Wednesday, November 14, 2012

ReturnCode in unix


Shell Command Exit Status

The return value of a command is its exit status, or 128 + N if the command is terminated by signal N. Exit status is used to check the result (success/failure) of the execution of the command. If the exit status is zero, then the command is success. If the command is failed the exit status will be non-zero.

Exit Value Exit Status
0 (Zero) Success
Non-zero Failure
2 Incorrect usage
127 Command Not found
126 Not an executable
$? Shell Variable

The shell variable name $? is a special built-in variable which has the exit status of the last command executed.

After the shell function execution, $? returns the exit status of the last command executed in a function.
After the shell script execution, $? returns the exit status of the last command executed in the script.
Sample Shell Script that Explains Shell-Command Exit Status

The following exitstatus.sh shell-script shows examples of various shell-command exit status.

$ cat exitstatus.sh
#! /bin/bash

echo -e "Successful execution"
echo -e "====================="
echo "hello world"
# Exit status returns 0, because the above command is a success.
echo "Exit status" $?

echo -e "Incorrect usage"
echo -e "====================="
ls --option
# Incorrect usage, so exit status will be 2.
echo "Exit status" $?

echo -e "Command Not found"
echo -e "====================="
bashscript
# Exit status returns 127, because bashscript command not found
echo "Exit status" $?

echo -e "Command is not an executable"
echo -e "============================="
ls -l execution.sh
./execution.sh
# Exit status returns 126, because its not an executable.
echo "Exit status" $?
Now, execute the above exitstatus.sh to see the various exit statues given by the sample shell script.

$ bash exitstatus.sh
Successful execution
=====================
hello world
Exit status 0
Incorrect usage
=====================
ls: unrecognized option `--option'
Try `ls --help' for more information.
Exit status 2
Command Not found
=====================
exitstaus.sh: line 15: bashscript: command not found
Exit status 127
Command is not an executable
=============================
-rw-r--r-- 1 root root 659 Mar  9 13:36 execution.sh
exitstatus.sh: line 21: ./execution.sh: Permission denied
Exit status 126

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