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<!-- TITLE=RED HAT LINUX 2ND EDITION //-->
<!-- AUTHOR=DAVID PITTS ET AL //-->
<!-- PUBLISHER=MACMILLAN //-->
<!-- IMPRINT=SAMS PUBLISHING //-->
<!-- PUBLICATION DATE=1998 //-->
<!-- CHAPTER=17 //-->
<!-- PAGES=0351-0372 //-->
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<P><CENTER>
<a href="0358-0361.html">Previous</A> | <a href="../ewtoc.html">Table of Contents</A> | <a href="0365-0367.html">Next</A>
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<A NAME="PAGENUM-362"><P>Page 362</P></A>
<P>There is no need to specify filenames to xargs because it reads these from the standard
input, so the xargs command will be
</P>
<!-- CODE SNIP //-->
<PRE>
xargs grep -l foo
</PRE>
<!-- END CODE SNIP //-->
<P>To get a list of files for xargs to give to
grep, use find to list the files in /usr/src/linux
that end in .c:
</P>
<!-- CODE SNIP //-->
<PRE>
find /usr/src/linux -name "*.c"
</PRE>
<!-- END CODE SNIP //-->
<P>Then attach the standard output of find to the standard input of
xargs with a pipe:
</P>
<!-- CODE SNIP //-->
<PRE>
find /usr/src/linux -name "*.c" | xargs grep -l foo
</PRE>
<!-- END CODE SNIP //-->
<P>Finally, tack on a wc -l to get a count and you have
</P>
<!-- CODE SNIP //-->
<PRE>
find /usr/src/linux -name "*.c" | xargs grep -l foo | wc -l
</PRE>
<!-- END CODE SNIP //-->
<P>On my system this took about 29 seconds, which is
considerably faster. The difference becomes even greater when more complex commands are run and the list of files is longer.
</P>
<P>You need to be careful about filenames that contain spaces in them. Many people believe
that spaces are not valid characters in UNIX filenames, but they are, and handling them correctly
is becoming an issue of greater importance because today many machines are able to mount
and read disks from systems that frequently use spaces in filenames.
</P>
<P>I routinely mount Mac HFS disks (Zip, floppy, hard disk) and many files on these disks
have spaces in their filenames. This will confuse
xargs because it uses the newline character (\n)
and the space character as filename delimiters. The GNU version of
xargs provides a pretty good workaround for this problem with the
--null and -0 options, which tell xargs to use the
null character (\0 or \000) as the delimiter. In order to generate filenames that end in
null, find can be given the -print0 option instead of
-print.
</P>
<P>As an illustration, here is an ls of my Mac's Zip Tools disk:
</P>
<!-- CODE SNIP //-->
<PRE>
./ .resource/ Desktop DB* System Folder/ ../
.rootinfo Desktop DF* Utilities/ .finderinfo/ Applications/ Icon:0d*
</PRE>
<!-- END CODE SNIP //-->
<P>Notice that the Desktop DB and the Desktop DF files
have spaces in them, as does the System Folder directory. If I wanted to run through this Zip disk and remove all files that end in
prefs copy, I would normally try
</P>
<!-- CODE SNIP //-->
<PRE>
find /mnt/zip -name "*prefs copy" -print | xargs rm
</PRE>
<!-- END CODE SNIP //-->
<P>However, this won't work because I have a filename with spaces, but if I add
-print0, I can do this with no problems:
</P>
<!-- CODE SNIP //-->
<PRE>
find /mnt/zip -name "*prefs copy" -print0 | xargs rm
</PRE>
<!-- END CODE SNIP //-->
<P>Two other options that are useful for xargs are the
-p option, which makes xargs interactive, and the -n
args option, which makes xargs run the specified command with only
args number of arguments.
</P>
<A NAME="PAGENUM-363"><P>Page 363</P></A>
<P>Some people wonder why the -p option. The reason is that
xargs runs the specified command on the filenames from its standard input, so interactive commands like
cp -i, mv -i, and rm -i don't work right. The -p option solves that problem. In the preceding example, the
-p option would have made the command safe because I could answer yes or no to each file.
So the real command I typed was the following:
</P>
<!-- CODE SNIP //-->
<PRE>
find /mnt/zip -name "*prefs copy" -print0 | xargs -p rm
</PRE>
<!-- END CODE SNIP //-->
<P>Many users frequently ask why xargs should be used when shell command substitution
archives the same results. The real drawback with commands such as
</P>
<!-- CODE SNIP //-->
<PRE>
grep -l foo `find /usr/src/linux -name "*.c"`
</PRE>
<!-- END CODE SNIP //-->
<P>is that if the set of files returned by find is longer than the system's command-line length
limit, the command will fail. The xargs approach gets around this problem because
xargs runs the command as many times as is required, instead of just once.
