Chapter 6. Exit and Exit Status
| | ...there are dark corners in the Bourne shell, and people use all
of them. |
| | Chet Ramey |
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 (previous to the
exit). #!/bin/bash
COMMAND_1
. . .
# Will exit with status of last command.
COMMAND_LAST
exit |
The equivalent of a bare exit is
exit $? or even just omitting the
exit. #!/bin/bash
COMMAND_1
. . .
# Will exit with status of last command.
COMMAND_LAST
exit $? |
#!/bin/bash
COMMAND1
. . .
# Will exit with status of last command.
COMMAND_LAST |
|
$? 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.
Example 6-1. exit / exit status
#!/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 12-32 and Example 12-17).
| 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 ! true # the "true" builtin.
echo "exit status of \"true\" = $?" # 0
! true
echo "exit status of \"! true\" = $?" # 1
# Note that the "!" needs a space.
# !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, St�phane Chazelas and Kristopher Newsome. |
|
 | Certain exit status codes have reserved meanings and should not
be user-specified in a script. |
