bash execute command in variable

Question

I have a command in a variable in Bash:

check_ifrunning=\`ps aux | grep "programmname" | grep -v "grep" | wc -l\`

The command checks if a specific program is running at the moment. Later in my script, I want to query the value of the variable on a point. If the specific program is running, the script should sleep for 15 minutes.

I solved it like this:

while [ $check_ifrunning -eq 1 ]; do
sleep 300
done

Will the script execute the command in the variable for each single loop-run or will the value in the variable stay after the first execution?

I have more variables in my script which can change their value. This was just one simple example of this.

Peter Mortensen · Accepted Answer · 2021-12-05 02:37:37Z

1

Notice that check_ifrunning is set only once, in

 check_ifrunning=`ps aux | grep "programmname" | grep -v "grep" | wc -l`

and that it is set before the loop:

 while [ $check_ifrunning -eq 1 ]; do
   sleep 300
 done

You could add, for debugging purposes, an echo check_ifrunning is $check_ifrunning statement inside your while loop just before the sleep ...

You probably simply want (using pidof(8)) - without defining or using any check_ifrunning Bash variable:

 while [ -n "$(pidof programname)" ]; do
    sleep 300
 done

Because you want to test if programname is running at every start of the loop!

You should use the more nestable and more readable $(...) instead of backquotes.

Consider reading the Advanced Bash Scripting Guide...

If you are writing a Bash script, consider to start it with

 #!/bin/bash -vx

while debugging. When you are satisfied, remove the -vx...

edited Dec 5, 2021 at 2:37

Peter Mortensen

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answered Jul 7, 2014 at 7:45

Basile Starynkevitch

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2 Comments

zorro42 Over a year ago

First of all thank you for answer. So I have to "re-set" the varibale everytime before a loop? Would this be possible with a function, which I call before every loop? As already mentioned, I have more variables like these in my script. They may change during the run of the script.

Basile Starynkevitch Over a year ago

Read more about bash scripting. Use the -vx trick. You seems very confused about what variables are in bash....

tripleee · Accepted Answer · 2014-07-07 08:20:05Z

If you want to encapsulate your commands, the proper way to do that is a function.

running () {
    ps aux | grep "$1" | grep -q -v grep
}

With grep -q you get the result as the exit code, not as output; you use it simply like

if running "$programname"; then
    :

Ideally, the second grep is unnecessary, but I did not want to complicate the code too much. It still won't work correctly if you are looking for grep. The proper solution is pidof.

See also http://mywiki.wooledge.org/BashFAQ/050

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