Bash – Provide default value if command returns with non-zero exit code

bashcatexit-statusshell

I have some configuration in file config and would like to cat that file. However, sometimes config doesn't exist. In this case, I would like to have my command output a default value.

Perhaps something that worked like this:

$ ls
$ cat config || echo 42
42
$ echo 73 > config
$ cat config || echo 42
73

Best Answer

Your construct is fine. You could even do someting like

cat config || cat defaultconfig

If you use some random command (like the ./get_config_from_web in comments), you'll have to make sure the command does give a sensible return status. That can be tricky, shell scripts just return the result of the last command executed, you'd have to do a exit if you want something else as result.

Related Solutions

Bash – How to catch a command exit code for later, in a Makefile

If you're not worried about the timing of data passing from foo to bar, and you're okay with a tempfile which will need to be handled in your clean target, then simply:

rcheck:
        foo | tee sometempfile
        -bar < sometempfile >/dev/null 2>/dev/null

If on the other hand you care a lot about timing then you could make bar repeat its input to stdout and try something like:

rcheck:
        -(foo; echo $$? > sometempfile) | bar
        exit $(cat sometempfile)

I'm sure there'll be cleaner ways, but the above came to mind. (Note, both are untested)

Shell – Cat all files in a folder including filename by using a for loop

for f in *; do
  printf '%s\n' "$f"
  paste /dev/null - < "$f"
done

Would print the file name followed by its content with each line preceded by a TAB character for each file in the directory.

Same with GNU awk:

gawk 'BEGINFILE{print FILENAME};{print "\t" $0}' ./*

Or to avoid printing the name of empty files (this one not GNU specific):

awk 'FNR==1 {print FILENAME}; {print "\t" $0}' ./*

Or with GNU sed:

sed -s '1F;s/^/\t/' ./*

Best Answer

Related Solutions

Bash – How to catch a command exit code for later, in a Makefile

Shell – Cat all files in a folder including filename by using a for loop

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