SUBSHELL

 

SUBSHELL
• When you log into a LINUX system, you execute an initial login shell.

 

• This initial shell executes any simple commands that you enter.
Current(parent) shell creates a new(child) shell to perform some tasks:

 
 When a grouped command such as ( ls; pwd; date ), is executed,  the parent shell create a child shell to execute the grouped commands. If the command is not executed in the background, the parent shell sleeps until the child shell terminates.

 
When a script is executed, a parent shell creates a child shell to execute the commands in the script. If the script is not executed in the background, the parent shell sleeps until the child shell terminates.

 
When a background job is executed, a parent shell creates a child shell to execute the background commands. The parent shell continues to run concurrently with the child shell.

 

 

SUBSHELL
  • A child shell is called a subshell
  • Just like any other LINUX process, a subshell has its own current working directory. Thus cd commands executed in a subshell do not   affect the working directory of the parent shell.

 
Example
   pwd             —> display my login shell’s current directory.

 

/home/pythonbaba

( cd /; pwd ) —> the subshell moves and executes pwd.

/                     —> output comes from the subshell.

pwd              —> my login shell never moved.

 

/home/pythonbaba

 

 

SUBSHELL
Every shell contains two data areas:

  •  An environment-variable space
  •  A local-variable space.
A child shell inherits a copy of its parent’s environment space and a clean local-variable space:

A child shell inherits a copy of its parent’s environment space
A child shell inherits a copy of its parent’s environment space