I'm looking for exception handling mechanism in shell script. Is there any try,catch equivalent mechanism in shell script ?
5 Answers
There is not really a try/catch in bash (i assume you're using bash), but you can achieve a quite similar behaviour using && or ||.
In this example, you want to run fallback_command if a_command fails (returns a non-zero value):
a_command || fallback_command
And in this example, you want to execute second_command if a_command is successful (returns 0):
a_command && second_command
They can easily be mixed together by using a subshell, for example, the following command will execute a_command, if it succeeds it will then run other_command, but if a_command or other_command fails, fallback_command will be executed:
(a_command && other_command) || fallback_command
2 Comments
B T
Bonus: if you want "finally" like behavior, use the no-op (: in bash) like this:
(a_command || : ) and the next line will run as if no error happened in a_command.
F. P. Freely
Not really because the semantics of finally is that it runs regardless of the exception state.
The if/else structure and exit codes can help you fake some of it. This should work in Bash or Bourne (sh).
if foo ; then
else
e=$? # return code from if
if [ "${e}" -eq "1"]; then
echo "Foo returned exit code 1"
elif [ "${e}" -gt "1"]; then
echo "Foo returned BAD exit code ${e}"
fi
fi
3 Comments
jlliagre
if foo ; then else can be simplified to if ! foo ; then, +1 anyway.
brightlancer
@jiliagre That does not work. "! foo" inverts $? from 0 to 1 and !0 to 0.
kouichi
can this part: if [ "${e}" -eq "1"]; then echo "Foo returned exit code 1" elif [ "${e}" -gt "1"]; then echo "Foo returned BAD exit code ${e}" be simplified to: if [ "${e}" -ge "1" ]; then echo "Foo returned BAD exit code ${e}" ?
{
# command which may fail and give an error
} || {
# command which should be run instead of the above failing command
}
1 Comment
Yokai
It would probably be
better if you explained to the one asking the question, the pseudo-code you post rather than just a block of cryptic code that they may not understand. If a child cannot understand it, it is not a good answer.Here are two simple bashfunctions which enable eventhandling in bash:
You could use it for basic exceptionhandling like this:
onFoo(){
echo "onFoo() called width arg $1!"
}
foo(){
[[ -f /tmp/somefile ]] || throw EXCEPTION_FOO_OCCURED "some arg"
}
addListener EXCEPTION_FOO_OCCURED onFoo
Exceptionhandling using try/catch blocks is not supported in bash, however, you might wanna try looking at the BANGSH framework which supports it (its a bit like jquery for bash).
However, exceptionhandling without cascading try/catch-blocks is similar to eventhandling, which is possible in almost any language with array-support.
If you want to keep your code nice and tidy (without if/else verbosity), I would recommend to use events.
The suggestion which MatToufoutu recommends (using || and &&) is not recommended for functions, but ok for simple commands. (see BashPitfalls about the risks)
Comments
Use following to handle error properly where error_exit is function that accepts one argument. In case if argument is not passed then it will throw unknown error with LineNo where actually error is happening. Please experiment before actually uses for production -
#!/bin/bash
PROGNAME=$(basename $0)
error_exit()
{
echo "${PROGNAME}: ${1:-"Unknown Error"}" 1>&2
exit 1
}
echo "Example of error with line number and message"
error_exit "$LINENO: An error has occurred."
exit 1