <span class="s">'</span><span class="se">\t</span><span class="s">stderr:'</span><span class="p">,</span> <span class="nb">repr</span><span class="p">(</span><span class="n">stderr_value</span><span class="p">)</span>
</pre></div>
</div>
<p>Notice that the program has to read from and close both stdout and
stderr <em>separately</em>. There are some related to flow control and
sequencing when dealing with I/O for multiple processes. The I/O is
buffered, and if the caller expects to be able to read all of the data
from a stream then the child process must close that stream to
indicate the end-of-file. For more information on these issues, refer
to the <a class="reference external" href="http://docs.python.org/library/popen2.html#popen2-flow-control">Flow Control Issues</a>
section of the Python library documentation.</p>
<div class="highlight-python"><pre>$ python -u os_popen3.py
popen3:
pass through: 'through stdin to stdout'
stderr: ';to stderr\n'</pre>
</div>
<p>And finally, <tt class="xref py py-func docutils literal"><span class="pre">popen4()</span></tt> returns 2 streams, stdin and a merged
stdout/stderr. This is useful when the results of the command need to
be logged, but not parsed directly.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">os</span>
<span class="k">print</span> <span class="s">'popen4:'</span>
<span class="n">pipe_stdin</span><span class="p">,</span> <span class="n">pipe_stdout_and_stderr</span> <span class="o">=</span> <span class="n">os</span><span class="o">.</span><span class="n">popen4</span><span class="p">(</span><span class="s">'cat -; echo ";to stderr" 1>&2'</span><span class="p">)</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">pipe_stdin</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="s">'through stdin to stdout'</span><span class="p">)</span>
<span class="k">finally</span><span class="p">:</span>
<span class="n">pipe_stdin</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">stdout_value</span> <span class="o">=</span> <span class="n">pipe_stdout_and_stderr</span><span class="o">.</span><span class="n">read</span><span class="p">()</span>
<span class="k">finally</span><span class="p">:</span>
<span class="n">pipe_stdout_and_stderr</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
<span class="k">print</span> <span class="s">'</span><span class="se">\t</span><span class="s">combined output:'</span><span class="p">,</span> <span class="nb">repr</span><span class="p">(</span><span class="n">stdout_value</span><span class="p">)</span>
</pre></div>
</div>
<p>All of the messages written to both stdout and stderr are read
together.</p>
<div class="highlight-python"><pre>$ python -u os_popen4.py
popen4:
combined output: 'through stdin to stdout;to stderr\n'</pre>
</div>
<p>Besides accepting a single string command to be given to the shell for
parsing, <tt class="xref py py-func docutils literal"><span class="pre">popen2()</span></tt>, <tt class="xref py py-func docutils literal"><span class="pre">popen3()</span></tt>, and <tt class="xref py py-func docutils literal"><span class="pre">popen4()</span></tt> also
accept a sequence of strings (command, followed by arguments).</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">os</span>
<span class="k">print</span> <span class="s">'popen2, cmd as sequence:'</span>
<span class="n">pipe_stdin</span><span class="p">,</span> <span class="n">pipe_stdout</span> <span class="o">=</span> <span class="n">os</span><span class="o">.</span><span class="n">popen2</span><span class="p">([</span><span class="s">'cat'</span><span class="p">,</span> <span class="s">'-'</span><span class="p">])</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">pipe_stdin</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="s">'through stdin to stdout'</span><span class="p">)</span>
<span class="k">finally</span><span class="p">:</span>
<span class="n">pipe_stdin</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">stdout_value</span> <span class="o">=</span> <span class="n">pipe_stdout</span><span class="o">.</span><span class="n">read</span><span class="p">()</span>
<span class="k">finally</span><span class="p">:</span>
<span class="n">pipe_stdout</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
<span class="k">print</span> <span class="s">'</span><span class="se">\t</span><span class="s">pass through:'</span><span class="p">,</span> <span class="nb">repr</span><span class="p">(</span><span class="n">stdout_value</span><span class="p">)</span>
</pre></div>
</div>
<p>In this case, the arguments are not processed by the shell.</p>
<div class="highlight-python"><pre>$ python -u os_popen2_seq.py
popen2, cmd as sequence:
pass through: 'through stdin to stdout'</pre>
</div>
</div>
<div class="section" id="file-descriptors">
<h2>File Descriptors<a class="headerlink" href="#file-descriptors" title="Permalink to this headline">¶</a></h2>
<p><a class="reference internal" href="#module-os" title="os: Portable access to operating system specific features."><tt class="xref py py-mod docutils literal"><span class="pre">os</span></tt></a> includes the standard set of functions for working with
low-level <em>file descriptors</em> (integers representing open files owned
by the current process). This is a lower-level API than is provided by
<tt class="xref py py-class docutils literal"><span class="pre">file</span></tt> objects. They are not covered here because it is
generally easier to work directly with <tt class="xref py py-class docutils literal"><span class="pre">file</span></tt> objects. Refer to
the library documentation for details.</p>
</div>
<div class="section" id="filesystem-permissions">
<h2>Filesystem Permissions<a class="headerlink" href="#filesystem-permissions" title="Permalink to this headline">¶</a></h2>
<p>The function <tt class="xref py py-func docutils literal"><span class="pre">access()</span></tt> can be used to test the access rights a
process has for a file.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">os</span>
<span class="k">print</span> <span class="s">'Testing:'</span><span class="p">,</span> <span class="n">__file__</span>
