Bash conditional on command exit code

Question

In bash, I want to say "if a file doesn't contain XYZ, then" do a bunch of things. The most natural way to transpose this into code is something like:

if [ ! grep --quiet XYZ "$MyFile" ] ; then
   ... do things ...
fi

But of course, that's not valid Bash syntax. I could use backticks, but then I'll be testing the output of the file. The two alternatives I can think of are:

grep --quiet XYZ "$MyFile"
if [ $? -ne 0 ]; then
   ... do things ...
fi

And

grep --quiet XYZ "$MyFile" ||
   (  ... do things ...
   )

I kind of prefer the second one, it's more Lispy and the || for control flow isn't that uncommon in scripting languages. I can see arguments for the first one too, although when the person reads the first line, they don't know why you're executing grep, it looks like you're executing it for it's main effect, rather than just to control a branch in script.

Is there a third, more direct way which uses an if statement and has the grep in the condition?

possible duplicate of encounter "unary operator expected" in bash script — tripleee
– tripleee, Commented Aug 20, 2013 at 4:49

user000001 · Accepted Answer · 2013-08-19 16:26:41Z

46

Yes there is:

if grep --quiet .....
then
    # If grep finds something
fi

or if the grep fails

if ! grep --quiet .....
then
    # If grep doesn't find something
fi

answered Aug 19, 2013 at 16:26

user000001

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4 Comments

Martin C. Martin Over a year ago

D'oh! Thanks. In my defense, I haven't seen that in any tutorials, even ones that talk about checking the exit code of a command.

coldfix Over a year ago

[ is kind of an alias for test (which also requires ]). So if [ ... ] is equivalent to if test ...

GaryO Over a year ago

What if you want to test multiple conditions, one of which is exit status of a command? As in: if [[ $OS == "windows" && command-exit-status==0 ]]; ... ? I can run the command first and save $? of course, but I'd like the shortcut && behavior so it doesn't run the command unless $OS==windows. Any good ideas?

user000001 Over a year ago

@GaryO: You could write it like if [[ $OS == "windows" ]] && command; then or if even without the if like [[ $OS == "windows" ]] && command && echo "All ok"

Costi Ciudatu · Accepted Answer · 2013-08-19 16:27:24Z

14

You don't need the [ ] (test) to check the return value of a command. Just try:

if ! grep --quiet XYZ "$MyFile" ; then

answered Aug 19, 2013 at 16:27

Costi Ciudatu

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Comments

Dr. Jan-Philip Gehrcke · Accepted Answer · 2013-08-19 16:27:18Z

This is a matter of taste since there obviously are multiple working solutions. When I deal with a problem like this, I usually apply wc -l after grep in order to count the lines that match. Then you have a single integer number that you can evaluate within a test condition. If the question only is whether there is a match at all (the number of matching lines does not matter), then applying wc probably is OTT and evaluation of grep's return code seems to be the best solution:

Normally, the exit status is 0 if selected lines are found and 1 otherwise. But the exit status is 2 if an error occurred, unless the -q or --quiet or --silent option is used and a selected line is found. Note, however, that POSIX only mandates, for programs such as grep, cmp, and diff, that the exit status in case of error be greater than 1; it is therefore advisable, for the sake of portability, to use logic that tests for this general condition instead of strict equality with 2.

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