Bash shell script tip: Run commands from a variable

There are many situations in which you may want to run different command in your shell script depending on requirements and circumstances. So how do you select and run a command?

The old way…

The old way is use the case statement or if..else..fi command. For example:

#!/bin/bash
if [ this -eq that ];then
   command1
else
   command2
fi

The new way…

BASH allows you to assign or store a command name in a variable. For example create a variable called CMD:

#!/bin/bash
[ this -eq that ] && CMD=”/bin/ls” || CMD="/bin/date"
$CMD

Above shell script is a simple example of this concept. CMD variable stores /bin/ls or /bin/date command. To run command you simply type a variable name. So you are using contents of a variable to run command itself depending on circumstances. For example write a backup script to backup data to tape, NAS/ftp server or using scp command. You can run a script as follows:
/path/to/backup.sh nas
/path/to/backup.sh tape
/path/to/backup.sh scp

Now depend upon circumstances you need to swap a command for backup task. Here is a sample script:

#!/bin/bash
CMD=""
DIRS="/etc /home /www /data1 /data2 /var/log /var/spool/mail"
FILE="/backup/$(hostname)-$(date +'%m-%d-%y').tar.gz"
[ "$1" == "nas" ] && CMD="lftp -u user,password -e 'cd /dump/; mput /backup/*; quit' nas.mylan.com" || :
[ "$1" == "scp" ] && CMD="scp /backup/* scponly@dumpserver:incoming' username" || :
[ "$1" == "tape" ] && CMD='tar -cf /dev/st0 /backup/*' || :
[ "$CMD" == "" ] && exit 1 || :

# make a backup
tar -zcvf  $FILE $DIRS
# Now depend upon circumstances run a backup command
$CMD

Now you can run a different backup command in your script depending on parameter passed to script itself. You can also use this trick to write a cross platform shell script.

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2 Responses to “Bash shell script tip: Run commands from a variable”

1. Bruno Says:
   April 24th, 2007 at 7:41 am

   It would be nice to write just 2 lines about the way it works : (condition-A && condition-B) : bash evaluate condition-B only if condition-A is true. If condition-A is false, the result is FALSE anyway and bash doesn’t need to evaluate condition-B. Then it goes thru all the || conditions.
   Or point to a page that explain this.
   Anyway, it is very nice to be able to read these tips on a regular basis.
   Thanks

2. Mahesh Says:
   May 4th, 2007 (5 weeks ago) at 6:54 am

   Hi

   I want to take tar-backup of user home folders on RH9 Linux.

   My short script is as follows :

   #!/bin/bash
   #pwd
   cd /home
   tar cvzf /backup/archive/usr1-`date +%C%y%m%d`.tar.gz usr1
   tar cvzf /backup/archive/usr2-`date +%C%y%m%d`.tar.gz usr2
   .
   .
   So On !!!!!!!!!

   If i am running this script from cmd , it works fine. But If i cron the job it stops aborted on 2nd directory itself with half-completed tar file.
   How to solve this problem? I am not so novice in Linux. Pls help.

   Regards /Mahesh