$? is used to return the return code / exit code of the last run command. For example, since whoami is a valid command the return code will be 0 (success).

~]# whoami
john.doe

~]# echo $?
0

On the other hand, since "bogus" is not a valid command, the return code will be something other than 0, often 127.

~]$ bogus
-bash: bogus: command not found

~]$ echo $?
127

Or, if you do not have permission to run a valid command, the return code should be 1.

~]# shutdown now
Permission denied

~]# echo $?
1

Or, a valid command that expects stdout to be returned but no stdout is returned may return 1 (failed).

~]$ ps -ef | grep -v grep | grep bogus
~]$ echo $?
1

Determine if a file contains a string

The grep command is used to search for a certain string in a file. If grep is able to find one or more occurrences of the string in the file, the exit code will be 0 (success).

~]# grep "hello world" example.file
hello world

~]# echo $?
0

On the other hand, if there are zero occurrences of the string in the file, the exit code will be 1 (failure).

~]# grep "hello world" example.file

~]# echo $?
1

This can be useful in a script, to do something when a file contains a certain string, or to do something else if the file does not contain a certain string.

#!/bin/bash

abc=$(grep "hello world" example.file)

result=$?

if [ $result == 0 ];
then
    echo "example.file does contain hello world"
else
    echo "example.file does not contain hello world"
fi

Exit code in bash script

Here is a basic example of how to do something in a bash shell script using the return code of a command.

#!/bin/bash
mkdir /tmp/my_temporary_directory

return_code=$?

if [ $return_code -eq 0 ]; then
  echo "successfully created directory /tmp/my_temporary_directory"
else
  echo "failed to create directory /tmp/my_temporary_directory - got return code $return_code"
  exit 1
fi

Or, sometimes something like this makes more sense.

#!/bin/bash
cmd="mkdir /tmp/my_temporary_directory"

return_code=$?

echo "command $cmd returned exit code $return_code"

if [ $return_code -ne 0 ]; then
  exit 1
fi

Be aware that if you set the Internal Field Separator (IFS), you may start getting exit code 127 returned. For example, like this.

#!/bin/bash
IFS=$'\n'
cmd="mkdir /tmp/example"
$cmd
rc=$?
echo return code $rc
exit

Running this script returned the following. So, perhaps just set Internal Field Separator (IFS) only if needed, and unset once done.

]$ bash example.sh
example.sh: line 3: mkdir /tmp/example: No such file or directory
return code 127