Robust expansion (i.e. crash) of bash variables with a typo

set -eu
By default bash expands an unbound variable to an empty string. This can be dangerous, if a critical variable name (a path prefix for example) has a typo. The -u option causes bash to treat this as an error, and the -e option causes it to exit in case of an error. These two together will make your scripts a lot safer against typos. The default behaviour can be explicitly requested using the ${NAME:-} syntax. A (less explicit) variation: #!/bin/bash -eu
Sample Output
$ cat safe-cacheclean.sh 
#!/bin/bash

set -eu
CACHEDIR=mycache
# a dangerous-looking typo, is /bin going to disappear:
rm -rf "$CACHEDI/bin"

$ ./safe-cacheclean.sh 
./safe-cacheclean.sh: line 7: CACHEDI: unbound variable

8
By: wipu
2010-04-07 11:54:40
set

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    AskApache · 2010-10-27 07:16:54 0
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  • Like 7171, but fixed typo, uses fewer variables, and even more cryptic! Show Sample Output


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  • Very helpful when you've got complex filenames and needs to change just some small parts of it. Renaming a file called "i-made-a-small-typo-right-here" to "i-made-a-big-typo-right-here": mv -vi i-made-a-{small,big}-typo-right-here You could also copy multiple files, edit, remove, process, etc. Show Sample Output


    1
    mv -vi file{,~}
    fsilveira · 2009-08-14 19:29:59 0

What Others Think

The -e and -u options are POSIX standard, so they should work with any /bin/sh, not just bash.
inof · 427 weeks and 5 days ago
Btw, you need to be careful with commands like grep and find that exit with nonzero when they don't find anything. To make matters worse, failures like these are completely silent. These kinds of commands need to be "guarded" by e.g. adding '|| echo "Nothing found"' or something after them. IMHO this kind of problem is less dangerous and easier to solve than the problems caused by not using set -eu, though.
wipu · 350 weeks ago

What do you think?

Any thoughts on this command? Does it work on your machine? Can you do the same thing with only 14 characters?

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