<span class="k">print</span> <span class="s">'Exists:'</span><span class="p">,</span> <span class="n">os</span><span class="o">.</span><span class="n">access</span><span class="p">(</span><span class="n">__file__</span><span class="p">,</span> <span class="n">os</span><span class="o">.</span><span class="n">F_OK</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'Readable:'</span><span class="p">,</span> <span class="n">os</span><span class="o">.</span><span class="n">access</span><span class="p">(</span><span class="n">__file__</span><span class="p">,</span> <span class="n">os</span><span class="o">.</span><span class="n">R_OK</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'Writable:'</span><span class="p">,</span> <span class="n">os</span><span class="o">.</span><span class="n">access</span><span class="p">(</span><span class="n">__file__</span><span class="p">,</span> <span class="n">os</span><span class="o">.</span><span class="n">W_OK</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'Executable:'</span><span class="p">,</span> <span class="n">os</span><span class="o">.</span><span class="n">access</span><span class="p">(</span><span class="n">__file__</span><span class="p">,</span> <span class="n">os</span><span class="o">.</span><span class="n">X_OK</span><span class="p">)</span>
</pre></div>
</div>
<p>The results will vary depending on how the example code is installed,
but it will look something like this:</p>
<div class="highlight-python"><pre>$ python os_access.py
Testing: os_access.py
Exists: True
Readable: True
Writable: True
Executable: False</pre>
</div>
<p>The library documentation for <tt class="xref py py-func docutils literal"><span class="pre">access()</span></tt> includes two special
warnings. First, there isn’t much sense in calling <tt class="xref py py-func docutils literal"><span class="pre">access()</span></tt>
to test whether a file can be opened before actually calling
<tt class="xref py py-func docutils literal"><span class="pre">open()</span></tt> on it. There is a small, but real, window of time
between the two calls during which the permissions on the file could
change. The other warning applies mostly to networked filesystems that
extend the POSIX permission semantics. Some filesystem types may
respond to the POSIX call that a process has permission to access a
file, then report a failure when the attempt is made using
<tt class="xref py py-func docutils literal"><span class="pre">open()</span></tt> for some reason not tested via the POSIX call. All in
all, it is better to call <tt class="xref py py-func docutils literal"><span class="pre">open()</span></tt> with the required mode and
catch the <a class="reference internal" href="../exceptions/index.html#exceptions-ioerror"><em>IOError</em></a> raised if there is a
problem.</p>
<p>More detailed information about the file can be accessed using
<tt class="xref py py-func docutils literal"><span class="pre">stat()</span></tt> or <tt class="xref py py-func docutils literal"><span class="pre">lstat()</span></tt> (for checking the status of
something that might be a symbolic link).</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">os</span>
<span class="kn">import</span> <span class="nn">sys</span>
<span class="kn">import</span> <span class="nn">time</span>
<span class="k">if</span> <span class="nb">len</span><span class="p">(</span><span class="n">sys</span><span class="o">.</span><span class="n">argv</span><span class="p">)</span> <span class="o">==</span> <span class="mi">1</span><span class="p">:</span>
<span class="n">filename</span> <span class="o">=</span> <span class="n">__file__</span>
<span class="k">else</span><span class="p">:</span>
<span class="n">filename</span> <span class="o">=</span> <span class="n">sys</span><span class="o">.</span><span class="n">argv</span><span class="p">[</span><span class="mi">1</span><span class="p">]</span>
<span class="n">stat_info</span> <span class="o">=</span> <span class="n">os</span><span class="o">.</span><span class="n">stat</span><span class="p">(</span><span class="n">filename</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'os.stat(</span><span class="si">%s</span><span class="s">):'</span> <span class="o">%</span> <span class="n">filename</span>
<span class="k">print</span> <span class="s">'</span><span class="se">\t</span><span class="s">Size:'</span><span class="p">,</span> <span class="n">stat_info</span><span class="o">.</span><span class="n">st_size</span>
<span class="k">print</span> <span class="s">'</span><span class="se">\t</span><span class="s">Permissions:'</span><span class="p">,</span> <span class="nb">oct</span><span class="p">(</span><span class="n">stat_info</span><span class="o">.</span><span class="n">st_mode</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'</span><span class="se">\t</span><span class="s">Owner:'</span><span class="p">,</span> <span class="n">stat_info</span><span class="o">.</span><span class="n">st_uid</span>
<span class="k">print</span> <span class="s">'</span><span class="se">\t</span><span class="s">Device:'</span><span class="p">,</span> <span class="n">stat_info</span><span class="o">.</span><span class="n">st_dev</span>
<span class="k">print</span> <span class="s">'</span><span class="se">\t</span><span class="s">Last modified:'</span><span class="p">,</span> <span class="n">time</span><span class="o">.</span><span class="n">ctime</span><span class="p">(</span><span class="n">stat_info</span><span class="o">.</span><span class="n">st_mtime</span><span class="p">)</span>
</pre></div>
</div>
<p>Once again, the output will vary depending on how the example code was
installed. Try passing different filenames on the command line to
<tt class="docutils literal"><span class="pre">os_stat.py</span></tt>.</p>
<div class="highlight-python"><pre>$ python os_stat.py
os.stat(os_stat.py):
Size: 1516
Permissions: 0100644
Owner: 527
Device: 234881025
Last modified: Sun Oct 10 09:12:21 2010</pre>
</div>
<p>On Unix-like systems, file permissions can be changed using
<tt class="xref py py-func docutils literal"><span class="pre">chmod()</span></tt>, passing the mode as an integer. Mode values can be
constructed using constants defined in the <tt class="xref py py-mod docutils literal"><span class="pre">stat</span></tt> module. This
example toggles the user’s execute permission bit:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">os</span>
<span class="kn">import</span> <span class="nn">stat</span>
<span class="n">filename</span> <span class="o">=</span> <span class="s">'os_stat_chmod_example.txt'</span>
<span class="k">if</span> <span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">exists</span><span class="p">(</span><span class="n">filename</span><span class="p">):</span>
<span class="n">os</span><span class="o">.</span><span class="n">unlink</span><span class="p">(</span><span class="n">filename</span><span class="p">)</span>
