Bash: How can I check the return value of a command

Question

I am new to bash scripting and want to write a short script, that checks if a certain program is running. If it runs, the script should bring the window to the foreground, if it does not run, the script should start it.

#!/bin/bash

if [ "$(wmctrl -l | grep Wunderlist)" = ""]; then
    /opt/google/chrome/google-chrome --profile-directory=Default --app-id=ojcflmmmcfpacggndoaaflkmcoblhnbh
else
    wmctrl -a Wunderlist
fi

My comparison is wrong, but I am not even sure what I should google to find a solution. My idea is, that the "$(wmctrl -l | grep Wunderlist)" will return an empty string, if the window does not exist. I get this error when I run the script:

~/bin » sh handle_wunderlist.sh                                       
handle_wunderlist.sh: 3: [: =: argument expected

shellcheck.net would have caught your syntax error; don't execute Bash scripts with sh, because sh isn't (always) Bash. — mklement0
– mklement0, Commented Jul 24, 2016 at 21:50

heemayl · Accepted Answer · 2016-07-24 21:34:01Z

4

You need a space before the closing argument, ], of the [ (test) command:

if [ "$(wmctrl -l | grep Wunderlist)" = "" ]; then
    ....
else
    ....
fi

As a side note, you have used the shebang as bash but running the script using sh (presumably dash, from the error message).

answered Jul 24, 2016 at 21:34

heemayl

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3 Comments

Natjo Over a year ago

A missing whitespace... Thank you, that solved the problem

mklement0 Over a year ago

++ for explaining the syntax problem and pointing out that Bash scripts shouldn't be executed with sh; taking a step back to look at the bigger picture: John1024's approach is the best choice.

chepner Over a year ago

The script itself is POSIX-compliant, so it's fine to run with sh. (The shebang is only relevant if running the script itself as a command. #!/bin/sh would be sufficient if that were the case.)

John1024 · Accepted Answer · 2016-07-24 21:42:02Z

3

Replace:

if [ "$(wmctrl -l | grep Wunderlist)" = ""]; then

With:

if ! wmctrl -l | grep -q Wunderlist; then

grep sets its exit condition to true (0) is a match was found and false (1) if it wasn't. Because you want the inverse of that, we placed ! at the beginning of the command to invert the exit code.

Normally, grep will send the matching text to standard out. We don't want that text, we just want to know if there was a match or not. Consequently, we added the -q option to make grep quiet.

Example

To illustrate the use of grep -q in an if statement:

$ if ! echo Wunderlist | grep -q Wunderlist; then echo Not found; else echo Found; fi
Found
$ if ! echo Wunderabcd | grep -q Wunderlist; then echo Not found; else echo Found; fi
Not found

answered Jul 24, 2016 at 21:42

John1024

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1 Comment

Natjo Over a year ago

Thank you for the elaborated answer!

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