Chapter 6. Exit and Exit Status

The exit command terminates 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 or exit code). A successful command returns a 0, while an unsuccessful one returns a non-zero value that usually can 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 an integer in the 0 - 255 range).

#!/bin/bash

COMMAND_1

. . .

COMMAND_LAST

# Will exit with status of last command.

exit

#!/bin/bash

COMMAND_1

. . .

COMMAND_LAST

# Will exit with status of last command.

exit $?

#!/bin/bash

COMMAND1

. . . 

COMMAND_LAST

# Will exit with status of last command.

$? 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." [1] 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.

#!/bin/bash

echo hello
echo $?    # Exit status 0 returned because command executed successfully.

lskdf      # Unrecognized command.
echo $?    # Non-zero exit status returned because command failed to execute.

echo

exit 113   # Will return 113 to shell.
           # To verify this, type "echo $?" after script terminates.

#  By convention, an 'exit 0' indicates success,
#+ 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 15-35 and Example 15-20).

true    # The "true" builtin.
echo "exit status of \"true\" = $?"     # 0

! true
echo "exit status of \"! true\" = $?"   # 1
# Note that the "!" needs a space between it and the command.
#    !true   leads to a "command not found" error
#
# The '!' operator prefixing a command invokes the Bash history mechanism.

true
!true
# No error this time, but no negation either.
# It just repeats the previous command (true).

# Thanks, StИphane Chazelas and Kristopher Newsome.