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>Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting</TH
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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="FTO"
></A
>7.2. File test operators</H1
><DIV
CLASS="VARIABLELIST"
><P
><B
><A
NAME="RTIF"
></A
>Returns true if...</B
></P
><DL
><DT
><SPAN
CLASS="TOKEN"
>-e</SPAN
></DT
><DD
><P
>file exists</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>-a</SPAN
></DT
><DD
><P
>file exists</P
><P
>This is identical in effect to <SPAN
CLASS="TOKEN"
>-e</SPAN
>.
	      It has been <SPAN
CLASS="QUOTE"
>"deprecated,"</SPAN
>

	        <A
NAME="AEN3289"
HREF="#FTN.AEN3289"
>[1]</A
>
	      
	      and its use is
	      discouraged.</P
></DD
><DT
><A
NAME="REGULARFILE"
></A
><SPAN
CLASS="TOKEN"
>-f</SPAN
></DT
><DD
><P
>file is a <TT
CLASS="REPLACEABLE"
><I
>regular</I
></TT
>
	      file (not a directory or <A
HREF="devproc.html#DEVFILEREF"
>device
	      file</A
>)</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>-s</SPAN
></DT
><DD
><P
>file is not zero size</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>-d</SPAN
></DT
><DD
><P
>file is a directory</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>-b</SPAN
></DT
><DD
><P
>file is a <A
HREF="devproc.html#BLOCKDEVREF"
>block
	    device</A
></P
><P
><A
NAME="BLOCKDEVTEST"
></A
></P
></DD
><DT
><SPAN
CLASS="TOKEN"
>-c</SPAN
></DT
><DD
><P
><A
NAME="CHARDEVTEST"
></A
>file is a <A
HREF="devproc.html#CHARDEVREF"
>character device</A
></P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;device0="/dev/sda2"    # /   (root directory)
   2&nbsp;if [ -b "$device0" ]
   3&nbsp;then
   4&nbsp;  echo "$device0 is a block device."
   5&nbsp;fi
   6&nbsp;
   7&nbsp;# /dev/sda2 is a block device.
   8&nbsp;
   9&nbsp;
  10&nbsp;
  11&nbsp;device1="/dev/ttyS1"   # PCMCIA modem card.
  12&nbsp;if [ -c "$device1" ]
  13&nbsp;then
  14&nbsp;  echo "$device1 is a character device."
  15&nbsp;fi
  16&nbsp;
  17&nbsp;# /dev/ttyS1 is a character device.</PRE
></TD
></TR
></TABLE
></P
></DD
><DT
><SPAN
CLASS="TOKEN"
>-p</SPAN
></DT
><DD
><P
>file is a <A
HREF="special-chars.html#PIPEREF"
>pipe</A
></P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;function show_input_type()
   2&nbsp;{
   3&nbsp;   [ -p /dev/fd/0 ] &#38;&#38; echo PIPE || echo STDIN
   4&nbsp;}
   5&nbsp;
   6&nbsp;show_input_type "Input"                           # STDIN
   7&nbsp;echo "Input" | show_input_type                    # PIPE
   8&nbsp;
   9&nbsp;# This example courtesy of Carl Anderson.</PRE
></TD
></TR
></TABLE
></P
></DD
><DT
><SPAN
CLASS="TOKEN"
>-h</SPAN
></DT
><DD
><P
>file is a <A
HREF="external.html#SYMLINKREF"
>symbolic
	      link</A
></P
></DD
><DT
><SPAN
CLASS="TOKEN"
>-L</SPAN
></DT
><DD
><P
>file is a symbolic link</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>-S</SPAN
></DT
><DD
><P
>file is a <A
HREF="devproc.html#SOCKETREF"
>socket</A
></P
></DD
><DT
><SPAN
CLASS="TOKEN"
>-t</SPAN
></DT
><DD
><P
><A
NAME="TERMTEST"
></A
>file (<A
HREF="io-redirection.html#FDREF"
>descriptor</A
>) is
	      associated with a terminal device</P
><P
>This test option <A
HREF="miscellany.html#II2TEST"
> may be used
	      to check</A
> whether the <TT
CLASS="FILENAME"
>stdin</TT
>
	      <TT
CLASS="USERINPUT"
><B
>[ -t 0 ]</B
></TT
> or
	      <TT
CLASS="FILENAME"
>stdout</TT
> <TT
CLASS="USERINPUT"
><B
>[ -t 1 ]</B
></TT
>
	      in a given script is a terminal.</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>-r</SPAN
></DT
><DD
><P
>file has read permission (<SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>for the
	      user running the test</I
></SPAN
>)</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>-w</SPAN
></DT
><DD
><P
>file has write permission (for the user running
	      the test)</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>-x</SPAN
></DT
><DD
><P
>file has execute permission (for the user running
	    the test)</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>-g</SPAN
></DT
><DD
><P
>set-group-id (sgid) flag set on file or directory</P
><P
>If a directory has the <TT
CLASS="REPLACEABLE"
><I
>sgid</I
></TT
>
	      flag set, then a file created within that directory belongs
	      to the group that owns the directory, not necessarily to
	      the group of the user who created the file. This may be
	      useful for a directory shared by a workgroup.</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>-u</SPAN
></DT
><DD
><P
><A
NAME="SUIDREF"
></A
></P
><P
>set-user-id (suid) flag set on file</P
><P
>A binary owned by <I
CLASS="FIRSTTERM"
>root</I
>
	      with <TT
CLASS="REPLACEABLE"
><I
>set-user-id</I
></TT
> flag set
	      runs with <I
CLASS="FIRSTTERM"
>root</I
> privileges, even
	      when an ordinary user invokes it.
