Storing an error message in bash only if it fails

Question

I am a beginner with bash so I apologize if this is a basic question or has syntax issues but I couldn't find the answer anywhere. I want to run a command, and if it fails, check if the error message matches a certain string. Here is what I have:

err_msg=`./command input.txt 2>&1`
if [$? -eq 1]
then 
   if [err_msg -eq "Error: timeout"]
   then
       #do something
   fi
fi

Is this the correct way to do this? If I'm using the first line to define the error message err_msg, will $? still store the pass/fail status of the command? Also, what will happen to if the command passes? Will err_msg just be blank?

You have some syntax errors: 1. leave a space around the characters [ and ]: these are builtins and need to have a space around them to be recognized as such. 2. Don't use -eq to test for equality of strings. Use = instead. Re. your question: yes, $? will expand to the return code of the command. — gniourf_gniourf
– gniourf_gniourf, Commented Dec 2, 2017 at 17:50
Thank you! Also, should err_msg be $err_msg in the 2nd if statement since it's a variable? — Sophie Foreman
– Sophie Foreman, Commented Dec 2, 2017 at 17:52
Yes, of course, you want to check for the expansion of err_msg. You also need to quote this expansion: "$err_msg", unless you use the more robust keyword [[. — gniourf_gniourf
– gniourf_gniourf, Commented Dec 2, 2017 at 17:54

PesaThe · Accepted Answer · 2017-12-02 19:11:43Z

2

Don't use the old-style backquote form of command substitution, use $(command) instead
Use [[ ]] for testing instead of [ ] as it has more features and fewer surprises
-eq is used for integer comparison. For string comparison, use = or ==
Note that command can fail with an exit status other than 1, thus it's better to test for non-zero status: ! [[ $? -eq 0 ]]
You can use $? for testing exit status of the last command or you can test it with if directly: if command; then echo CMD OK; fi
You can assign output of a command to a variable while testing its exit code

Using the points above, you can rewrite your code like this:

if ! err_msg=$(./command input.txt 2>&1)
then 
   if [[ $err_msg = "Error: timeout" ]]; then
      echo "Doing something.."
   fi
fi

edited Dec 2, 2017 at 19:11

answered Dec 2, 2017 at 17:54

PesaThe

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2 Comments

chepner Over a year ago

You can also use ! err_msg=$(...) && [[ $err_msg = "..." ]] to avoid the nested if statement.

PesaThe Over a year ago

@chepner Exactly. However, I found that a bit too confusing for a beginner.

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