Trap Shell-Script Errors

An easy way to protect shell scripts from creating havoc when they go wrong due to a missing directory is to replace the shell's cd command by a function declared at the start of the script:

cd(){
    if ! builtin cd $1
    then    echo "Failed to cd $1 !!!" >&2
            exit 1
    fi
}

No other changes are needed in the script. An alternative option is to add

set -e

...at the start of a script, then any failing command (outside a construct that tests the return code like if ...) will cause the script to stop. It also encourages you to make sure any other scripts that you call will exit with an appropriate return code: 0 for ok, and anything else for failure.

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What does "builtin" in the

Anonymous's picture

What does "builtin" in the script do?

Thanks

Another BIG help is 'set

Anonymous's picture

Another BIG help is 'set -u', which treats unset variables as an error. I trashed my system once using rsync: i misspelled a variable name, it expanded to nothing, the trailing / caused it to select the root directory as the destination, and the --delete flag promptly deleted everything on my filesystem. :-(

These are the first non-comment lines in all of my shell scripts now:
set -e
set -u

:-)

Disclaimer

Shawn Powers's picture

So your disclaimer is for my file cleanup script, isn't it?

"rm -rf /"

Here's the real tip: Don't listen to Shawn. :)

And now for my disclaimer. If you're new to Linux, I was joking. rm -rf is NOT your friend. It will kick your dog, steal your significant other, and call you ugly. Oh, and delete all your files.

Shawn Powers is a Linux Journal Associate Editor. You might find him on IRC, Twitter, or training IT pros at CBT Nuggets.

Slight tweak

Andrew Kirkpatrick's picture

If you use "$@" instead of $1, it will work with directory names containing whitespace, as well as handle multiple arguments such as cd -P /path/with/symlink


cd(){
if ! builtin cd "$@"
then echo "Failed to cd $@ !!!" >&2
exit 1
fi
}

Cheers

How is this any better

Anonymous's picture

How is this any better than:

if cd some-place
then
ok
else
fail
fi

Multiple uses

Phil Hughes's picture

If you script only has one cd in it, it isn't. But, by defining the function at the start of the script you can protect all cd commands within the script.

Phil Hughes

In a correctly written

CoolHand's picture

In a correctly written script, one should be checking the existance and permissions of a directory before trying to change to it - and, if necessary, creating the directory if it doesn't.

Where do these kids learn to program

Anonymous's picture

Thank you.