Is there any way to check if there is an error in executing a command?
Example :
test1=`sed -i "/:@/c connection.url=jdbc:oracle:thin:@$ip:1521:$dataBase" $search`
valid $test1
function valid () { if $test -eq 1; then echo "OK" else echo "ERROR" fi
}I already tried do that but it seems it isn't working. I don't how do that.
39 Answers
The return value is stored in $?. 0 indicates success, others indicates error.
some_command
if [ $? -eq 0 ]; then echo OK
else echo FAIL
fiLike any other textual value, you can store it in a variable for future comparison:
some_command
retval=$?
do_something $retval
if [ $retval -ne 0 ]; then echo "Return code was not zero but $retval"
fiFor possible comparison operators, see man test.
If you only need to know if the command succeeded or failed, don't bother testing $?, just test the command directly. E.g.:
if some_command; then printf 'some_command succeeded\n'
else printf 'some_command failed\n'
fiAnd assigning the output to a variable doesn't change the return value (well, unless it behaves differently when stdout isn't a terminal of course).
if output=$(some_command); then printf 'some_command succeded, the output was «%s»\n' "$output"
fi explains if in more detail.
command && echo OK || echo Failed$? should contain the exit status of the previous command, which should be zero for no error.
So, something like;
cd /nonexistant
if [ $? -ne 0 ]
then echo failed
else echo success!
fifor most cases, it's easier to use the && construct to chain commands that need to depend on each other. So cd /nonexistant && echo success! would not echo success because the command breaks before &&. The corollary of this is ||, where cd /nonexistant || echo fail would echo fail because cd failed. (this becomes useful if you use something like ||exit, which will end the script if the previous command failed.)
command && echo $? || echo $?It should be noted that if...then...fi and &&/|| type of approach deals with exit status returned by command we want to test( 0 on success ); however, some commands don't return a non-zero exit status if command failed or couldn't deal with input. This means that the usual if and &&/|| approaches won't work for those particular commands.
For instance, on Linux GNU file still exits with 0 if it received a non-existing file as argument and find couldn't locate the file user specified.
$ find . -name "not_existing_file"
$ echo $?
0
$ file ./not_existing_file
./not_existing_file: cannot open `./not_existing_file' (No such file or directory)
$ echo $?
0In such cases, one potential way we could handle the situation is by reading stderr/stdin messages, e.g. those that returned by file command, or parse output of the command like in find. For that purposes, case statement could be used.
$ file ./doesntexist | while IFS= read -r output; do
> case "$output" in
> *"No such file or directory"*) printf "%s\n" "This will show up if failed";;
> *) printf "%s\n" "This will show up if succeeded" ;;
> esac
> done
This will show up if failed
$ find . -name "doesn'texist" | if ! read IFS= out; then echo "File not found"; fi
File not found( This is a repost of my own answer on related question at unix.stackexchange.com )
As mentioned in many other answers, a simple test of $? will do, like this
if [ $? -eq 0 ]; then something; fiIf you want to test if the command failed, you can use a shorter version in bash (but perhaps overkill) as follows:
if (($?)); then something; fiThis works by using the (( )) arithmetic mode, so if the command returned success, i.e. $? = 0 then the test evaluates to ((0)) which tests as false, otherwise it will return true.
To test for success, you could use:
if ! (($?)); then something; fibut it's already not much shorter than the first example.
0For easier debugging, I make commands to only output errors with:
so [-tag] [commands]
After that $? is 0 if success, otherwise failure.
You can also configure bash to exit if a command fails, useful to avoid checks after every command. Place this at beginning of your script:
set -e
