Bash shell script tip: Run commands from a variable

by on April 22, 2007 · 25 comments· LAST UPDATED April 22, 2007

in , ,

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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{ 25 comments… read them below or add one }

1 Bruno April 24, 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

Reply

2 Mahesh May 4, 2007 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

Reply

3 raihan September 22, 2007 at 12:35 pm

can you guys please send me linux shell script commands for backup documents home folder, i need it
very badly because i need to submit it on 24/9/07 to my teacher..please any one kindly send me this..
my e-mail address raihan_alam@hotmail.com

Reply

4 corex December 8, 2007 at 2:32 pm

I experienced another problem.
If I call script files, input methods (read ) are ignored.

find /media/private -iname "*.bash" | while read file
do
echo "Executing: $file"
$file -m
if [ "$?" -ne 0 ]
then
echo "Error executing $file"
exit 1
fi

echo "done"

Reply

5 Finnbarr Murphy March 24, 2008 at 10:02 am

This is not just a bash feature as you imply. $CMD works with Korn shell and most other shells.

Reply

6 charusrinivasan April 2, 2009 at 1:01 pm

hai i want a example linux program for case statement

Reply

7 pavium April 22, 2009 at 1:26 pm

It may be slightly more clever to use variables this way, but what are we sacrificing? READABILITY.

You’ll be sorry when to look at the script a few months from now.

Reply

8 kalem December 14, 2009 at 11:30 am

hi, i am trying to make a precess to run by itself whenever a media is plugged.
the process finds and eliminate (deletes) programs ,viruses .
since i use UNIX/lunix , these programs don’t run on my machine.
can any one say some ?
thanks in advance

Reply

9 Hanish May 17, 2010 at 2:39 pm

if I give a PIPE to a command then it does not work like

CMD=’ps -ef|grep vnc’

Help me out…

Reply

10 Hanish May 17, 2010 at 4:00 pm

Hi investigated and eval did a trick.

CMD=’ps -elf|grep vnc’
eval $CMD

Thanks to all

Reply

11 dev May 10, 2011 at 5:51 pm

Hi,

I am trying to execute some list of commands.So,I have added my commands inside a csv file.I am trying to read that command and execute the command and store the result of the command in the variable.

cmd=`echo $line | awk -F ‘;’ ‘{print $1}’`
After this “cmd” gets the value like “cat filename | wc -l ”

I am trying to store in a variable like this
count=`$cmd`.

Since there is a pipe i am getting error.
Can some one help me on this

Reply

12 Rhongomiant November 17, 2011 at 2:40 pm

dev,

cmd=`echo $line | awk -F ‘;’ ‘{print $1}’`
count=`$cmd`

The variable cmd is storing the results of a command because you have the command encapsulated by backticks “. Based on this information it seems like you are doubling your work. I would modify as follows.

cmd=`echo $line | awk -F ‘;’ ‘{print $1}’`
count=”$cmd”

or

count=`echo $line | awk -F ‘;’ ‘{print $1}’`

If you really need it setup this way, use double or single quotes to encapsulate the command defined by cmd.

cmd=”echo $line | awk -F ‘;’ ‘{print $1}’”

I have not had luck storing commands that contain awk in a variable and calling it later. I usually use a function.

cmd () {
echo $line | awk -F ‘;’ ‘{print $1}’
}

count=`cmd`

Reply

13 dev May 10, 2011 at 5:58 pm

Hanish,

You did a great investigation man. My problem also solved

by giving like this count =`eval $cmd`

Thanks a lot hanish

Reply

14 samira October 2, 2011 at 6:34 pm

Hanish May 17, 2010
Hi investigated and eval did a trick.

CMD=’ps -elf|grep vnc’
eval $CMD

Thanks to all

————————–
thankss soooo muccchhhhhhhh it is workeed

Reply

15 Petridish January 25, 2012 at 3:53 pm

I’m a bit new to linux scripting, but I come from a programming background (c/c++). Out of curiosity, is there a different scripting language besides bash? The syntax is horrible, and I find it incredibly difficult to use.

Reply

16 Hanish Madan April 13, 2012 at 7:46 am

Since you are from c/c++ background try to program in CSH or TCSH shell.

Reply

17 Sanket April 11, 2012 at 9:21 am

Hi,

I am trying to execute command (in ksh) stored in varialbe and want to capture output in second variable. For eg,

var1=”ls -l $RMD_REPORTS/nat_logs/*${REPORT_DATE}*.xml 2>/dev/null | awk ‘{print $NF}’”;
var2=`eval $var1`;
echo $var2

I was expecting only file names as output of above command, however “awk” filter in above command is not working and output of “echo” is same as “ls -l” command. Can anyone tell what’s wrong in above commands?

Reply

18 Rhongomiant April 11, 2012 at 12:29 pm

Sanket,

I believe you need to escape the $ in the awk statement, so \$NF. This works for bash, but I do not know ksh.

var1=”ls -l $RMD_REPORTS/nat_logs/*${REPORT_DATE}*.xml 2>/dev/null | awk ‘{print \$NF}’”;

Reply

19 Sanket April 12, 2012 at 6:31 am

Thanks mate!
Yes, it works in ksh as well. :)
Sorry, but didn’t get reson why eval can’t interpret awk variable ($NF)?

Reply

20 Hanish Madan April 13, 2012 at 10:52 am

try below: –

var1=”ls -l $RMD_REPORTS/nat_logs/*${REPORT_DATE}*.xml 2>/dev/null | awk ‘{print $NF}’”;

echo $var1

Reply

21 Hanish Madan April 13, 2012 at 11:02 am

okay below worked for me: –

#!/bin/ksh

var1=”ls -l ~/|awk ‘{print \$1}’”
var2=`eval $var1`
echo $var2

Reply

22 Lakshmi July 17, 2012 at 7:13 am

Any one please let me know how to call a variable of a shell script in another shell script without using export

Reply

23 Rhongomiant July 18, 2012 at 4:30 am

Lakshmi,

Given the very general nature of your question, it is impossible for anyone to provide you with helpful information. We need to know what you are doing, why you can’t use export, etc. Basically you are asking how you can call someone in a different area code without entering the area code. You can’t call them, but if you use the same central repository, twitter, facebook, etc. you can share information. If you can’t use export and using the source option will not work then you need a central repository for information shared between scripts. This could be a file, a database, etc.

Reply

24 6uru October 29, 2012 at 12:45 pm

which shell script
command use to run databases

Reply

25 Vinod November 21, 2012 at 10:26 am

If I understand your Q right, say you are executing b.sh from within a.sh and like to use the variables set in b.sh. Then you need to source b.sh from within a.sh

#———–in a.sh —————–
. ./b.sh
#now all variables set in b.sh will be available here

Reply

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