</P>
<H3><A NAME="ch17_ 11">
Shell Utilities
</A></H3>
<P>The GNU shell utilities are a package of small shell programming utilities. The following
programs are included in the package:
</P>
<TABLE WIDTH="360">
<TR><TD>
basename
</TD><TD>
printenv
</TD></TR>
<TR><TD>
chroot
</TD><TD>
printf
</TD></TR>
<TR><TD>
date
</TD><TD>
pwd
</TD></TR>
<TR><TD>
dirname
</TD><TD>
seq
</TD></TR>
<TR><TD>
echo
</TD><TD>
sleep
</TD></TR>
<TR><TD>
env
</TD><TD>
stty
</TD></TR>
<TR><TD>
expr
</TD><TD>
su
</TD></TR>
<TR><TD>
factor
</TD><TD>
tee
</TD></TR>
<TR><TD>
false
</TD><TD>
test
</TD></TR>
<TR><TD>
groups
</TD><TD>
true
</TD></TR>
<TR><TD>
hostname
</TD><TD>
tty
</TD></TR>
<TR><TD>
id
</TD><TD>
uname
</TD></TR>
<TR><TD>
logname
</TD><TD>
users
</TD></TR>
<TR><TD>
nice
</TD><TD>
who
</TD></TR>
<TR><TD>
nohup
</TD><TD>
whoami
</TD></TR>
<TR><TD>
pathchk
</TD><TD>
yes
</TD></TR>
</TABLE>
<H4><A NAME="ch17_ 12">
Who's Who in GNU
</A></H4>
<P>One of the first things many people do when they log on to a new machine is to see who
else is logged on. The GNU shell utilities provide the
commands who and users to give
</P>
<A NAME="PAGENUM-364"><P>Page 364</P></A>
<P>information about which users are currently logged on and what they are doing. The
users command just prints out a list of names of people who are logged on. The
who command is much more sophisticated.
</P>
<P>In addition to giving information about who is logged on,
who makes it possible to find out how long people have been idle, when they logged on, and if they are allowing other people
to talk. Some of the options that who recognizes are as follows:
</P>
<TABLE WIDTH="360">
<TR><TD>
-H or --heading
</TD><TD>
Prints a heading
</TD></TR>
<TR><TD>
-T, -w, or--mesg
</TD><TD>
Adds user message status as
+, -, or ?
</TD></TR>
<TR><TD>
-i, -u, or --idle
</TD><TD>
Adds user idle time as
HOURS:MINUTES, . (less than a minute), or old (greater than a day)
</TD></TR>
</TABLE>
<P>One of the useful features of who over users is that because
who outputs one user entry per line, it can be used with commands like
grep or sed to process and format its output with ease.
</P>
<H4><A NAME="ch17_ 13">
The id Commands
</A></H4>
<P>The next set of frequently used commands are the
id commands.
</P>
<P>Knowing your own uid and gid and having the ability to determine other users'
uids and gids are very handy skills.
</P>
<P>Almost all users know about the commands whoami and
groups, but many have never heard of id, which encompasses their functionality and adds the ability to determine user
information about other people on the system.
</P>
<P>By default, the id command prints out all the identification information about the
current user. When I run id on my system, I get (as myself)
</P>
<!-- CODE SNIP //-->
<PRE>
uid=500(ranga) gid=100(users) groups=100(users)
</PRE>
<!-- END CODE SNIP //-->
<P>But, I can also run id on my brother's login name:
</P>
<!-- CODE SNIP //-->
<PRE>
id vathsa
</PRE>
<!-- END CODE SNIP //-->
<P>and I get
</P>
<!-- CODE SNIP //-->
<PRE>
uid=501(vathsa) gid=100(users) groups=500(vathsa)
</PRE>
<!-- END CODE SNIP //-->
<P>I could have determined all the preceding information by looking in
/etc/passwd, but id is a much easier way of accomplishing the task.
</P>
<P>In addition, the output of id can be tailored using the following options:
</P>
<TABLE WIDTH="360">
<TR><TD>
-g or --group
</TD><TD>
Prints only the group ID
</TD></TR>
<TR><TD>
-G or --groups
</TD><TD>
Prints only the supplementary groups
</TD></TR>
<TR><TD>
-n or --name
</TD><TD>
Prints a name instead of a number
</TD></TR>
</TABLE>
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