What does if -eq 0 mean for shell scripts

Question

There is this line in a shell script i have seen   grep -e ERROR   LOG DIR PATH    LOG NAME    gt   dev null if      -eq 0

User · Answer

It is an extremely overused way to check for the success/failure of a command. Typically, the code snippet you give would be refactored as:

if grep -e ERROR ${LOG_DIR_PATH}/${LOG_NAME} > /dev/null; then
   ...
fi

(Although you can use 'grep -q' in some instances instead of redirecting to /dev/null, doing so is not portable. Many implementations of grep do not support the -q option, so your script may fail if you use it.)

User · Answer

It s checking the return value      of grep  In this case it s comparing it to 0  success    Usually when you see something like this  checking the return value of grep  it s checking to see whether the particular string was detected  Although the redirect to  dev null isn t necessary  the same thing can be accomplished using -q

User · Answer

is the exit status of the most recently-executed command  by convention  0 means success and anything else indicates failure   That line is testing whether the grep command succeeded   The grep manpage states      The exit status is 0 if selected lines are found  and 1 if  not  found   If an error occurred the exit status is 2    Note  POSIX error handling code should check for  2  or greater     So in this case it s checking whether any ERROR lines were found

[bash] What does if [ $? -eq 0 ] mean for shell scripts?

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