<span class="n">f</span> <span class="o">=</span> <span class="nb">open</span><span class="p">(</span><span class="n">filename</span><span class="p">,</span> <span class="s">'wt'</span><span class="p">)</span>
<span class="n">f</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="s">'contents'</span><span class="p">)</span>
<span class="n">f</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
<span class="c"># Determine what permissions are already set using stat</span>
<span class="n">existing_permissions</span> <span class="o">=</span> <span class="n">stat</span><span class="o">.</span><span class="n">S_IMODE</span><span class="p">(</span><span class="n">os</span><span class="o">.</span><span class="n">stat</span><span class="p">(</span><span class="n">filename</span><span class="p">)</span><span class="o">.</span><span class="n">st_mode</span><span class="p">)</span>
<span class="k">if</span> <span class="ow">not</span> <span class="n">os</span><span class="o">.</span><span class="n">access</span><span class="p">(</span><span class="n">filename</span><span class="p">,</span> <span class="n">os</span><span class="o">.</span><span class="n">X_OK</span><span class="p">):</span>
<span class="k">print</span> <span class="s">'Adding execute permission'</span>
<span class="n">new_permissions</span> <span class="o">=</span> <span class="n">existing_permissions</span> <span class="o">|</span> <span class="n">stat</span><span class="o">.</span><span class="n">S_IXUSR</span>
<span class="k">else</span><span class="p">:</span>
<span class="k">print</span> <span class="s">'Removing execute permission'</span>
<span class="c"># use xor to remove the user execute permission</span>
<span class="n">new_permissions</span> <span class="o">=</span> <span class="n">existing_permissions</span> <span class="o">^</span> <span class="n">stat</span><span class="o">.</span><span class="n">S_IXUSR</span>
<span class="n">os</span><span class="o">.</span><span class="n">chmod</span><span class="p">(</span><span class="n">filename</span><span class="p">,</span> <span class="n">new_permissions</span><span class="p">)</span>
</pre></div>
</div>
<p>The script assumes it has the permissions necessary to modify the mode
of the file when run.</p>
<div class="highlight-python"><pre>$ python os_stat_chmod.py
Adding execute permission</pre>
</div>
</div>
<div class="section" id="directories">
<span id="os-directories"></span><h2>Directories<a class="headerlink" href="#directories" title="Permalink to this headline">¶</a></h2>
<p>There are several functions for working with directories on the filesystem,
including creating, listing contents, and removing them.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">os</span>
<span class="n">dir_name</span> <span class="o">=</span> <span class="s">'os_directories_example'</span>
<span class="k">print</span> <span class="s">'Creating'</span><span class="p">,</span> <span class="n">dir_name</span>
<span class="n">os</span><span class="o">.</span><span class="n">makedirs</span><span class="p">(</span><span class="n">dir_name</span><span class="p">)</span>
<span class="n">file_name</span> <span class="o">=</span> <span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">join</span><span class="p">(</span><span class="n">dir_name</span><span class="p">,</span> <span class="s">'example.txt'</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'Creating'</span><span class="p">,</span> <span class="n">file_name</span>
<span class="n">f</span> <span class="o">=</span> <span class="nb">open</span><span class="p">(</span><span class="n">file_name</span><span class="p">,</span> <span class="s">'wt'</span><span class="p">)</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">f</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="s">'example file'</span><span class="p">)</span>
<span class="k">finally</span><span class="p">:</span>
<span class="n">f</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
<span class="k">print</span> <span class="s">'Listing'</span><span class="p">,</span> <span class="n">dir_name</span>
<span class="k">print</span> <span class="n">os</span><span class="o">.</span><span class="n">listdir</span><span class="p">(</span><span class="n">dir_name</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'Cleaning up'</span>
<span class="n">os</span><span class="o">.</span><span class="n">unlink</span><span class="p">(</span><span class="n">file_name</span><span class="p">)</span>
<span class="n">os</span><span class="o">.</span><span class="n">rmdir</span><span class="p">(</span><span class="n">dir_name</span><span class="p">)</span>
</pre></div>
</div>
<p>There are two sets of functions for creating and deleting
directories. When creating a new directory with <tt class="xref py py-func docutils literal"><span class="pre">mkdir()</span></tt>,
all of the parent directories must already exist. When removing a
directory with <tt class="xref py py-func docutils literal"><span class="pre">rmdir()</span></tt>, only the leaf directory (the last
part of the path) is actually removed. In contrast,
<tt class="xref py py-func docutils literal"><span class="pre">makedirs()</span></tt> and <tt class="xref py py-func docutils literal"><span class="pre">removedirs()</span></tt> operate on all of
the nodes in the path. <tt class="xref py py-func docutils literal"><span class="pre">makedirs()</span></tt> will create any parts of
the path which do not exist, and <tt class="xref py py-func docutils literal"><span class="pre">removedirs()</span></tt> will remove
all of the parent directories (assuming it can).</p>
<div class="highlight-python"><pre>$ python os_directories.py
Creating os_directories_example
Creating os_directories_example/example.txt
Listing os_directories_example
['example.txt']
Cleaning up</pre>
</div>
</div>
<div class="section" id="symbolic-links">
<h2>Symbolic Links<a class="headerlink" href="#symbolic-links" title="Permalink to this headline">¶</a></h2>
<p>For platforms and filesystems that support them, there are functions
for working with symlinks.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">os</span><span class="o">,</span> <span class="nn">tempfile</span>
<span class="n">link_name</span> <span class="o">=</span> <span class="n">tempfile</span><span class="o">.</span><span class="n">mktemp</span><span class="p">()</span>
