Trap Shell-Script Errors
December 2nd, 2007 by LJ Staff
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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If last month's Infrastrucuture issue was too "big" for you then try on this month's Embedded issue. Find out how to use Player for programming mobile robots, build a humidity controller for your root cellar, find out how to reduce the boot time of your embedded system, and if you're new to embedded systems find out the basics that go into one. You can also read about the Beagle Board, the Mesh Potato and a spate of other interestingly named items. And along with our regular columns don't miss our new monthly column: Economy Size Geek.
What does "builtin" in the
On December 26th, 2007 Anonymous (not verified) says:
What does "builtin" in the script do?
Thanks
Another BIG help is 'set
On December 6th, 2007 Anonymous (not verified) says:
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
On December 6th, 2007 Shawn Powers says:
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.
Slight tweak
On December 3rd, 2007 Andrew Kirkpatrick (not verified) says:
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
On November 30th, 2007 Anonymous (not verified) says:
How is this any better than:
if cd some-place
then
ok
else
fail
fi
Multiple uses
On November 30th, 2007 Phil Hughes says:
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.
In a correctly written
On December 5th, 2007 CoolHand says:
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
On December 10th, 2007 Anonymous (not verified) says:
Thank you.
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