Learn Linux Shell Scripting - Fundamentals of Bash 4.4

Fun with exclamation marks

Exclamations marks are normally used to give text some emphasis, but under Bash they are actually a shell keyword:

reader@ubuntu:~$ type -a !! is a shell keyword

While the term "shell keyword" does not really give us a great indication of what it does, there are multiple things we can accomplish with the exclamation mark. One we have already seen: if we want to negate a test, we can supply the exclamation mark within the check. If you'd like to verify this on your Terminal, try the following with either true or false:

reader@ubuntu:~$ truereader@ubuntu:~$ echo $?0reader@ubuntu:~$ ! truereader@ubuntu:~$ echo $?1

As you can see, the exclamation mark reverses the exit status: true becomes false, and false becomes true. ...