chmod.html

unix 下的C开发手册,还用详细的例程。

values will be set.

</dl>
<p>
When using the symbolic mode form on a regular file,
it is implementation-dependent whether or not:
<p>
<ul>
<p>
<li>
Requests to set the set-user-ID-on-execution or set-group-ID-on-execution
bit when all execute bits are currently clear and
none are being set are ignored.
<p>
<li>
Requests to clear all execute bits also clear the set-user-ID-on-execution
and set-group-ID-on-execution bits.
<p>
<li>
Requests to clear the set-user-ID-on-execution or set-group-ID-on-execution
bits when all execute bits are currently clear are ignored.
However, if the command
<i><a href="ls.html">ls</a></i>
<b>-l</b>
<i>file</i>
writes an
s
in the position indicating that the
set-user-ID-on-execution or set-group-ID-on-execution is set, the
commands
<i>chmod</i>
<b>u-s</b>
<i>file</i>
or
<i>chmod</i>
<b>g-s</b>
<i>file</i>,
respectively, will not be ignored.
<p>
</ul>
<p>
When using the symbolic mode form on other file types, it is
implementation-dependent whether or not requests to set or clear
the set-user-ID-on-execution or set-group-ID-on-execution bits
are honoured.
<p>
If the
<b>who</b>
symbol
o
is used in conjunction with the
<b>perm</b>
symbol
s
with no
other
<b>who</b>
symbols being specified, the set-user-ID-on-execution and
set-group-ID-on-execution
bits will not be modified.
It will not be an
error to specify the
<b>who</b>
symbol
o
in conjunction with the
<b>perm</b>
symbol
s.
<p>
For an octal integer
<i>mode</i>
operand, the file mode bits will be set absolutely.
<p>
For each bit set in the octal number, the corresponding
file permission bit shown in
the following table will be set;
all other file permission bits will be cleared.
For regular files, for
each bit set in the octal number corresponding to the
set-user-ID-on-execution or the set-group-ID-on-execution bits
shown in the following table will be set;
if these bits are not
set in the octal number, they will be cleared.
For other file
types, it is implementation-dependent whether or not requests to
set or clear the set-user-ID-on-execution or
set-group-ID-on-execution bits are honoured.
<pre>
<table  bordercolor=#000000 border=1<tr valign=top><th align=center><b>Octal</b>
<th align=left>Mode bit
<th align=center><b>Octal</b>
<th align=center><b>Mode bit</b>
<th align=left>Octal
<th align=center><b>Mode bit</b>
<th align=center><b>Octal</b>
<th align=left>Mode bit
<tr valign=top><td align=left>4000
<td align=left>S_ISUID
<td align=left>0400
<td align=left>S_IRUSR
<td align=left>0040
<td align=left>S_IRGRP
<td align=left>0004
<td align=left>S_IROTH
<tr valign=top><td align=left>2000
<td align=left>S_ISGID
<td align=left>0200
<td align=left>S_IWUSR
<td align=left>0020
<td align=left>S_IWGRP
<td align=left>0002
<td align=left>S_IWOTH
<tr valign=top><td align=left>&nbsp;
<td align=left>&nbsp;
<td align=left>0100
<td align=left>S_IXUSR
<td align=left>0010
<td align=left>S_IXGRP
<td align=left>0001
<td align=left>S_IXOTH
</table>
</pre>
<p>
When bits are set in the octal number other
than those listed in the table above, the behaviour is unspecified.
<h5><a name = "tag_001_014_047_001">&nbsp;</a>Grammar for chmod</h5>
The grammar and lexical conventions in this section describe the syntax
for the
<b>symbolic_mode</b>
operand.
The general conventions for this style of grammar are described in
<xref href=grammar></xref>.
A valid
<b>symbolic_mode</b>
can be represented
as the non-terminal symbol
<b>symbolic_mode</b>
in the grammar.
This formal syntax takes precedence over
the preceding text syntax description.
<p>
The lexical processing will be based entirely on single characters.
Implementations need not allow
blank characters
within the single argument being processed.
<p>
<pre>
<code>
%start             symbolic_mode
%%

symbolic_mode    : section
                 | symbolic_mode ',' section
                 ;