<span class="k">print</span> <span class="s">'Creating link </span><span class="si">%s</span><span class="s"> -> </span><span class="si">%s</span><span class="s">'</span> <span class="o">%</span> <span class="p">(</span><span class="n">link_name</span><span class="p">,</span> <span class="n">__file__</span><span class="p">)</span>
<span class="n">os</span><span class="o">.</span><span class="n">symlink</span><span class="p">(</span><span class="n">__file__</span><span class="p">,</span> <span class="n">link_name</span><span class="p">)</span>
<span class="n">stat_info</span> <span class="o">=</span> <span class="n">os</span><span class="o">.</span><span class="n">lstat</span><span class="p">(</span><span class="n">link_name</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'Permissions:'</span><span class="p">,</span> <span class="nb">oct</span><span class="p">(</span><span class="n">stat_info</span><span class="o">.</span><span class="n">st_mode</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'Points to:'</span><span class="p">,</span> <span class="n">os</span><span class="o">.</span><span class="n">readlink</span><span class="p">(</span><span class="n">link_name</span><span class="p">)</span>
<span class="c"># Cleanup</span>
<span class="n">os</span><span class="o">.</span><span class="n">unlink</span><span class="p">(</span><span class="n">link_name</span><span class="p">)</span>
</pre></div>
</div>
<p>Although <a class="reference internal" href="#module-os" title="os: Portable access to operating system specific features."><tt class="xref py py-mod docutils literal"><span class="pre">os</span></tt></a> includes <tt class="xref py py-func docutils literal"><span class="pre">tempnam()</span></tt> for creating
temporary filenames, it is not as secure as the <a class="reference internal" href="../tempfile/index.html#module-tempfile" title="tempfile: Create temporary filesystem resources."><tt class="xref py py-mod docutils literal"><span class="pre">tempfile</span></tt></a> module
and produces a <a class="reference internal" href="../exceptions/index.html#exceptions-runtimewarning"><em>RuntimeWarning</em></a>
message when it is used. In general it is better to use
<a class="reference internal" href="../tempfile/index.html#module-tempfile" title="tempfile: Create temporary filesystem resources."><tt class="xref py py-mod docutils literal"><span class="pre">tempfile</span></tt></a>, as in this example.</p>
<div class="highlight-python"><pre>$ python os_symlinks.py
Creating link /var/folders/9R/9R1t+tR02Raxzk+F71Q50U+++Uw/-Tmp-/tmpMnxrGF -> os_symlinks.py
Permissions: 0120755
Points to: os_symlinks.py</pre>
</div>
</div>
<div class="section" id="walking-a-directory-tree">
<h2>Walking a Directory Tree<a class="headerlink" href="#walking-a-directory-tree" title="Permalink to this headline">¶</a></h2>
<p>The function <tt class="xref py py-func docutils literal"><span class="pre">walk()</span></tt> traverses a directory recursively and
for each directory generates a tuple containing the directory path,
any immediate sub-directories of that path, and the names of any files
in that directory.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">os</span><span class="o">,</span> <span class="nn">sys</span>
<span class="c"># If we are not given a path to list, use /tmp</span>
<span class="k">if</span> <span class="nb">len</span><span class="p">(</span><span class="n">sys</span><span class="o">.</span><span class="n">argv</span><span class="p">)</span> <span class="o">==</span> <span class="mi">1</span><span class="p">:</span>
<span class="n">root</span> <span class="o">=</span> <span class="s">'/tmp'</span>
<span class="k">else</span><span class="p">:</span>
<span class="n">root</span> <span class="o">=</span> <span class="n">sys</span><span class="o">.</span><span class="n">argv</span><span class="p">[</span><span class="mi">1</span><span class="p">]</span>
<span class="k">for</span> <span class="n">dir_name</span><span class="p">,</span> <span class="n">sub_dirs</span><span class="p">,</span> <span class="n">files</span> <span class="ow">in</span> <span class="n">os</span><span class="o">.</span><span class="n">walk</span><span class="p">(</span><span class="n">root</span><span class="p">):</span>
<span class="k">print</span> <span class="s">'</span><span class="se">\n</span><span class="s">'</span><span class="p">,</span> <span class="n">dir_name</span>
<span class="c"># Make the subdirectory names stand out with /</span>
<span class="n">sub_dirs</span> <span class="o">=</span> <span class="p">[</span> <span class="s">'</span><span class="si">%s</span><span class="s">/'</span> <span class="o">%</span> <span class="n">n</span> <span class="k">for</span> <span class="n">n</span> <span class="ow">in</span> <span class="n">sub_dirs</span> <span class="p">]</span>
<span class="c"># Mix the directory contents together</span>
<span class="n">contents</span> <span class="o">=</span> <span class="n">sub_dirs</span> <span class="o">+</span> <span class="n">files</span>
<span class="n">contents</span><span class="o">.</span><span class="n">sort</span><span class="p">()</span>
<span class="c"># Show the contents</span>
<span class="k">for</span> <span class="n">c</span> <span class="ow">in</span> <span class="n">contents</span><span class="p">:</span>
<span class="k">print</span> <span class="s">'</span><span class="se">\t</span><span class="si">%s</span><span class="s">'</span> <span class="o">%</span> <span class="n">c</span>
</pre></div>
</div>
<p>This example shows a recursive directory listing.</p>
<div class="highlight-python"><pre>$ python os_walk.py ../zipimport
../zipimport
__init__.py
__init__.pyc
example_package/
index.rst
zipimport_example.zip
zipimport_find_module.py
zipimport_find_module.pyc
zipimport_get_code.py
zipimport_get_code.pyc
zipimport_get_data.py
zipimport_get_data.pyc
zipimport_get_data_nozip.py
zipimport_get_data_nozip.pyc
zipimport_get_data_zip.py
zipimport_get_data_zip.pyc
zipimport_get_source.py
zipimport_get_source.pyc
zipimport_is_package.py
zipimport_is_package.pyc
zipimport_load_module.py
zipimport_load_module.pyc
zipimport_make_example.py
zipimport_make_example.pyc
../zipimport/example_package
README.txt
__init__.py
__init__.pyc</pre>
</div>
</div>
<div class="section" id="running-external-commands">
<span id="os-system"></span><h2>Running External Commands<a class="headerlink" href="#running-external-commands" title="Permalink to this headline">¶</a></h2>
<div class="admonition warning">
<p class="first admonition-title">Warning</p>
<p class="last">Many of these functions for working with processes have limited
