Bash, using exit status for decisions

action_owl · 03-03-2010, 05:02 AM

How exactly can I use the exit status of a command in an if statement ?

Something like this:

Code:

#!/bin/bash

this=that

if [ $(whatis $this) ]; then

        echo "I know what $this is"

else
	echo "I don't know what $this is"

fi

colucix · 03-03-2010, 05:17 AM

Two ways. One explicit:

Code:

#!/bin/bash
command
if [ $? -eq 0 ]
then
  echo command successful
else
  echo command not successful
fi

one implicit

Code:

#!/bin/bash
if command
then
  echo command successful
else
  echo command not successful
fi

Note that even [ is a command.

druuna · 03-03-2010, 05:18 AM

Hi,

The exit status of a program is held in the following variable: $?

If the content of that variable is 0 (zero) it means all is OK. If it is anything else than that something is "wrong" (if it is actually wrong depends on the program and the number given, you need to check the manpage of that program to see what the number means).

Try this:

Quote:

$ ls -l /etc/fstab
-rw-r--r-- 1 root root 2799 Jan 25 16:23 /etc/fstab
$ echo $?
0

$ ls -l /etc/dummy
ls: /etc/dummy: No such file or directory
$ echo $?
2

About your example:

Code:

#!/bin/bash

this="that"
#this="perl"

whatis $this 1>/dev/null 2>&1
ExitStatus="$?"

if [ $ExitStatus == "0" ]; then
  echo "I know what $this is"
else
  echo "I don't know what $this is"
fi

Hope this helps.

action_owl · 03-03-2010, 05:20 AM

Thank you