section          : actionlist
                 | wholist actionlist
                 ;

wholist          : who
                 | wholist who
                 ;

who              : 'u' | 'g' | 'o' | 'a'
                 ;

actionlist       : action
                 | actionlist action
                 ;

action           : op
                 | op permlist
                 | op permcopy
                 ;

permcopy         : 'u' | 'g' | 'o'
                 ;

op               : '+' | '-' | '='
                 ;

permlist         : perm
                 | perm permlist
                 ;

perm             : 'r' | 'w' | 'x' | 'X' | 's'
                 ;
</code>
</pre>
</blockquote><h4><a name = "tag_001_014_048">&nbsp;</a>EXIT STATUS</h4><blockquote>
The following exit values are returned:
<dl compact>

<dt>0<dd>The utility executed successfully and all requested changes were made.

<dt>&gt;0<dd>An error occurred.

</dl>
<br>
</blockquote><h4><a name = "tag_001_014_049">&nbsp;</a>CONSEQUENCES OF ERRORS</h4><blockquote>
If, when invoked with the
<b>-R</b>
option,
<i>chmod</i>
attempts but fails to change
the mode of a particular file in a specified file hierarchy, it
will continue to process the remaining files in the hierarchy, affecting the
final exit status.
If
<i>chmod</i>
cannot read or search a directory within a
hierarchy, it will continue to process the other parts of the hierarchy
that are accessible.
</blockquote><h4><a name = "tag_001_014_050">&nbsp;</a>APPLICATION USAGE</h4><blockquote>
The references to octal modes are supported
because, although they are obsolescent in the ISO/IEC 9945-2:1993 standard,
XSI-conformant systems have committed to maintaining them
for portable applications until further notice.
<p>
Some implementations of the
<i>chmod</i>
utility change the mode of a
directory before the files in the directory when performing a recursive
(<b>-R</b>
option) change; others change the directory mode after the files in
the directory.
If an application tries to remove read or search permission for a
file hierarchy, the removal attempt will fail if the directory is changed
first; on the other hand, trying to re-enable permissions to a restricted
hierarchy will fail if directories are changed last.
Users should not try to make a
hierarchy inaccessible to themselves.
<p>
Some implementations of
<i>chmod</i>
never used the process'
<i>umask</i>
when changing modes; systems conformant with this specification do so when
<b>who</b>
is not specified.
Note the difference between:
<pre>
<code>
chmod a-w file
</code>
</pre>
which removes all write permissions, and:
<pre>
<code>
chmod -- -w file
</code>
</pre>
<br>
which removes write permissions that would be allowed if
<b>file</b>
was created
with the same
<i>umask.</i>
<p>
Portable applications
should never assume that they know how the
set-user-ID and set-group-ID bits on directories
will be interpreted.
<br>
</blockquote><h4><a name = "tag_001_014_051">&nbsp;</a>EXAMPLES</h4><blockquote>
<pre>
<table  bordercolor=#000000 border=1 align=center><tr valign=top><th align=center><b>Mode</b>
<th align=center><b>Results</b>
<tr valign=top><td align=left><b>a+=</b>
<td align=left> Equivalent to clears all file mode bits. 
<tr valign=top><td align=left><b>go+-w</b>
<td align=left> Equivalent to clears group and other write bits. 
<tr valign=top><td align=left><b>g=o-w</b>
<td align=left> Equivalent to sets group bit to match other bits and then clears group write bit. 
<tr valign=top><td align=left><b>g-r+w</b>
<td align=left> Equivalent to clears group read bit and sets group write bit. 
<tr valign=top><td align=left><b>=g</b>
<td align=left>Sets owner bits to match group bits and sets other bits to match group bits
</table>
</pre>
</blockquote><h4><a name = "tag_001_014_052">&nbsp;</a>FUTURE DIRECTIONS</h4><blockquote>
None.
</blockquote><h4><a name = "tag_001_014_053">&nbsp;</a>SEE ALSO</h4><blockquote>
<i><a href="ls.html">ls</a></i>,
<i><a href="umask.html">umask</a></i>,
the <b>XSH</b> specification description of
<i><a href="../xsh/chmod.html">chmod()</a></i>.
</blockquote><hr size=2 noshade>
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