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19.2.2022 22:08:16 reposted from linux

Double dollar sign ($$) in bash

It substitutes to PID of the current running shell. For example running

ps -Alf|grep $$

generates interesting results, as shown below

$ ps -Alf|grep $$
0 S juozas     11125   11120  0  80   0 -  3391 do_wai 22:43 pts/0    00:00:00 bash
4 R juozas     11197   11125  0  80   0 -  3506 -      22:46 pts/0    00:00:00 ps -Alf
0 S juozas     11198   11125  0  80   0 -  2859 pipe_w 22:46 pts/0    00:00:00 grep --color=auto 11125

Another example is to use it in the file names where it gets substituted with the PID of the current running shell.

$ cat >f$$.txt <<EOF
This is a test. The pid of running shell is $$
EOF
$ cat f$$.txt
This is a test. The pid of running shell is 11125
$ ls f$$.txt
f11125.txt
$ LC_ALL=C rm -v f$$.txt
removed 'f11125.txt'
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