Chapter 6. Exit and Exit Status

The exit command may be used to terminate a script, just as in a C program. It can also return a value, which is available to the script's parent process.

Every command returns an exit status (sometimes referred to as a return status ). A successful command returns a 0, while an unsuccessful one returns a non-zero value that usually may be interpreted as an error code. Well-behaved UNIX commands, programs, and utilities return a 0 exit code upon successful completion, though there are some exceptions.

Likewise, functions within a script and the script itself return an exit status. The last command executed in the function or script determines the exit status. Within a script, an exit nnn command may be used to deliver an nnn exit status to the shell (nnn must be a decimal number in the 0 - 255 range).

When a script ends with an exit that has no parameter, the exit status of the script is the exit status of the last command executed in the script (not counting the exit).

$? reads the exit status of the last command executed. After a function returns, $? gives the exit status of the last command executed in the function. This is Bash's way of giving functions a "return value". After a script terminates, a $? from the command line gives the exit status of the script, that is, the last command executed in the script, which is, by convention, 0 on success or an integer in the range 1 - 255 on error.

   1 #!/bin/bash
   2 
   3 echo hello
   4 echo $?    # Exit status 0 returned because command executed successfully.
   5 
   6 lskdf      # Unrecognized command.
   7 echo $?    # Non-zero exit status returned because command failed to execute.
   8 
   9 echo
  10 
  11 exit 113   # Will return 113 to shell.
  12            # To verify this, type "echo $?" after script terminates.
  13 
  14 #  By convention, an 'exit 0' indicates success,
  15 #+ while a non-zero exit value means an error or anomalous condition.

$? is especially useful for testing the result of a command in a script (see Example 12-27 and Example 12-13).

The !, the logical "not" qualifier, reverses the outcome of a test or command, and this affects its exit status. Example 6-2. Negating a condition using ! 1 true # the "true" builtin. 2 echo "exit status of \"true\" = $?" # 0 3  4 ! true 5 echo "exit status of \"! true\" = $?" # 1 6 # Note that the "!" needs a space. 7 # !true leads to a "command not found" error 8  9 # Thanks, S.C.

Certain exit status codes have reserved meanings and should not be user-specified in a script.