Sunday, 29 January 2012


Xargs  is a command which can pass arguments from the output of one command to another. It can run multiline commands by passing arguments in a single file. The  most important feature is that it  creates a  list of arguments to be passed to the command and runs them.
Consider that you have many files  and  each  have to be  renamed  with a common subscript letter.

/home/Krishna $ ls    account_[0-9].txt

Then to rename  all these files  as  < filename>_old  , use xargs as follows.

/home/Krishna$ ls  account_[0-9].txt  | xargs  -I  { }   mv   { }   { }_prev
/home/Krishna $ls    account_[0-9]*

Here, xargs passed the output of ls, which is  a list  as  an argument list which is denoted by 
‘{  }’   to  the  mv  command .the  the entire line can be reconstructed as
mv  account_4   account_4_prev
mv   account_5  account_5_prev

On few occasions, many unwanted processes  of a common type  may be running and  it is necessary to kill all of them without killing any other  process. One method would be to kill all of them with their individual PIDs, but  it may be not possible to do this in scripts, xargs does the job.

When  it is required to kill all processes run by your login name that have names containing  ftp in it,use
ps  -fu    $LOGNAME  |  grep  ftp  |  awk  ‘{ print  $2 }’  | xargs  -I   { }   kill  -9  { }

Example 3)
 You want to copy  a large number of  files  into  a directory  placing all the filenames  in a list file.
Consider a file containing  list of all the filenames .end line contains the destination directory

/home/Krishna$ cat   candidates_logs.txt

You want to copy all these files to a directory  /var/tmp/logs with timestamp. use  xargs as follows.

/home/Krishna$ xargs   cp  –p  <  candidates_logs.txt
The command  structure created by xargs was
cp  -p   Arjuna.log  Yudhistir.log  Bhim.log  Nakul.log  Sahadev.log    candidates_logs.txt

This avoided the typing of all the files in an entire line. You may use this form of xargs in scripts where it is easier to make the list file using  simple echo and  “>>” .

Example 4)
You are redirecting  output containing contents of a file to another and you want a delimiting character  after every  n words , where n is any natural number, use xargs as follows.
a point to note here is that  a word can be a single line if it contains  only one word per line.

/home/Krishna$  cat   Train-Time.list
S29        F             12           Mumbai_CST     Karjat                   7:00pm  
T113     F             12 (x)   Mumbai_CST     Thane                   7:04pm  
K95        S              9 (x)      Mumbai_CST     Kalyan                  7:06pm  
A59        S              9             Mumbai_CST     Ambernath         7:15pm  
BL39     F             12           Mumbai_CST     Badlapur             7:17pm  
T115     S              9 (x)      Mumbai_CST     Thane                   7:20pm  
N27       F             12           Mumbai_CST     Kalyan                  7:21pm  

You can use awk to delimit the fields with  a “|” or any other delimiting character, but  xargs can  perform it too.
/home/Krishna$ cat   Train-Time.list | xargs  –n1  | xargs   -I  { }  echo  “{ }|”
S29|       F|            12|         Mumbai_CST|   Karjat|                  7:00pm|  
T113|   F|            12|         Mumbai_CST|   Thane|                 7:04pm|  
K95|      S|            9|            Mumbai_CST|   Kalyan|                7:06pm|  
A59|      S|            9|            Mumbai_CST|   Ambernath         7:15pm|  
BL39|   F|            12|         Mumbai_CST|   Badlapur|            7:17pm|  
T115|   S|            9|            Mumbai_CST|   Thane|                                 7:20pm|  
N27|      F|            12|         Mumbai_CST|   Kalyan|                7:21pm|  

You can trim the ending  “|” character by using sed .

Here you could have also used values greater than 1 after  n  so that the character “|” might be inserted after   those  many   words instead of 1.

Example 5)
When running commands by  passing multiple arguments , it sometimes becomes necessary to interactively  ask you to run it for each argument as seen in the commands  cp  -i  and  mv  -i.
xargs  using  its  -p option, can perform this on any command you want to run  interactively.

Consider a case where you need to take  tar  of files in a directory  by adding  every file  interactively  into the archive file.

/home/Krishna$ ls  |  xargs   -p   -n 1   tar -cvf   /backup/ALL_KRISHNA.tar
tar -cvf   /backup/ALL_KRISHNA.tar  account_4_prev   ?...y
tar -cvf   /backup/ALL_KRISHNA.tar  account_5_prev   ?...y
tar -cvf   /backup/ALL_KRISHNA.tar  account_7_prev   ?...n  ----------> your reply.
tar -cvf   /backup/ALL_KRISHNA.tar   jokes_old   ?...y

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