04. Executing Shell Command

What is a Subshell?¶

A subshell is a separate shell process spawned by the current shell to execute a command. The output produced by that command can then be captured and used elsewhere in the script.

Command substitution allows Bash to:

Execute a command
Capture its output
Replace the command with its result

Syntax¶

$(command)

The command inside $( ) runs in a subshell, and its output replaces the entire expression.

Example Script¶

`subshell.sh`¶

#!/bin/bash
# 04. Executing Shell Command
echo $(uname -o)

# executing bash command without subshell
echo uname -o

Execution¶

Running the command directly¶

$ uname -o
GNU/Linux

Running the script¶

$ ./subshell.sh
GNU/Linux
uname -o

Explanation¶

1. Using Subshell `$(uname -o)`¶

echo $(uname -o)

Step-by-step:

Bash creates a subshell.
The command uname -o runs inside it.
The output (GNU/Linux) is captured.
The captured output replaces $(uname -o).
echo prints the result.

Effectively Bash interprets it as:

echo GNU/Linux

2. Without Subshell¶

echo uname -o

Here:

No command execution happens.
Bash treats uname -o as plain text.
The text is printed literally.

Output:

uname -o

Key Difference¶

Command	Behavior	Result
`echo $(uname -o)`	Executes command in subshell	Prints command output
`echo uname -o`	Treated as text	Prints literal string

Why Use Command Substitution?¶

Command substitution is useful when you need to:

Store command output in variables
Combine commands
Automate workflows
Dynamically generate values

Example:

os=$(uname -o)
echo "Operating System: $os"

Output:

Operating System: GNU/Linux

Old vs Modern Syntax¶

Older Bash used backticks:

echo `uname -o`

Modern recommended syntax:

echo $(uname -o)

Reasons to prefer $( ):

More readable
Easier nesting
Less error-prone

Nested Command Example¶

echo $(echo $(uname -o))

Bash evaluates from the innermost command outward.

Summary¶

$( ) performs command substitution using a subshell.
The command runs and its output replaces the expression.
Without $( ), text is printed literally.
Modern Bash scripts should always prefer $( ) over backticks.

This mechanism is fundamental for writing dynamic and automated Bash scripts.