portability. For a more consistent way to work with processes in a
platform independent manner, see the <a class="reference internal" href="../subprocess/index.html#module-subprocess" title="subprocess: Work with additional processes"><tt class="xref py py-mod docutils literal"><span class="pre">subprocess</span></tt></a> module
instead.</p>
</div>
<p>The simplest way to run a separate command, without interacting with
it at all, is <tt class="xref py py-func docutils literal"><span class="pre">system()</span></tt>. It takes a single string which is
the command line to be executed by a sub-process running a shell.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">os</span>
<span class="c"># Simple command</span>
<span class="n">os</span><span class="o">.</span><span class="n">system</span><span class="p">(</span><span class="s">'ls -l'</span><span class="p">)</span>
</pre></div>
</div>
<p>The return value of <tt class="xref py py-func docutils literal"><span class="pre">system()</span></tt> is the exit value of the shell
running the program packed into a 16 bit number, with the high byte
the exit status and the low byte the signal number that caused the
process to die, or zero.</p>
<div class="highlight-python"><pre>$ python -u os_system_example.py
total 248
-rw-r--r-- 1 dhellmann dhellmann 0 Oct 10 09:12 __init__.py
-rw-r--r-- 1 dhellmann dhellmann 22702 Oct 24 08:51 index.rst
-rw-r--r-- 1 dhellmann dhellmann 1360 Oct 10 09:12 os_access.py
-rw-r--r-- 1 dhellmann dhellmann 1292 Oct 16 10:56 os_cwd_example.py
-rw-r--r-- 1 dhellmann dhellmann 1499 Oct 10 09:12 os_directories.py
-rw-r--r-- 1 dhellmann dhellmann 1573 Oct 10 09:12 os_environ_example.py
-rw-r--r-- 1 dhellmann dhellmann 1241 Oct 10 09:12 os_exec_example.py
-rw-r--r-- 1 dhellmann dhellmann 1267 Oct 10 09:12 os_fork_example.py
-rw-r--r-- 1 dhellmann dhellmann 1703 Oct 10 09:12 os_kill_example.py
-rw-r--r-- 1 dhellmann dhellmann 1476 Oct 10 09:12 os_popen.py
-rw-r--r-- 1 dhellmann dhellmann 1506 Oct 16 10:56 os_popen2.py
-rw-r--r-- 1 dhellmann dhellmann 1528 Oct 16 10:56 os_popen2_seq.py
-rw-r--r-- 1 dhellmann dhellmann 1658 Oct 16 10:56 os_popen3.py
-rw-r--r-- 1 dhellmann dhellmann 1567 Oct 16 10:56 os_popen4.py
-rw-r--r-- 1 dhellmann dhellmann 1395 Oct 10 09:12 os_process_id_example.py
-rw-r--r-- 1 dhellmann dhellmann 1896 Oct 16 10:56 os_process_user_example.py
-rw-r--r-- 1 dhellmann dhellmann 1206 Oct 10 09:12 os_spawn_example.py
-rw-r--r-- 1 dhellmann dhellmann 1516 Oct 10 09:12 os_stat.py
-rw-r--r-- 1 dhellmann dhellmann 1751 Oct 10 09:12 os_stat_chmod.py
-rwxr--r-- 1 dhellmann dhellmann 8 Oct 24 08:54 os_stat_chmod_example.txt
-rw-r--r-- 1 dhellmann dhellmann 1421 Oct 16 10:56 os_symlinks.py
-rw-r--r-- 1 dhellmann dhellmann 1250 Oct 10 09:12 os_system_background.py
-rw-r--r-- 1 dhellmann dhellmann 1191 Oct 10 09:12 os_system_example.py
-rw-r--r-- 1 dhellmann dhellmann 1214 Oct 10 09:12 os_system_shell.py
-rw-r--r-- 1 dhellmann dhellmann 1499 Oct 10 09:12 os_wait_example.py
-rw-r--r-- 1 dhellmann dhellmann 1555 Oct 10 09:12 os_waitpid_example.py
-rw-r--r-- 1 dhellmann dhellmann 1643 Oct 10 09:12 os_walk.py</pre>
</div>
<p>Since the command is passed directly to the shell for processing, it can even
include shell syntax such as globbing or environment variables:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">os</span>
<span class="c"># Command with shell expansion</span>
<span class="n">os</span><span class="o">.</span><span class="n">system</span><span class="p">(</span><span class="s">'ls -ld $TMPDIR'</span><span class="p">)</span>
</pre></div>
</div>
<div class="highlight-python"><pre>$ python -u os_system_shell.py
drwx------ 41 dhellmann dhellmann 1394 Oct 24 08:54 /var/folders/9R/9R1t+tR02Raxzk+F71Q50U+++Uw/-Tmp-/</pre>
</div>
<p>Unless the command is explicitly run in the background, the call to
<tt class="xref py py-func docutils literal"><span class="pre">system()</span></tt> blocks until it is complete. Standard input,
output, and error from the child process are tied to the appropriate
streams owned by the caller by default, but can be redirected using
shell syntax.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">os</span>
<span class="kn">import</span> <span class="nn">time</span>
<span class="k">print</span> <span class="s">'Calling...'</span>
<span class="n">os</span><span class="o">.</span><span class="n">system</span><span class="p">(</span><span class="s">'date; (sleep 3; date) &'</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'Sleeping...'</span>
<span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mi">5</span><span class="p">)</span>
</pre></div>
</div>
<p>This is getting into shell trickery, though, and there are better ways to
accomplish the same thing.</p>
<div class="highlight-python"><pre>$ python -u os_system_background.py
Calling...
Sun Oct 24 08:54:06 EDT 2010
Sleeping...
Sun Oct 24 08:54:09 EDT 2010</pre>
</div>
</div>
<div class="section" id="creating-processes-with-os-fork">
<span id="id1"></span><h2>Creating Processes with os.fork()<a class="headerlink" href="#creating-processes-with-os-fork" title="Permalink to this headline">¶</a></h2>
<p>The POSIX functions <tt class="xref py py-func docutils literal"><span class="pre">fork()</span></tt> and <tt class="xref py py-func docutils literal"><span class="pre">exec*()</span></tt> (available
under Mac OS X, Linux, and other UNIX variants) are exposed via the
<a class="reference internal" href="#module-os" title="os: Portable access to operating system specific features."><tt class="xref py py-mod docutils literal"><span class="pre">os</span></tt></a> module. Entire books have been written about reliably using
these functions, so check the library or bookstore for more details
than are presented here.</p>
<p>To create a new process as a clone of the current process, use
<tt class="xref py py-func docutils literal"><span class="pre">fork()</span></tt>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">os</span>
<span class="n">pid</span> <span class="o">=</span> <span class="n">os</span><span class="o">.</span><span class="n">fork</span><span class="p">()</span>
<span class="k">if</span> <span class="n">pid</span><span class="p">:</span>
<span class="k">print</span> <span class="s">'Child process id:'</span><span class="p">,</span> <span class="n">pid</span>
<span class="k">else</span><span class="p">:</span>
<span class="k">print</span> <span class="s">'I am the child'</span>
</pre></div>
</div>
