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| <<Anchor(faq105)>> == Why doesn't set -e (set -o errexit) do what I expected? == `set -e` was an attempt to add "automatic error detection" to the shell. Its ''goal'' was to cause the shell to abort any time an error occurred, so you don't have to put `|| exit 1` after each important command. That goal is non-trivial, because many commands are ''supposed'' to return non-zero. For example, {{{ if [ -d /foo ]; then ... else ... fi }}} Clearly we don't want to abort when `[ -d /foo ]` returns non-zero (because the directory does not exist) -- our script wants to handle that in the `else` part. So the implementors decided to make a bunch of special rules, like "commands that are part of an `if` test are immune", or "commands in a pipeline, other than the last one, are immune". These rules are extremely convoluted, and they still fail to catch even some remarkably simple cases. Even worse, the rules ''change'' from one Bash version to another, as Bash attempts to track the extremely slippery POSIX definition of this "feature". When a SubShell is involved, it gets worse still -- the behavior changes depending on whether Bash is invoked in POSIX mode. [[http://fvue.nl/wiki/Bash:_Error_handling|Another wiki]] has a page that covers this in more detail. Be sure to check the caveats. ''Exercise for the reader: why doesn't this example print anything?'' {{{#!highlight bash #!/bin/bash set -e i=0 let i++ echo "i is $i" }}} ''Exercise 2: why does ''this'' one sometimes appear to work? In which versions of bash does it work, and in which versions does it fail?'' {{{#!highlight bash #!/bin/bash set -e i=0 ((i++)) echo "i is $i" }}} ([[/Answers|Answers]]) GreyCat's personal recommendation is simple: don't use `set -e`. Add your own error checking instead. |
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