XARGS

xargs

A filter for feeding arguments to a command, and also a tool for assembling the commands themselves. It breaks a data stream into small enough chunks for filters and commands to process. Consider it as a powerful replacement for backquotes. In situations where backquotes fail with a too many arguments error, substituting xargs often works. Normally, xargs reads from stdin or from a pipe, but it can also be given the output of a file.

The default command for xargs is echo. This means that input piped to xargs may have linefeeds and other whitespace characters stripped out.

 bash$ ls -l
 total 0
 -rw-rw-r--    1 bozo  bozo         0 Jan 29 23:58 file1
 -rw-rw-r--    1 bozo  bozo         0 Jan 29 23:58 file2


 bash$ ls -l | xargs
 total 0 -rw-rw-r-- 1 bozo bozo 0 Jan 29 23:58 file1 -rw-rw-r-- 1 bozo bozo 0 Jan 29 23:58 file2
              

ls | xargs -p -l gzip gzips every file in current directory, one at a time, prompting before each operation.

An interesting xargs option is -n NN, which limits to NN the number of arguments passed. ls | xargs -n 8 echo lists the files in the current directory in 8 columns.

Another useful option is -0, in combination with find -print0 or grep -lZ. This allows handling arguments containing whitespace or quotes. find / -type f -print0 | xargs -0 grep -liwZ GUI | xargs -0 rm -f grep -rliwZ GUI / | xargs -0 rm -f Either of the above will remove any file containing "GUI". (Thanks, S.C.)

Example 12-4. Logfile using xargs to monitor system log

   1 #!/bin/bash
   2
   3 # Generates a log file in current directory
   4 # from the tail end of /var/log/messages.
   5
   6 # Note: /var/log/messages must be world readable
   7 # if this script invoked by an ordinary user.
   8 #         #root chmod 644 /var/log/messages
   9
  10 LINES=5
  11
  12 ( date; uname -a ) >>logfile
  13 # Time and machine name
  14 echo --------------------------------------------------------------------- >>logfile
  15 tail -$LINES /var/log/messages | xargs |  fmt -s >>logfile
  16 echo >>logfile
  17 echo >>logfile
  18
  19 exit 0
  20
  21 # Exercise:
  22 # --------
  23 #  Modify this script to track changes in /var/log/messages at intervals
  24 #+ of 20 minutes.
  25 #  Hint: Use the "watch" command.

Example 12-5. copydir, copying files in current directory to another, using xargs

   1 #!/bin/bash
   2
   3 # Copy (verbose) all files in current directory
   4 # to directory specified on command line.
   5
   6 if [ -z "$1" ]   # Exit if no argument given.
   7 then
   8   echo "Usage: `basename $0` directory-to-copy-to"
   9   exit 65
  10 fi 
  11
  12 ls . | xargs -i -t cp ./{} $1
  13 # This is the exact equivalent of
  14 #    cp * $1
  15 # unless any of the filenames has "whitespace" characters.
  16
  17 exit 0