sh.html

IEEE 1003.1-2003, Single Unix Specification v3

#
# !INSTALLSHELLPATH -p
#
scripts="a b c"
#
# Transform each script
#
for i in ${scripts}
do
    sed -e "s|INSTALLSHELLPATH|${Pshell}|" < ${i}.source > ${i}
done
</tt>
</pre>
</blockquote>

<h4><a name="tag_04_128_17"></a>EXAMPLES</h4>

<blockquote>
<ol>
<li>
<p>Execute a shell command from a string:</p>

<pre>
<tt>sh -c "cat myfile"
</tt>
</pre>
</li>

<li>
<p>Execute a shell script from a file in the current directory:</p>

<pre>
<tt>sh my_shell_cmds
</tt>
</pre>
</li>
</ol>
</blockquote>

<h4><a name="tag_04_128_18"></a>RATIONALE</h4>

<blockquote>
<p>The <i>sh</i> utility and the <a href="../utilities/set.html"><i>set</i></a> special built-in utility share a common
set of options.</p>

<p>The KornShell ignores the contents of <i>IFS</i> upon entry to the script. A conforming application cannot rely on importing
<i>IFS .</i> One justification for this, beyond security considerations, is to assist possible future shell compilers. Allowing
<i>IFS</i> to be imported from the environment prevents many optimizations that might otherwise be performed via dataflow analysis
of the script itself.</p>

<p>The text in the STDIN section about non-blocking reads concerns an instance of <i>sh</i> that has been invoked, probably by a
C-language program, with standard input that has been opened using the O_NONBLOCK flag; see <a href=
"../functions/open.html"><i>open</i>()</a> in the System Interfaces volume of IEEE&nbsp;Std&nbsp;1003.1-2001. If the shell did not
reset this flag, it would immediately terminate because no input data would be available yet and that would be considered the same
as end-of-file.</p>

<p>The options associated with a <i>restricted shell</i> (command name <i>rsh</i> and the <b>-r</b> option) were excluded because
the standard developers considered that the implied level of security could not be achieved and they did not want to raise false
expectations.</p>

<p>On systems that support set-user-ID scripts, a historical trapdoor has been to link a script to the name <b>-i</b>. When it is
called by a sequence such as:</p>

<pre>
<tt>sh -
</tt>
</pre>

<p>or by:</p>

<pre>
<tt>#! usr/bin/sh -
</tt>
</pre>

<p>the historical systems have assumed that no option letters follow. Thus, this volume of IEEE&nbsp;Std&nbsp;1003.1-2001 allows
the single hyphen to mark the end of the options, in addition to the use of the regular <tt>"--"</tt> argument, because it was
considered that the older practice was so pervasive. An alternative approach is taken by the KornShell, where real and effective
user/group IDs must match for an interactive shell; this behavior is specifically allowed by this volume of
IEEE&nbsp;Std&nbsp;1003.1-2001. <basefont size="2"></p>

<dl>
<dt><b>Note:</b></dt>

<dd>There are other problems with set-user-ID scripts that the two approaches described here do not resolve.</dd>
</dl>

<basefont size="3"> 

<p>The initialization process for the history file can be dependent on the system start-up files, in that they may contain commands
that effectively preempt the user's settings of <i>HISTFILE</i> and <i>HISTSIZE .</i> For example, function definition commands are
recorded in the history file, unless the <a href="../utilities/set.html"><i>set</i></a> <b>-o</b> <i>nolog</i> option is
set. If the system administrator includes function definitions in some system start-up file called before the <i>ENV</i> file, the
history file is initialized before the user gets a chance to influence its characteristics. In some historical shells, the history
file is initialized just after the <i>ENV</i> file has been processed. Therefore, it is implementation-defined whether changes made
to <i>HISTFILE</i> after the history file has been initialized are effective.</p>

<p>The default messages for the various <i>MAIL -related</i> messages are unspecified because they vary across implementations.
Typical messages are:</p>

<pre>
<tt>"you have mail\n"
</tt>
</pre>

<p>or:</p>

<pre>
<tt>"you have new mail\n"
</tt>
</pre>

<p>It is important that the descriptions of command line editing refer to the same shell as that in IEEE&nbsp;Std&nbsp;1003.1-2001
so that interactive users can also be application programmers without having to deal with programmatic differences in their two
environments. It is also essential that the utility name <i>sh</i> be specified because this explicit utility name is too firmly
rooted in historical practice of application programs for it to change.</p>

<p>Consideration was given to mandating a diagnostic message when attempting to set <a href=
"../utilities/vi.html"><i>vi</i></a>-mode on terminals that do not support command line editing. However, it is not historical
practice for the shell to be cognizant of all terminal types and thus be able to detect inappropriate terminals in all cases.
Implementations are encouraged to supply diagnostics in this case whenever possible, rather than leaving the user in a state where
editing commands work incorrectly.</p>