	      
		<A
NAME="AEN3400"
HREF="#FTN.AEN3400"
>[2]</A
>

	      This is useful for executables (such as
	      <B
CLASS="COMMAND"
>pppd</B
> and <B
CLASS="COMMAND"
>cdrecord</B
>)
	      that need to access system hardware. Lacking the
	      <I
CLASS="FIRSTTERM"
>suid</I
> flag, these binaries could not
	      be invoked by a <I
CLASS="FIRSTTERM"
>non-root</I
> user.</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> 	      <TT
CLASS="COMPUTEROUTPUT"
>-rwsr-xr-t    1 root       178236 Oct  2  2000 /usr/sbin/pppd</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	      </P
><P
>A file with the <TT
CLASS="REPLACEABLE"
><I
>suid</I
></TT
>
		flag set shows an <I
CLASS="FIRSTTERM"
>s</I
> in its
		permissions.</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>-k</SPAN
></DT
><DD
><P
><TT
CLASS="REPLACEABLE"
><I
>sticky bit</I
></TT
> set</P
><P
>Commonly known as the <I
CLASS="FIRSTTERM"
>sticky bit,</I
>
	      the <I
CLASS="FIRSTTERM"
>save-text-mode</I
> flag is a special
	      type of file permission. If a file has this flag set,
	      that file will be kept in cache memory, for quicker access.
	        <A
NAME="AEN3423"
HREF="#FTN.AEN3423"
>[3]</A
>
	      If set on a directory, it restricts write permission.
	      Setting the sticky bit adds a <I
CLASS="FIRSTTERM"
>t</I
>
	      to the permissions on the file or directory listing.
	      This restricts altering or deleting specific files 
	      in that directory to the owner of those files.</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> 	      <TT
CLASS="COMPUTEROUTPUT"
>drwxrwxrwt    7 root         1024 May 19 21:26 tmp/</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	      </P
><P
>If a user does not own a directory that has the sticky
		bit set, but has write permission in that directory, she
		can only delete those files that she owns in it. This
		keeps users from inadvertently overwriting or deleting
		each other's files in a publicly accessible directory,
		such as <TT
CLASS="FILENAME"
>/tmp</TT
>.
		(The <I
CLASS="FIRSTTERM"
>owner</I
> of the directory or
		<I
CLASS="FIRSTTERM"
>root</I
> can, of course, delete or
		rename files there.)</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>-O</SPAN
></DT
><DD
><P
>you are owner of file</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>-G</SPAN
></DT
><DD
><P
>group-id of file same as yours</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>-N</SPAN
></DT
><DD
><P
>file modified since it was last read</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>f1 -nt f2</SPAN
></DT
><DD
><P
>file <TT
CLASS="REPLACEABLE"
><I
>f1</I
></TT
> is newer than
		<TT
CLASS="REPLACEABLE"
><I
>f2</I
></TT
></P
></DD
><DT
><SPAN
CLASS="TOKEN"
>f1 -ot f2</SPAN
></DT
><DD
><P
>file <TT
CLASS="REPLACEABLE"
><I
>f1</I
></TT
> is older than
		<TT
CLASS="REPLACEABLE"
><I
>f2</I
></TT
></P
></DD
><DT
><SPAN
CLASS="TOKEN"
>f1 -ef f2</SPAN
></DT
><DD
><P
>files <TT
CLASS="REPLACEABLE"
><I
>f1</I
></TT
> and
		<TT
CLASS="REPLACEABLE"
><I
>f2</I
></TT
> are hard links to the same
		file</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>!</SPAN
></DT
><DD
><P
><SPAN
CLASS="QUOTE"
>"not"</SPAN
> -- reverses the sense of the
	    tests above (returns true if condition absent).</P
></DD
></DL
></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="BROKENLINK"
></A
><P
><B
>Example 7-4. Testing for broken links</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# broken-link.sh
   3&nbsp;# Written by Lee bigelow &#60;ligelowbee@yahoo.com&#62;
   4&nbsp;# Used in ABS Guide with permission.
