How to check if a command succeeded?

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.

9 Answers

The return value is stored in $?. 0 indicates success, others indicates error.

some_command if [ $? -eq 0 ]; then echo OK else echo FAIL fi 

Like 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" fi 

For 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' fi 

And 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! fi 

for 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 $? 0 

In 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; fi 

If you want to test if the command failed, you can use a shorter version in bash (but perhaps overkill) as follows:

if (($?)); then something; fi 

This 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; fi 

but it's already not much shorter than the first example.

For 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