What will echo $$ command give


 
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# 1  
What will echo $$ command give

Hi

I tried giving the following command
echo $$a

I got an output like 32178a

Can some one please explain why echo $$ is returning 32178

Thanks in advance
# 2  
That is your terminal process ID ..
Code:
$ echo $$
19095
$ tty
/dev/pts/39
$ ps -ef | grep "/dev/pts/39"
userid  1052 19095  0 07:36:14 pts/39   0:00 grep /dev/pts/39
$

# 3  
Thank you
# 4  
@Sri3001: I see what you're tying to do here. You have something stored in variable 'a', the stored value if used as a variable will point to something else, right. Try this:

Code:
$ a=x
$ x=hello
$ echo $a
x
$ echo $x
hello
$ eval echo \$$a
hello

And as enunciated by jayan_jay, $$ refers to the pid of current shell. So a plain echo $$awould no doubt fetch you a pid concatenated with 'a'.
# 5  
Quote:
Originally Posted by jayan_jay
That is your terminal process ID ..
Code:
$ echo $$
19095
$ tty
/dev/pts/39
$ ps -ef | grep "/dev/pts/39"
userid  1052 19095  0 07:36:14 pts/39   0:00 grep /dev/pts/39
$

Why we are not able to combine both 2nd and 3rd commands?
i have tried the below:
Code:
pandeeswaran@ubuntu:~$ ps -ef | grep `tty`
grep: a: No such file or directory
grep: tty: No such file or directory
pandeeswaran@ubuntu:~$ ps -ef | grep `tty|sh`
sh: not: not found
Usage: grep [OPTION]... PATTERN [FILE]...
Try `grep --help' for more information.

What i am doing wrong?

Thanks

---------- Post updated at 12:05 AM ---------- Previous update was at 12:02 AM ----------

I have tried the below also:
Code:
pandeeswaran@ubuntu:~$ ps -ef|grep "`tty`"
1000      3059  2221  0 13:35 pts/0    00:00:00 grep --color=auto not a tty

---------- Post updated at 12:09 AM ---------- Previous update was at 12:05 AM ----------

Finally got it:
Code:
ps -ef|grep "`tty`"|cut -d" " -f9

is equiavlent to
Code:
echo $$

But still i don't know why
Code:
ps -ef|grep "`tty`"

returns some
Code:
grep --color=auto not a tty

at the end,

Thanks
# 6  
Quote:
Originally Posted by pandeesh
Why we are not able to combine both 2nd and 3rd commands?
i have tried the below:
Code:
pandeeswaran@ubuntu:~$ ps -ef | grep `tty`
grep: a: No such file or directory
grep: tty: No such file or directory
  
--- snip---


But still i don't know why 
Code:
ps -ef|grep "`tty`"
returns some
Code:
grep --color=auto not a tty
at the end, Thanks


If you execute with set -x or replace grep with echo, it might make the results more clear. The grep $(tty) is being run in a child process whch isn't associated with the tty and the output from the command is 'not a tty'. This output is being passed to grep, so the command line looks like this after the tty runs:
Code:
ps -ef  | grep "not" "a" "tty"

And grep then searches for the word 'not' in the files 'a' and 'tty' resulting in the no such file messages.


If you want this will work:


Code:
eval ps -ef \|grep $(tty)


The tty command is executed first, in the current shell associated with the tty and then the pipeline is executed such that grep is given one search argument.
# 7  
@pandeesh
Please stop hijacking threads.

Code:
echo $$

This is process ID of the current shell. It is described in the "man" for your Shell.

Quote:
ps -ef|grep "`tty`"|cut -d" " -f3
is equiavlent to

Code:
echo $$
Though the most awful piece of code, it demonstrates that parent process of the "grep" command is the current shell. It does nothing else. The only thing that grep found was itself.
 

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