Variables, Conditions & Loops

What is it?

Bash scripting glues commands with logic: variables hold values, conditions ([ ... ] / test) branch, and loops (for/while) repeat over lists or lines. It is the everyday automation language of Linux.

Why it matters

Repetitive admin work — over many files, hosts or log lines — is where mistakes and toil live. A small script does it consistently, every time, without a slip.

Where you see it

`name=value` (no spaces), `if [ -f /etc/x ]; then ...; fi`, `for f in *.log; do ...; done` — the building blocks of every ops script.

What normal looks like

Quoted variables ("$f"), explicit conditions, and loops that handle the empty case — a script that behaves the same on 0, 1 or 1000 items.

What suspicious looks like

Unquoted variables that break on spaces, an off-by-one condition, or a loop that does the wrong thing when the list is empty — subtle bugs that bite at scale.

How analysts investigate

Read a script by tracing one input through it, always quote variable expansions, and test on the tricky inputs (spaces, empty, many) before trusting it.

Common beginner mistakes

Writing name = value with spaces (Bash treats it as a command).
Leaving $var unquoted so a filename with a space splits into two.

Logic over commands

  #!/usr/bin/env bash
  threshold=90
  usage=$(df / | awk 'NR==2{print $5}' | tr -d '%')
  if [ "$usage" -ge "$threshold" ]; then
    echo "WARN: root disk at ${usage}%"
  fi
  quote expansions; no spaces around =

Variables + conditions + loops turn one-off commands into repeatable, quoted, safe automation.

Quick check

Why quote "$f" in a loop over filenames?

A quick self-check — it doesn't affect your XP or progress.