Shell Scripting – Subshell

Shell scripting is a powerful tool for automating tasks and simplifying the management of systems and applications. One important concept in shell scripting is the use of subshells, which allow you to execute commands within a separate shell environment.

A subshell is a child shell that is spawned by the main shell (also known as the parent shell). It is a separate process with its own set of variables and command history, and it allows you to execute commands and perform operations within a separate environment.

There are several ways to create a subshell in shell scripting. One way is to use the parentheses operator, like this:

( command1 ; command2 ; command3 )

• This will execute the commands within the parentheses in a subshell. The commands are separated by semicolons, and the subshell will terminate when all commands have completed execution.

Another way to create a subshell is to use the bash command, like this:

bash -c "command1; command2; command3"

• This will execute the commands within the quotation marks in a subshell created by the bash command.
• One useful feature of subshells is that they allow you to isolate the effects of certain commands or operations. For example, you can use a subshell to set temporary variables or change directories without affecting the parent shell’s environment.

Using a subshell to set a temporary variable

Here’s an example of using a subshell to set a temporary variable:

# Set a variable in the parent shell
FOO=bar

# Create a subshell and set a different value for the same variable
( FOO=baz ; echo $FOO )

# The value of FOO in the parent shell is unchanged
echo $FOO

The output of this script would be:

baz
bar

As you can see, the value of the FOO variable is set to baz within the subshell, but the value in the parent shell remains unchanged.

Change directories without affecting the parent shell’s current directory

Another example of using a subshell is to change directories without affecting the parent shell’s current directory.

# Show the current directory in the parent shell

pwd



# Create a subshell and change to a different directory

( cd /tmp ; pwd )



# The current directory in the parent shell is unchanged

pwd

The output of this script would be:

/home/user
/tmp
/home/user

As you can see, the subshell changes to the /tmp directory, but the parent shell’s current directory remains unchanged.

There are many other uses for subshells in shell scripting, including executing commands in the background, creating subshells with modified environments, and more. With the ability to execute commands in a separate shell environment, subshells provide a powerful tool for managing systems and automating tasks in shell scripts.

Full Shell Script- subshell:

#!/bin/bash

# This line sets the variable 'filename' to the first argument passed to the script
filename=$1

# This line prints the hostname of the current system
echo "Hostname: $(hostname)"

# This line creates a subshell and runs the 'ls' command inside it.
# The output of the 'ls' command is then stored in the 'ls_output' variable
ls_output=$(ls)

# This line prints the output of the 'ls' command
echo "Output of 'ls' command: $ls_output"

# This line checks if the file specified by the 'filename' variable exists
# and is a regular file (not a directory). If the file exists, it prints the contents
# of the file using the 'cat' command. If the file does not exist, it prints an error message
if [ -f "$filename" ]; then

  cat "$filename"

else

  echo "Error: File not found"

fi

# This line prints the current working directory
echo "Current working directory: $(pwd)"

# This line prints the current user's home directory
echo "Home directory: $HOME"

Here is an example output of the script, given a file named “test.txt” that exists in the current directory and contains the text “Hello, World!”:

Hostname: myhostname
Output of 'ls' command: bashscriptname.sh
test.txt
Hello, World!
Current working directory: /home/user/documents
Home directory: /home/user

If the file “test.txt” did not exist in the current directory, the output would be:

Hostname: myhostname
Output of 'ls' command: example.sh
Error: File not found
Current working directory: /home/user/documents
Home directory: /home/user

Conclusion

In conclusion, subshells are an important concept in shell scripting that allows you to execute commands within a separate shell environment. They can be created using the parentheses operator or the bash command, and they provide a way to isolate the effects of certain commands or operations. Subshells can be useful for a variety of purposes, including setting temporary variables, changing directories, executing commands in the background, and more. By understanding how to use subshells in your shell scripts, you can greatly improve your ability to automate tasks and manage systems efficiently.