Your first simple shell scripts

A shell script is just a text file of commands that the shell runs in order. Instead of typing ten commands every Monday, you save them once and run them with one line. Scripts are the next rung on the automation ladder, and they can change many files in a second, so we will learn them with a safe habit from the first line: every script that changes things must be able to show what it would do before it does it. Everything in this lesson happens only inside your practice folder, with made-up files.

A script is a saved list of commands

Earlier, you typed commands such as ls and mv one at a time and watched each result. A script holds the same commands in a plain text file. When you run it, the shell reads the lines from the top and carries them out in order. Scripts also let you add a few programming ideas you met in the programming lessons: variables, loops and conditions.

Set up a practice folder

We will make a folder called auto-demo inside your practice folder, with three empty files named like monthly reports. The script will add the prefix 2025- to each name, so report-jan.txt becomes 2025-report-jan.txt. The files are empty and made up, so nothing can be lost.

mkdir -p ~/practice/auto-demo
cd ~/practice/auto-demo
touch report-jan.txt report-feb.txt report-mar.txt
ls

report-feb.txt  report-jan.txt  report-mar.txt

New-Item -ItemType Directory -Path "$HOME\practice\auto-demo" -Force
Set-Location "$HOME\practice\auto-demo"
New-Item -ItemType File -Path report-jan.txt, report-feb.txt, report-mar.txt
Get-ChildItem

Ubuntu: your first script

We will use nano, a simple text editor that runs inside the terminal. Type nano rename.sh and press Enter. A blank editor opens. Type or paste the script below. To save, press Ctrl+O and then Enter to confirm the file name. To leave, press Ctrl+X.

#!/bin/bash
# rename.sh: add the prefix 2025- to files named report-*.txt
# "bash rename.sh" shows a PREVIEW. "bash rename.sh --go" really renames.
mode="$1"
for f in report-*.txt; do
  [ -e "$f" ] || continue
  new="2025-$f"
  if [ "$mode" = "--go" ]; then
    mv -n -- "$f" "$new"
    echo "Renamed: $f -> $new"
  else
    echo "Would rename: $f -> $new"
  fi
done
if [ "$mode" != "--go" ]; then
  echo "Dry run only. Nothing was changed."
fi

Read it line by line before you run anything. The first line, starting #!, names the program that should run the file. Lines starting with # are comments for humans. mode="$1" stores the first word you type after the script name, so --go or nothing. The line for f in report-*.txt; do is a loop: it takes each file whose name starts with report- and ends with .txt, one at a time, calling it f. The line [ -e "$f" ] || continue skips the loop turn if no such file exists. new="2025-$f" builds the new name. The if line chooses between real renaming and printing. In mv -n -- "$f" "$new", -n means do not overwrite an existing file and -- marks the end of options, so odd file names are not mistaken for options.

Windows: the same job in PowerShell

PowerShell has a built-in safety switch. Many commands that change things accept -WhatIf, which means: describe what you would do and do nothing. We use it first. Here Get-ChildItem -Filter "report-*.txt" lists matching files, the brackets collect the list before any renaming begins, the pipe | hands each file to Rename-Item, and -NewName { "2025-" + $_.Name } builds a new name from the file's current name ($_ means the item being handled).

(Get-ChildItem -Filter "report-*.txt") | Rename-Item -NewName { "2025-" + $_.Name } -WhatIf

You should see three lines like the one below, one for each file. Only the first is shown here. Your folder path will show your own user name.

What if: Performing the operation "Rename File" on target "Item: C:\Users\Amina\practice\auto-demo\report-feb.txt Destination: C:\Users\Amina\practice\auto-demo\2025-report-feb.txt".

If the preview matches your plan, run the same command without -WhatIf. It prints nothing when it succeeds. Then list the folder to check.

(Get-ChildItem -Filter "report-*.txt") | Rename-Item -NewName { "2025-" + $_.Name }
Get-ChildItem

Habits of a careful script writer

• Preview first. Build in a dry run, or use -WhatIf, before any command that renames, moves or overwrites.
• Be specific. report-*.txt is safer than *, which matches everything.
• Do not overwrite. Options such as mv -n refuse to replace an existing file.
• Work in the practice folder. Check where you are with pwd (Linux) or Get-Location (Windows) before you run anything.
• Avoid deleting. A script that deletes should be written much later, after you have a backup routine. Prefer moving files to a new folder.
• Comment your script. Write one line saying what it does, and how to run it. You will forget in a month.

To clean up, open your file manager, go to the practice folder and move auto-demo to the Trash or Recycle Bin. Doing it with the file manager is deliberate: you can see exactly what you are removing, and you can restore it if you moved the wrong folder.