<p>In early proposals, the KornShell-derived <i>emacs</i> mode of command line editing was included, even though the <i>emacs</i>
editor itself was not. The community of <i>emacs</i> proponents was adamant that the full <i>emacs</i> editor not be standardized
because they were concerned that an attempt to standardize this very powerful environment would encourage vendors to ship strictly
conforming versions lacking the extensibility required by the community. The author of the original <i>emacs</i> program also
expressed his desire to omit the program. Furthermore, there were a number of historical systems that did not include <i>emacs</i>,
or included it without supporting it, but there were very few that did not include and support <a href=
"../utilities/vi.html"><i>vi</i></a>. The shell <i>emacs</i> command line editing mode was finally omitted because it became
apparent that the KornShell version and the editor being distributed with the GNU system had diverged in some respects. The author
of <i>emacs</i> requested that the POSIX <i>emacs</i> mode either be deleted or have a significant number of unspecified
conditions. Although the KornShell author agreed to consider changes to bring the shell into alignment, the standard developers
decided to defer specification at that time. At the time, it was assumed that convergence on an acceptable definition would occur
for a subsequent draft, but that has not happened, and there appears to be no impetus to do so. In any case, implementations are
free to offer additional command line editing modes based on the exact models of editors their users are most comfortable with.</p>

<p>Early proposals had the following list entry in <a href="#tag_04_128_13_04">vi Line Editing Insert Mode</a> :</p>

<dl compact>
<dt><tt>\</tt></dt>

<dd>If followed by the <i>erase</i> or <i>kill</i> character, that character shall be inserted into the input line. Otherwise, the
backslash itself shall be inserted into the input line.</dd>
</dl>

<p>However, this is not actually a feature of <i>sh</i> command line editing insert mode, but one of some historical terminal line
drivers. Some conforming implementations continue to do this when the <a href="../utilities/stty.html"><i>stty</i></a>
<b>iexten</b> flag is set.</p>
</blockquote>

<h4><a name="tag_04_128_19"></a>FUTURE DIRECTIONS</h4>

<blockquote>
<p>None.</p>
</blockquote>

<h4><a name="tag_04_128_20"></a>SEE ALSO</h4>

<blockquote>
<p><a href="xcu_chap02.html#tag_02"><i>Shell Command Language</i></a> , <a href="cd.html"><i>cd</i></a> , <a href=
"echo.html"><i>echo</i></a> , <a href="exit.html#tag_04_47"><i>exit</i>()</a> , <a href="fc.html"><i>fc</i></a> , <a href=
"pwd.html"><i>pwd</i></a> , <a href="read.html"><i>read</i>()</a> , <a href="set.html#tag_04_127"><i>set</i></a> , <a href=
"stty.html"><i>stty</i></a> , <a href="test.html"><i>test</i></a> , <a href="umask.html"><i>umask</i>()</a> , <a href=
"vi.html"><i>vi</i></a> , the System Interfaces volume of IEEE&nbsp;Std&nbsp;1003.1-2001, <a href=
"../functions/dup.html"><i>dup</i>()</a>, <i>exec</i>, <a href="../functions/exit.html"><i>exit</i>()</a>, <a href=
"../functions/fork.html"><i>fork</i>()</a>, <a href="../functions/open.html"><i>open</i>()</a>, <a href=
"../functions/pipe.html"><i>pipe</i>()</a>, <a href="../functions/signal.html"><i>signal</i>()</a>, <a href=
"../functions/system.html"><i>system</i>()</a>, <a href="../functions/ulimit.html"><i>ulimit</i>()</a>, <a href=
"../functions/umask.html"><i>umask</i>()</a>, <a href="../functions/wait.html"><i>wait</i>()</a></p>
</blockquote>

<h4><a name="tag_04_128_21"></a>CHANGE HISTORY</h4>

<blockquote>
<p>First released in Issue 2.</p>
</blockquote>

<h4><a name="tag_04_128_22"></a>Issue 5</h4>

<blockquote>
<p>The FUTURE DIRECTIONS section is added.</p>

<p>Text is added to the DESCRIPTION for the Large File Summit proposal.</p>
</blockquote>

<h4><a name="tag_04_128_23"></a>Issue 6</h4>

<blockquote>
<p>The Open Group Corrigendum U029/2 is applied, correcting the second SYNOPSIS.</p>

<p>The Open Group Corrigendum U027/3 is applied, correcting a typographical error.</p>

<p>The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:</p>

<ul>
<li>
<p>The option letters derived from the <a href="../utilities/set.html"><i>set</i></a> special built-in are also accepted
with a leading plus sign ( <tt>'+'</tt> ).</p>
</li>

<li>
<p>Large file extensions are added:</p>

<ul>
<li>
<p>Pathname expansion does not fail due to the size of a file.</p>
</li>

<li>
<p>Shell input and output redirections have an implementation-defined offset maximum that is established in the open file
description.</p>
</li>
</ul>
</li>
</ul>

<p>In the ENVIRONMENT VARIABLES section, the text &quot;user's home directory&quot; is updated to &quot;directory referred to by the
<i>HOME</i> environment variable&quot;.</p>

<p>Descriptions for the <i>ENV</i> and <i>PWD</i> environment variables are included to align with the IEEE&nbsp;P1003.2b draft
standard.</p>

<p>The normative text is reworded to avoid use of the term &quot;must&quot; for application requirements.</p>
</blockquote>

<div class="box"><em>End of informative text.</em></div>

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