In UNIX shell scripting: What does $! expand to?

Question

What is the meaning for $! in shell or shell scripting? I am trying to understand a script which has the something like the following.

local@usr> a=1
local@usr> echo $a
1
local@usr> echo $!a
a

It is printing the variable back. Is it all for that? What are the other $x options we have? Few I know are $$, $*, $?. If anyone can point me to a good source, it will be helpful. BTW, This is in Sun OS 5.8, KSH.

Answer 1

The various $… variables are described in Bash manual. According to the manual $! expands to the PID of the last process launched in background. See:

$ echo "Foo"
Foo
$ echo $!

$ true&
[1] 67064
$ echo $!
67064
[1]+  Done                    true

In ksh it seems to do the same.

Answer 2

From the ksh man page on my system:

  ${!vname}
      Expands  to the name of the variable referred to by vname.  This
      will be vname except when vname is a name reference.

Answer 3

For the shell you are asking, ksh, use the the ksh manual, and read this:

Parameter Substitution
A parameter is an identifier, one or more digits, or any of the characters *, @, #, ?, -, $, and !.

It is clear that those are the accepted options $*, $@, $#, $?, $-, $$, and $!.
More could be included in the future.

---

For the parameter $!, from the manual:

"!" The process number of the last background command invoked.

if you start a background process, like sleep 60 &, then there will be a process number for such process, and the parameter $! will print its number.

$ sleep 60 &
[1] 12329
$ echo "$!"
12329

If there is no background process in execution (as when the shell starts), the exansion is empty. It has a null value.

$ ksh -c 'echo $!'

If there is a background process, it will expand to the PID of such process:

$  ksh -c 'sleep 30 & echo $!'
42586

That is why echo $!a expanded to a. It is because there is no PID to report:

$  ksh -c 'echo $!a'
a

---

Other shells may have a different (usually pretty similar) list of expansions (a parameter with only one $ and one next character).
For example, bash recognize this *@#?-$!0_ as "Special parameters". Search the Bash manual for the heading "3.4.2 Special Parameters".

Special Parameters
The shell treats several parameters specially.

Answer 4

It gives the Process id of last backgroundjob or background function Please go through this link below

http://www.well.ox.ac.uk/~johnb/comp/unix/ksh.html#specvar

Answer 5

! is a reference operator in unix, though it is not called with that name.

It always refers to a mother process. Try typing :! in vi, it takes you to command prompt and you can execute commands as usual until exit command.

! in SQLPLUS also executes the command from the command prompt. try this in sqlplus

SQL> !ls --- this gives the list of files inthe current dir.

$! - obviously gives the process id of the current/latest process.