How to check if a command succeeded?

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

http://mywiki.wooledge.org/BashGuide/TestsAndConditionals explains if in more detail.

command && echo OK || echo Failed

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