A detailed description of Shell $$

On the surface it means: The current bash PID, but the various shell environments to understand thoroughly.

Official explanation:

Bashpid

Expands to the process ID of the current bash process. This differs from

$$ under certain circumstances, such as subshells that does not require bash

To is re-initialized.

Outputs the PID of the current bash process. This is different in the following case, such as not getting to bash's child shell, will be reinitialized.

Example 1:

[[email protected] ~]$ ps-ef |grep bash |grep-v grep

Andy 30245 30244 0 Jul17 pts/0 00:00:00-bash

[Email protected] ~]$ echo $$

30245

Note: $$ represents the process ID of the login shell (a shell that is directly logged in by an Andy user ssh connection)

[Email protected] ~]$

Example 2:

[Email protected] std]# ps-ef |grep Bash

Root 4422 4421 0 Jul23 pts/0 00:00:02-bash

Root 13791 4422 0 22:14 pts/0 00:00:00 grep--color=auto Bash

Andy 30245 30244 0 Jul17 pts/0 00:00:00-bash

[Email protected] std]# echo $$

4422

Note: $$ is the PID that logged into the interactive shell (I was Andy Login, then sudo su-to root)

Example 3:

[Email protected] std]# echo $$

4422

[email protected] std]# cat test.sh

#/bin/bash

echo "Fei Jiao hu Shi Shell ' S process pid:$$"

[Email protected] std]# sh test.sh

Fei Jiao hu Shi Shell ' S process pid:13858

Note: This is a non-interactive shell, and the shell,$$ that is executed by the script is this PID

Example 4:

[Email protected] ~]$ echo $$

30245

[Email protected] ~]$ echo "abc" | {echo "abc"; Echo $$;}

Abc

30245

[Email protected] ~]$

Note: The Subshell in the pipeline, although it is a child shell environment, but does not get the-bash process, so the parent process is still bashid. (This is a bit of a detour, look at the official explanation of the English language to slowly understand it.) )

This article is from the "learning communication, the first job ops" blog, please be sure to keep this source http://cuidehua.blog.51cto.com/5449828/1831910

A detailed description of Shell $$