Linux Shell - Scripting

Introduction

What is a Script?

A shell script is simply a text file containing commands that you could type yourself. The power comes from chaining commands together and reusing them.

Think of it like recording a macro in Excel: instead of repeating steps by hand, you write them down once and run them whenever needed.

Structure of a Script

1. Shebang line: tells Linux which interpreter to use.

   #!/bin/bash

   Without this, the system might use /bin/sh (which may behave differently).

2. Commands: everything you would type manually.

3. Comments: begin with #. Comments are ignored by the shell but help humans understand.

Running a Script

1. Save it as myscript.sh.
2. Make it executable:

   chmod +x myscript.sh

3. Run it:

   ./myscript.sh

   The ./ ensures the shell looks in the current directory.

Alternatively, you can run it explicitly with an interpreter:

bash myscript.sh

Variables

Variables let you store values.

• Create:

  name="Alice"

• Use:

  echo "Hello $name"

By default, variables exist only inside the current shell. If you want them to be inherited by child processes, you need export:

export PATH=$PATH:/usr/local/bin

Script Arguments

Scripts can accept inputs. This is essential if you want them to be reusable.

• $0 → script name
• $1, $2 … → first, second argument
• $@ → all arguments

Example:

#!/bin/bash
echo "This script is: $0"
echo "First arg: $1"
echo "All args: $@"

Run it:

./myscript.sh apple banana

Output:

This script is: ./myscript.sh
First arg: apple
All args: apple banana

This is similar to function parameters in other languages.

Exit Codes

Every command returns a number ($?).

• 0 means success.
• Anything else means an error.

This is how you can make scripts react to success or failure.

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Logic and Testing in Scripts

Scripts become powerful when they can make decisions. That’s where conditions come in.

If/Else Statements

if [ condition ]; then
    echo "True"
else
    echo "False"
fi

The [ ] is actually a command called test.

File Tests

The test command can check properties of files:

• -f file → exists and is a regular file.
• -d dir → exists and is a directory.
• -x file → file exists and is executable.

Example:

if [ -f /etc/passwd ]; then
  echo "passwd file exists"
fi

This is used all the time in installation or configuration scripts to check whether a file or directory already exists before overwriting it.

String and Number Tests

• [ "$a" = "$b" ] → strings equal
• [ "$a" != "$b" ] → strings not equal
• [ 5 -gt 3 ] → greater than
• [ 2 -le 7 ] → less or equal

Case Statements

Instead of many if statements, you can use case to handle multiple possibilities:

case $1 in
  start) echo "Starting service" ;;
  stop)  echo "Stopping service" ;;
  *)     echo "Usage: $0 {start|stop}" ;;
esac

This is often used for system scripts where the user might type ./script.sh start or ./script.sh stop. It improves readability compared to multiple if/else blocks.

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Loops and Automation

Loops let you repeat actions automatically.

For Loops

Best for iterating over lists.

for file in *.txt; do
  echo "Processing $file"
done

This is equivalent to PowerShell’s foreach ($f in Get-ChildItem).

While Loops

Run as long as a condition is true.

count=1
while [ $count -le 5 ]; do
  echo "Count = $count"
  count=$((count + 1))
done

This is like PowerShell’s while ($true) { ... }, except you normally put a real condition instead of $true.

Until Loops

The opposite of while: run until the condition becomes true.

until [ -f /tmp/stopfile ]; do
  echo "Waiting..."
  sleep 5
done

This is useful for waiting on a process to finish, or a file to appear, without writing complicated checks.

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