<p>The output will vary based on the state of the system each time the
example is run, but it will look something like:</p>
<div class="highlight-python"><pre>$ python -u os_fork_example.py
Child process id: 14215
I am the child</pre>
</div>
<p>After the fork, there are two processes running the same code. For a
program to tell which one it is in, it needs to check the return value
of <tt class="xref py py-func docutils literal"><span class="pre">fork()</span></tt>. If the value is <tt class="docutils literal"><span class="pre">0</span></tt>, the current process is the
child. If it is not <tt class="docutils literal"><span class="pre">0</span></tt>, the program is running in the parent
process and the return value is the process id of the child process.</p>
<p>From the parent process, it is possible to send the child
signals. This is a bit more complicated to set up, and uses the
<a class="reference internal" href="../signal/index.html#module-signal" title="signal: Receive notification of asynchronous system events"><tt class="xref py py-mod docutils literal"><span class="pre">signal</span></tt></a> module. First, define a signal handler to be invoked
when the signal is received.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">os</span>
<span class="kn">import</span> <span class="nn">signal</span>
<span class="kn">import</span> <span class="nn">time</span>
<span class="k">def</span> <span class="nf">signal_usr1</span><span class="p">(</span><span class="n">signum</span><span class="p">,</span> <span class="n">frame</span><span class="p">):</span>
<span class="s">"Callback invoked when a signal is received"</span>
<span class="n">pid</span> <span class="o">=</span> <span class="n">os</span><span class="o">.</span><span class="n">getpid</span><span class="p">()</span>
<span class="k">print</span> <span class="s">'Received USR1 in process </span><span class="si">%s</span><span class="s">'</span> <span class="o">%</span> <span class="n">pid</span>
</pre></div>
</div>
<p>Then <tt class="xref py py-func docutils literal"><span class="pre">fork()</span></tt>, and in the parent pause a short amount of time
before sending a <tt class="xref py py-const docutils literal"><span class="pre">USR1</span></tt> signal using <tt class="xref py py-func docutils literal"><span class="pre">kill()</span></tt>. The
short pause gives the child process time to set up the signal handler.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">print</span> <span class="s">'Forking...'</span>
<span class="n">child_pid</span> <span class="o">=</span> <span class="n">os</span><span class="o">.</span><span class="n">fork</span><span class="p">()</span>
<span class="k">if</span> <span class="n">child_pid</span><span class="p">:</span>
<span class="k">print</span> <span class="s">'PARENT: Pausing before sending signal...'</span>
<span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'PARENT: Signaling </span><span class="si">%s</span><span class="s">'</span> <span class="o">%</span> <span class="n">child_pid</span>
<span class="n">os</span><span class="o">.</span><span class="n">kill</span><span class="p">(</span><span class="n">child_pid</span><span class="p">,</span> <span class="n">signal</span><span class="o">.</span><span class="n">SIGUSR1</span><span class="p">)</span>
</pre></div>
</div>
<p>In the child, set up the signal handler and go to sleep for a while to
give the parent time to send the signal:</p>
<div class="highlight-python"><pre>else:
print 'CHILD: Setting up signal handler'
signal.signal(signal.SIGUSR1, signal_usr1)
print 'CHILD: Pausing to wait for signal'
time.sleep(5)
</pre>
</div>
<p>A real application, wouldn’t need (or want) to call <tt class="xref py py-func docutils literal"><span class="pre">sleep()</span></tt>.</p>
<div class="highlight-python"><pre>$ python os_kill_example.py
Forking...
PARENT: Pausing before sending signal...
PARENT: Signaling 14218
Forking...
CHILD: Setting up signal handler
CHILD: Pausing to wait for signal
Received USR1 in process 14218</pre>
</div>
<p>A simple way to handle separate behavior in the child process is to
check the return value of <tt class="xref py py-func docutils literal"><span class="pre">fork()</span></tt> and branch. More complex
behavior may call for more code separation than a simple branch. In
other cases, there may be an existing program that needs to be
wrapped. For both of these situations, the <tt class="xref py py-func docutils literal"><span class="pre">exec*()</span></tt> series
of functions can be used to run another program.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">os</span>
<span class="n">child_pid</span> <span class="o">=</span> <span class="n">os</span><span class="o">.</span><span class="n">fork</span><span class="p">()</span>
<span class="k">if</span> <span class="n">child_pid</span><span class="p">:</span>
<span class="n">os</span><span class="o">.</span><span class="n">waitpid</span><span class="p">(</span><span class="n">child_pid</span><span class="p">,</span> <span class="mi">0</span><span class="p">)</span>
<span class="k">else</span><span class="p">:</span>
<span class="n">os</span><span class="o">.</span><span class="n">execlp</span><span class="p">(</span><span class="s">'ls'</span><span class="p">,</span> <span class="s">'ls'</span><span class="p">,</span> <span class="s">'-l'</span><span class="p">,</span> <span class="s">'/tmp/'</span><span class="p">)</span>
</pre></div>
</div>
<p>When a program is “execed”, the code from that program replaces the
code from the existing process.</p>
<div class="highlight-python"><pre>$ python os_exec_example.py
total 88
lrwxr-xr-x 1 root wheel 64 Oct 9 16:44 4cb0d41dcb773 -> /private/etc/cups/ppd/HP_LaserJet_P2015_Series__Development_.ppd
lrwxr-xr-x 1 root wheel 64 Oct 16 16:44 4cba0eaed2f64 -> /private/etc/cups/ppd/HP_LaserJet_P2015_Series__Development_.ppd
drwx------ 2 dhellmann wheel 68 Oct 20 18:49 CrashUpload-JncPM
-rw------- 1 root wheel 1507 Oct 22 03:00 ccc_exclude.RicTKq
-rw------- 1 root wheel 1507 Oct 21 03:00 ccc_exclude.WoHUlT
drwx------ 3 dhellmann wheel 102 Oct 3 08:19 emacs527
-rw-r--r-- 1 dhellmann wheel 12288 Oct 24 08:53 example.db
srwxr-xr-x 1 dhellmann wheel 0 Oct 10 08:34 icssuis527
drwx------ 3 dhellmann wheel 102 Oct 3 07:57 launch-0gVBko
drwx------ 3 dhellmann wheel 102 Oct 3 07:57 launch-T9WHDU
drwx------ 3 dhellmann wheel 102 Oct 3 07:57 launch-aLrEc7
drwx------ 3 dhellmann wheel 102 Oct 3 07:57 launchd-166.12Vldj
-rw-r--r-- 1 dhellmann wheel 12288 Oct 21 05:59 pymotw_import_example.shelve
