Bash: if [ tar = error ]

Question

I want a shell script to write a message depending on whether tar succeeds or fails, but this script doesn't work correctly:

TAR=$(tar -cf Archiv.tar myfolder/ myotherfolder/)
if [ ! "$TAR" = "/dev/null" ]; then
   echo "success"
else
  echo "error - no such Directory or file"
fi

What is the problem?

Answer 1

I'm not aware of a situation where tar will output /dev/null to signal an error, which is how your code attempts to detect the error. This is the correct way to check for errors:

if tar -cf Archiv.tar myfolder/ myotherfolder/
then
    echo "success"
else
    echo "error - no such Directory or file"
fi

You probably don't need the else case because tar itself will complain in that case. If your use of /dev/null was supposed to suppress tar complaints so that you can substitute your own message, then the first line should be:

if tar -cf Archiv.tar myfolder/ myotherfolder/ > /dev/null 2>&1

Notice that we're not using square brackets here. That's actually an alias for a built-in shell command called test(1) which does logical operations and returns 0 or nonzero to signal success or failure, respectively. if in turn checks for this 0/nonzero return, so in order to check for the success of a program like tar that returns nonzero on failure, you don't need to involve test or its alias [.

If you were trained on a programming language that requires some kind of punctuation surrounding the if expression so that the lack of punctuation bothers you here, you could rewrite the first line like this:

tar -cf Archiv.tar myfolder/ myotherfolder/
if [ $? = 0 ]

These two lines together do the same thing a the version above because the shell built-in variable $? holds the status code of that last program run.

This is perhaps clearer, but a bit wasteful.

Answer 2

As usual there is more then one way to do what you were trying to.
This is a (maybe compact) way to do it:

#!/bin/bash
TAR_COMMAND='tar -cf Archiv.tar myfolder/ myotherfolder/ 2>/dev/null'
MSG_OK='### Work well DONE ### '   # A more common way to use a variable
MSG_ERR='### OPS IT FAILS ###  '    

$TAR_COMMAND && echo $MSG_OK || echo $MSG_ERR

Short notes

• If you have no need to store the exit status (or the exit code) of your command you can use immediately after the operators AND, &&, and OR, || ^[1].
  The logic: cmd1 && cmd2, cmd2 will executed only if cmd1 exits without errors (with exit code 0). Instead with cmd1 || cmd3, cmd3 will be executed only if cmd1 will exits with an error (error code different from 0).

• If, instead, you want to store the exit code of a command you have to do it immediately after its execution, because with the next command execution this value will be updated. It's enough a variable assignation

  mycommand               # you execute your command
  ExitCodeToUseLater=$?   # So I can use it later in the script
  # ... other stuffs ...
  [ $ExitCodeToUseLater = 0  ]  && echo "It was gone ok " 
  # ... again other stuffs ...
  [ $ExitCodeToUseLater = 0  ] || echo "# Never a time that it goes as I want"
  

• In bash scripting, as well as in the makefiles ^[2a,2b], you can find a command with many options written in a variable in order to make more readable the code (as you can see above for the TAR_COMMAND variable). Writing in a shell the variable $MyVar it will be the same to write in the shell what is written inside ^[3]. It works in a script too. You may have to pay attention to the bash expansion and substitution rules about which you can read more from man bash.

• /dev/null is the Null device ^[4], "a device file that discards all data written to it but reports that the write operation succeeded". It can be used to give an empty input to a command (</dev/null) or to pass in "absolute silence" the standard output or the standard error ^[5].