   5&nbsp;
   6&nbsp;#  A pure shell script to find dead symlinks and output them quoted
   7&nbsp;#+ so they can be fed to xargs and dealt with :)
   8&nbsp;#+ eg. sh broken-link.sh /somedir /someotherdir|xargs rm
   9&nbsp;#
  10&nbsp;#  This, however, is a better method:
  11&nbsp;#
  12&nbsp;#  find "somedir" -type l -print0|\
  13&nbsp;#  xargs -r0 file|\
  14&nbsp;#  grep "broken symbolic"|
  15&nbsp;#  sed -e 's/^\|: *broken symbolic.*$/"/g'
  16&nbsp;#
  17&nbsp;#+ but that wouldn't be pure Bash, now would it.
  18&nbsp;#  Caution: beware the /proc file system and any circular links!
  19&nbsp;################################################################
  20&nbsp;
  21&nbsp;
  22&nbsp;#  If no args are passed to the script set directories-to-search 
  23&nbsp;#+ to current directory.  Otherwise set the directories-to-search 
  24&nbsp;#+ to the args passed.
  25&nbsp;######################
  26&nbsp;
  27&nbsp;[ $# -eq 0 ] &#38;&#38; directorys=`pwd` || directorys=$@
  28&nbsp;
  29&nbsp;
  30&nbsp;#  Setup the function linkchk to check the directory it is passed 
  31&nbsp;#+ for files that are links and don't exist, then print them quoted.
  32&nbsp;#  If one of the elements in the directory is a subdirectory then 
  33&nbsp;#+ send that subdirectory to the linkcheck function.
  34&nbsp;##########
  35&nbsp;
  36&nbsp;linkchk () {
  37&nbsp;    for element in $1/*; do
  38&nbsp;      [ -h "$element" -a ! -e "$element" ] &#38;&#38; echo \"$element\"
  39&nbsp;      [ -d "$element" ] &#38;&#38; linkchk $element
  40&nbsp;    # Of course, '-h' tests for symbolic link, '-d' for directory.
  41&nbsp;    done
  42&nbsp;}
  43&nbsp;
  44&nbsp;#  Send each arg that was passed to the script to the linkchk() function
  45&nbsp;#+ if it is a valid directoy.  If not, then print the error message
  46&nbsp;#+ and usage info.
  47&nbsp;##################
  48&nbsp;for directory in $directorys; do
  49&nbsp;    if [ -d $directory ]
  50&nbsp;	then linkchk $directory
  51&nbsp;	else 
  52&nbsp;	    echo "$directory is not a directory"
  53&nbsp;	    echo "Usage: $0 dir1 dir2 ..."
  54&nbsp;    fi
  55&nbsp;done
  56&nbsp;
  57&nbsp;exit $?</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
><A
HREF="zeros.html#COOKIES"
>Example 31-1</A
>, <A
HREF="loops.html#BINGREP"
>Example 11-8</A
>,
	  <A
HREF="loops.html#FILEINFO"
>Example 11-3</A
>, <A
HREF="zeros.html#RAMDISK"
>Example 31-3</A
>, and <A
HREF="contributed-scripts.html#MAILFORMAT"
>Example A-1</A
> also illustrate uses of the file test
	  operators.</P
></DIV
><H3
CLASS="FOOTNOTES"
>Notes</H3
><TABLE
BORDER="0"
CLASS="FOOTNOTES"
WIDTH="100%"
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN3289"
HREF="fto.html#AEN3289"
>[1]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>		Per the 1913 edition of <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>Webster's
		Dictionary</I
></SPAN
>:
		<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;Deprecate
   2&nbsp;...
   3&nbsp;
   4&nbsp;To pray against, as an evil;
   5&nbsp;to seek to avert by prayer;
   6&nbsp;to desire the removal of;
   7&nbsp;to seek deliverance from;
   8&nbsp;to express deep regret for;
   9&nbsp;to disapprove of strongly.</PRE
></TD
></TR
></TABLE
>
	        </P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN3400"
HREF="fto.html#AEN3400"
>[2]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>Be aware that <I
CLASS="FIRSTTERM"
>suid</I
>
		  binaries may open security holes. The
		  <I
CLASS="FIRSTTERM"
>suid</I
> flag has no effect on
		  shell scripts.</P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN3423"
HREF="fto.html#AEN3423"
>[3]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>On Linux systems, the sticky
		  bit is no longer used for files, only on
		  directories.</P
></TD
></TR
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