-rw-r--r-- 1 dhellmann wheel 448 Oct 21 06:00 trace_example.recurse.cover
drwxr-xr-x 2 dhellmann dhellmann 68 Oct 23 06:50 var_backups</pre>
</div>
<p>There are many variations of <tt class="xref py py-func docutils literal"><span class="pre">exec*()</span></tt>, depending on the form in
which the arguments are available, whether the path and environment of
the parent process should be be copied to the child, etc. Refer to the
library documentation for complete details.</p>
<p>For all variations, the first argument is a path or filename and the
remaining arguments control how that program runs. They are either
passed as command line arguments or override the process “environment”
(see <tt class="xref py py-data docutils literal"><span class="pre">os.environ</span></tt> and <tt class="xref py py-data docutils literal"><span class="pre">os.getenv</span></tt>).</p>
</div>
<div class="section" id="waiting-for-a-child">
<h2>Waiting for a Child<a class="headerlink" href="#waiting-for-a-child" title="Permalink to this headline">¶</a></h2>
<p>Many computationally intensive programs use multiple processes to work
around the threading limitations of Python and the Global Interpreter
Lock. When starting several processes to run separate tasks, the
master will need to wait for one or more of them to finish before
starting new ones, to avoid overloading the server. There are a few
different ways to do that using <tt class="xref py py-func docutils literal"><span class="pre">wait()</span></tt> and related functions.</p>
<p>When it does not matter which child process might exit first, use
<tt class="xref py py-func docutils literal"><span class="pre">wait()</span></tt>. It returns as soon as any child process exits.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">os</span>
<span class="kn">import</span> <span class="nn">sys</span>
<span class="kn">import</span> <span class="nn">time</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">3</span><span class="p">):</span>
<span class="k">print</span> <span class="s">'PARENT: Forking </span><span class="si">%s</span><span class="s">'</span> <span class="o">%</span> <span class="n">i</span>
<span class="n">worker_pid</span> <span class="o">=</span> <span class="n">os</span><span class="o">.</span><span class="n">fork</span><span class="p">()</span>
<span class="k">if</span> <span class="ow">not</span> <span class="n">worker_pid</span><span class="p">:</span>
<span class="k">print</span> <span class="s">'WORKER </span><span class="si">%s</span><span class="s">: Starting'</span> <span class="o">%</span> <span class="n">i</span>
<span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mi">2</span> <span class="o">+</span> <span class="n">i</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'WORKER </span><span class="si">%s</span><span class="s">: Finishing'</span> <span class="o">%</span> <span class="n">i</span>
<span class="n">sys</span><span class="o">.</span><span class="n">exit</span><span class="p">(</span><span class="n">i</span><span class="p">)</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">3</span><span class="p">):</span>
<span class="k">print</span> <span class="s">'PARENT: Waiting for </span><span class="si">%s</span><span class="s">'</span> <span class="o">%</span> <span class="n">i</span>
<span class="n">done</span> <span class="o">=</span> <span class="n">os</span><span class="o">.</span><span class="n">wait</span><span class="p">()</span>
<span class="k">print</span> <span class="s">'PARENT:'</span><span class="p">,</span> <span class="n">done</span>
</pre></div>
</div>
<p>The return value from <tt class="xref py py-func docutils literal"><span class="pre">wait()</span></tt> is a tuple containing the
process id and exit status (“a 16-bit number, whose low byte is the
signal number that killed the process, and whose high byte is the exit
status”).</p>
<div class="highlight-python"><pre>$ python os_wait_example.py
PARENT: Forking 0
WORKER 0: Starting
WORKER 0: Finishing
PARENT: Forking 0
PARENT: Forking 1
WORKER 1: Starting
WORKER 1: Finishing
PARENT: Forking 0
PARENT: Forking 1
PARENT: Forking 2
WORKER 2: Starting
WORKER 2: Finishing
PARENT: Forking 0
PARENT: Forking 1
PARENT: Forking 2
PARENT: Waiting for 0
PARENT: (14224, 0)
PARENT: Waiting for 1
PARENT: (14225, 256)
PARENT: Waiting for 2
PARENT: (14226, 512)</pre>
</div>
<p>To wait for a specific process, use <tt class="xref py py-func docutils literal"><span class="pre">waitpid()</span></tt>.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">os</span>
<span class="kn">import</span> <span class="nn">sys</span>
<span class="kn">import</span> <span class="nn">time</span>
<span class="n">workers</span> <span class="o">=</span> <span class="p">[]</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">3</span><span class="p">):</span>
<span class="k">print</span> <span class="s">'PARENT: Forking </span><span class="si">%s</span><span class="s">'</span> <span class="o">%</span> <span class="n">i</span>
<span class="n">worker_pid</span> <span class="o">=</span> <span class="n">os</span><span class="o">.</span><span class="n">fork</span><span class="p">()</span>
<span class="k">if</span> <span class="ow">not</span> <span class="n">worker_pid</span><span class="p">:</span>
<span class="k">print</span> <span class="s">'WORKER </span><span class="si">%s</span><span class="s">: Starting'</span> <span class="o">%</span> <span class="n">i</span>
<span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mi">2</span> <span class="o">+</span> <span class="n">i</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'WORKER </span><span class="si">%s</span><span class="s">: Finishing'</span> <span class="o">%</span> <span class="n">i</span>
<span class="n">sys</span><span class="o">.</span><span class="n">exit</span><span class="p">(</span><span class="n">i</span><span class="p">)</span>
<span class="n">workers</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">worker_pid</span><span class="p">)</span>
<span class="k">for</span> <span class="n">pid</span> <span class="ow">in</span> <span class="n">workers</span><span class="p">:</span>
<span class="k">print</span> <span class="s">'PARENT: Waiting for </span><span class="si">%s</span><span class="s">'</span> <span class="o">%</span> <span class="n">pid</span>
<span class="n">done</span> <span class="o">=</span> <span class="n">os</span><span class="o">.</span><span class="n">waitpid</span><span class="p">(</span><span class="n">pid</span><span class="p">,</span> <span class="mi">0</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'PARENT:'</span><span class="p">,</span> <span class="n">done</span>
