Shell Scripting

So far, we’ve typed commands one by one. Shell scripting allows you to save a sequence of commands into a file, add logic (like if statements and loops), and run them as a single program.

A Simple Script

A shell script is just a text file. It usually starts with a shebang (#!) telling the system which shell to use.

#!/bin/bash
# This is a comment

NAME="Learner"
echo "Hello, $NAME!"
echo "Today is $(date)"

Variables and Logic

• Variables: No spaces around the =. Access them with $.
• Arguments: $1, $2, etc., refer to the first, second arguments passed to the script. $# is the count of arguments.
• Conditionals:

  if [ $USER == "root" ]; then
    echo "Welcome, admin."
  else
    echo "Standard user."
  fi

Loops

Automating a task over many files is where scripting shines:

# Rename all .txt files to .bak
for file in *.txt; do
  mv "$file" "${file%.txt}.bak"
done

Why Script?

1. Automation: Never type a complex 5-command pipe more than once.
2. Reproducibility: Document exactly how a build or deployment is performed.
3. Power: Combine small, specialized tools (grep, sed, awk) into a custom solution for your specific data.

Takeaways

• Scripts are executable files containing a sequence of shell commands.
• They support standard programming constructs like variables, loops, and logic.
• They are the “glue” that holds together complex server-side workflows.