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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You may also be interested in other helpful articles:
- BASH shell scripting tip: Set default values for variable
- Executing script or command on the last day of a month
- Linux / UNIX: display time of different time zones using TZ environment variable
- Take action or execute a command based upon shell script name
- nixCraft FAQ roundup - Jan / 15 / 2007
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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
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
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
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”
This is not just a bash feature as you imply. $CMD works with Korn shell and most other shells.