</pre></div>
</div>
<p>Pass the process id of the target process, and <tt class="xref py py-func docutils literal"><span class="pre">waitpid()</span></tt> blocks
until that process exits.</p>
<div class="highlight-python"><pre>$ python os_waitpid_example.py
PARENT: Forking 0
WORKER 0: Starting
WORKER 0: Finishing
PARENT: Forking 0
PARENT: Forking 1
WORKER 1: Starting
WORKER 1: Finishing
PARENT: Forking 0
PARENT: Forking 1
PARENT: Forking 2
WORKER 2: Starting
WORKER 2: Finishing
PARENT: Forking 0
PARENT: Forking 1
PARENT: Forking 2
PARENT: Waiting for 14229
PARENT: (14229, 0)
PARENT: Waiting for 14230
PARENT: (14230, 256)
PARENT: Waiting for 14231
PARENT: (14231, 512)</pre>
</div>
<p><tt class="xref py py-func docutils literal"><span class="pre">wait3()</span></tt> and <tt class="xref py py-func docutils literal"><span class="pre">wait4()</span></tt> work in a similar manner, but
return more detailed information about the child process with the pid,
exit status, and resource usage.</p>
</div>
<div class="section" id="spawn">
<h2>Spawn<a class="headerlink" href="#spawn" title="Permalink to this headline">¶</a></h2>
<p>As a convenience, the <tt class="xref py py-func docutils literal"><span class="pre">spawn*()</span></tt> family of functions handles the
<tt class="xref py py-func docutils literal"><span class="pre">fork()</span></tt> and <tt class="xref py py-func docutils literal"><span class="pre">exec*()</span></tt> in one statement:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">os</span>
<span class="n">os</span><span class="o">.</span><span class="n">spawnlp</span><span class="p">(</span><span class="n">os</span><span class="o">.</span><span class="n">P_WAIT</span><span class="p">,</span> <span class="s">'ls'</span><span class="p">,</span> <span class="s">'ls'</span><span class="p">,</span> <span class="s">'-l'</span><span class="p">,</span> <span class="s">'/tmp/'</span><span class="p">)</span>
</pre></div>
</div>
<p>The first argument is a mode indicating whether or not to wait for the
process to finish before returning. This example waits. Use
<tt class="xref py py-const docutils literal"><span class="pre">P_NOWAIT</span></tt> to let the other process start, but then resume in
the current process.</p>
<div class="highlight-python"><pre>$ python os_spawn_example.py
total 88
lrwxr-xr-x 1 root wheel 64 Oct 9 16:44 4cb0d41dcb773 -> /private/etc/cups/ppd/HP_LaserJet_P2015_Series__Development_.ppd
lrwxr-xr-x 1 root wheel 64 Oct 16 16:44 4cba0eaed2f64 -> /private/etc/cups/ppd/HP_LaserJet_P2015_Series__Development_.ppd
drwx------ 2 dhellmann wheel 68 Oct 20 18:49 CrashUpload-JncPM
-rw------- 1 root wheel 1507 Oct 22 03:00 ccc_exclude.RicTKq
-rw------- 1 root wheel 1507 Oct 21 03:00 ccc_exclude.WoHUlT
drwx------ 3 dhellmann wheel 102 Oct 3 08:19 emacs527
-rw-r--r-- 1 dhellmann wheel 12288 Oct 24 08:53 example.db
srwxr-xr-x 1 dhellmann wheel 0 Oct 10 08:34 icssuis527
drwx------ 3 dhellmann wheel 102 Oct 3 07:57 launch-0gVBko
drwx------ 3 dhellmann wheel 102 Oct 3 07:57 launch-T9WHDU
drwx------ 3 dhellmann wheel 102 Oct 3 07:57 launch-aLrEc7
drwx------ 3 dhellmann wheel 102 Oct 3 07:57 launchd-166.12Vldj
-rw-r--r-- 1 dhellmann wheel 12288 Oct 21 05:59 pymotw_import_example.shelve
-rw-r--r-- 1 dhellmann wheel 448 Oct 21 06:00 trace_example.recurse.cover
drwxr-xr-x 2 dhellmann dhellmann 68 Oct 23 06:50 var_backups</pre>
</div>
<div class="admonition-see-also admonition seealso">
<p class="first admonition-title">See also</p>
<dl class="docutils">
<dt><a class="reference external" href="http://docs.python.org/lib/module-os.html">os</a></dt>
<dd>Standard library documentation for this module.</dd>
<dt><a class="reference internal" href="../subprocess/index.html#module-subprocess" title="subprocess: Work with additional processes"><tt class="xref py py-mod docutils literal"><span class="pre">subprocess</span></tt></a></dt>
<dd>The subprocess module supersedes os.popen().</dd>
<dt><a class="reference internal" href="../multiprocessing/index.html#module-multiprocessing" title="multiprocessing: Manage processes like threads."><tt class="xref py py-mod docutils literal"><span class="pre">multiprocessing</span></tt></a></dt>
<dd>The multiprocessing module makes working with extra processes
easier than doing all of the work yourself.</dd>
<dt><a class="reference internal" href="../tempfile/index.html#module-tempfile" title="tempfile: Create temporary filesystem resources."><tt class="xref py py-mod docutils literal"><span class="pre">tempfile</span></tt></a></dt>
<dd>The tempfile module for working with temporary files.</dd>
<dt><em>Unix Manual Page Introduction</em></dt>
<dd><p class="first">Includes definitions of real and effective ids, etc.</p>
<p class="last"><a class="reference external" href="http://www.scit.wlv.ac.uk/cgi-bin/mansec?2+intro">http://www.scit.wlv.ac.uk/cgi-bin/mansec?2+intro</a></p>
</dd>
<dt><em>Speaking UNIX, Part 8.</em></dt>
<dd><p class="first">Learn how UNIX multitasks.</p>
<p class="last"><a class="reference external" href="http://www.ibm.com/developerworks/aix/library/au-speakingunix8/index.html">http://www.ibm.com/developerworks/aix/library/au-speakingunix8/index.html</a></p>
</dd>
<dt><em>Unix Concepts</em></dt>
<dd><p class="first">For more discussion of stdin, stdout, and stderr.</p>
<p class="last"><a class="reference external" href="http://www.linuxhq.com/guides/LUG/node67.html">http://www.linuxhq.com/guides/LUG/node67.html</a></p>
</dd>
<dt><em>Delve into Unix Process Creation</em></dt>
<dd><p class="first">Explains the life cycle of a UNIX process.</p>
<p class="last"><a class="reference external" href="http://www.ibm.com/developerworks/aix/library/au-unixprocess.html">http://www.ibm.com/developerworks/aix/library/au-unixprocess.html</a></p>
</dd>
<dt><a class="reference external" href="http://www.amazon.com/Programming-Environment-Addison-Wesley-Professional-Computing/dp/0201433079/ref=pd_bbs_3/002-2842372-4768037?ie=UTF8&s=books&amp;qid=1182098757&sr=8-3">Advanced Programming in the UNIX(R) Environment</a></dt>
<dd>Covers working with multiple processes, such as handling signals, closing duplicated
file descriptors, etc.</dd>
</dl>
<p class="last"><a class="reference internal" href="../articles/file_access.html#article-file-access"><em>File Access</em></a></p>
</div>
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