While going through:
info coreutils 'dd invocation'
I came across:
dd if=/dev/zero of=/dev/null count=10MB & pid=$!
What is $! used for?
Asked
Active
Viewed 318 times
3
goldilocks
- 86,451
- 30
- 200
- 258
Bleeding Fingers
- 971
- 3
- 12
- 23
-
Are you using bash? – slm Oct 22 '13 at 20:02
-
@slm how am I supposed to find out? – Bleeding Fingers Oct 22 '13 at 20:05
-
You could try this command: `echo $SHELL` – slm Oct 22 '13 at 20:07
-
@slm yes, it's bash. `/bin/bash`. – Bleeding Fingers Oct 22 '13 at 20:08
-
`$!` is (at least) POSIX so not specific to `bash`. I'd assume it's universal to `sh`. – goldilocks Oct 23 '13 at 15:52
-
another possible duplicate: [In UNIX shell scripting: What does $! expand to?](http://stackoverflow.com/q/1672960/1309352) – Bleeding Fingers Oct 23 '13 at 16:36
2 Answers
13
If you're talking about Bash they're in the "Special Parameters" section of the Bash man page.
! Expands to the process ID of the most recently executed background
(asynchronous) command.
Example
$ sleep 10 &
[1] 22257
$ echo $!
22257
Your command
So with this command:
$ dd if=/dev/zero of=/dev/null count=10MB & pid=$!
The dd command is backgrounded, and the resulting process ID ($!) is stored in a variable pid for use there after.
References
slm
- 363,520
- 117
- 767
- 871
6
$! is used to get the PID(process identifier) of the most recent background command.
There is also !$:
!$ is used to get last argument for the last executed command.
slm
- 363,520
- 117
- 767
- 871
Ionut Ciucanu
- 101
- 2