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Bash scripting: variables, input, conditions, loops, functions, error handling and real automation scripts

A Bash script is a text file full of commands that runs from top to bottom, like a recipe. Anything you type in the terminal can go in a script, and then it can run again and again: nightly backups, deploying a website, creating user accounts for a whole class, checking disk space and emailing an alert, renaming hundreds of photos. Scripting is what turns a Linux user into a Linux administrator.

Your first script

Terminal

nano hello.sh

Terminal

#!/bin/bash
# hello.sh: my first script
echo "Hello, $USER!"
echo "Today is $(date +%A), $(date +%d-%m-%Y)"
echo "You are in: $(pwd)"

Terminal

chmod +x hello.sh      # make it executable
./hello.sh             # run it (./ means "in this folder")
bash hello.sh          # or run it with bash directly

The first line #!/bin/bash is the shebang: it tells the system which interpreter runs the file. Lines starting with # are comments.

Variables

Terminal

NAME="Wanjiku"           # no spaces around =
COUNT=5
echo "Hello $NAME, you have $COUNT messages"
echo "File: ${NAME}_report.txt"     # braces separate the name from text after it

Double quotes expand variables: "Hi $NAME" → Hi Wanjiku.
Single quotes don't: 'Hi $NAME' → Hi $NAME.
Command substitution stores a command's output: TODAY=$(date +%F).
Arithmetic: TOTAL=$((PRICE * QTY)).

Terminal

PRICE=250
QTY=4
echo "Total: KSh $((PRICE * QTY))"
FILES=$(ls | wc -l)
echo "There are $FILES items here"

Environment variables ($HOME, $USER, $PATH) are available in every script. Export your own for child processes: export API_URL="https://...".

Input and arguments

Terminal

read -p "Enter your name: " NAME
read -sp "Password: " PASS; echo     # -s hides typing
echo "Welcome, $NAME"

Arguments passed on the command line (./greet.sh Amina Mombasa):

Variable	Meaning
`$0`	Script name
`$1`, `$2`...	First, second argument
`$#`	Number of arguments
`$@`	All arguments (use `"$@"` in quotes)
`$?`	Exit code of the last command
`$$`	The script's PID

Terminal

#!/bin/bash
if [ $# -lt 2 ]; then
  echo "Usage: $0 NAME TOWN"
  exit 1
fi
echo "Hello $1 from $2"

Exit codes

Every command returns an exit code: 0 means success, anything else means failure.

Terminal

ls /etc > /dev/null; echo $?        # 0
ls /nope 2> /dev/null; echo $?      # 2 (error)

mkdir backup && echo "created"      # && runs only on success
ping -c1 google.com > /dev/null || echo "No internet!"   # || runs only on failure

Scripts should end with exit 0 on success and a non-zero code on errors, so other tools (cron, CI) know what happened.

Conditions

Terminal

if [ condition ]; then
  ...
elif [ other ]; then
  ...
else
  ...
fi

Test	True if
`-f file`	file exists and is a regular file
`-d dir`	directory exists
`-e path`	anything exists
`-r / -w / -x file`	readable / writable / executable
`-z "$s"`	string is empty
`"$a" = "$b"` / `!=`	strings equal / different
`$a -eq $b`	numbers equal (`-ne`, `-lt`, `-le`, `-gt`, `-ge`)

Terminal

#!/bin/bash
FILE="/etc/nginx/nginx.conf"
if [ -f "$FILE" ]; then
  echo "Nginx config found"
else
  echo "Nginx not installed"
fi

USAGE=$(df / --output=pcent | tail -1 | tr -dc '0-9')
if [ "$USAGE" -ge 90 ]; then
  echo "CRITICAL: disk ${USAGE}% full"
elif [ "$USAGE" -ge 75 ]; then
  echo "Warning: disk ${USAGE}% full"
else
  echo "Disk OK (${USAGE}%)"
fi

Bash also has [[ ... ]], which is more forgiving (supports &&, || and patterns): if [[ $NAME == W* && -d "$HOME" ]]; then ....

Loops

Terminal

# for over a list
for TOWN in Nairobi Mombasa Kisumu Nakuru; do
  echo "Branch: $TOWN"
done

# for over files
for f in *.jpg; do
  echo "Processing $f"
done

# numeric range
for i in {1..5}; do
  echo "Attempt $i"
done

# C-style
for ((i = 1; i <= 3; i++)); do
  echo "Round $i"
done

# while: read a file line by line
while IFS= read -r line; do
  echo "User: $line"
done < users.txt

# until: keep trying until success
until ping -c1 8.8.8.8 > /dev/null; do
  echo "Waiting for network..."
  sleep 5
done

break exits a loop; continue skips to the next round.

case: menus and choices

Terminal

read -p "Choose: [s]tart, [t]op, [r]estart: " CHOICE
case "$CHOICE" in
  s|start)   sudo systemctl start nginx ;;
  t|stop)    sudo systemctl stop nginx ;;
  r|restart) sudo systemctl restart nginx ;;
  *)         echo "Unknown option" ;;
esac

Functions

Terminal

log() {
  echo "$(date '+%F %T') $*" >> /var/log/myscript.log
}

backup_folder() {
  local SRC="$1"            # local keeps the variable inside the function
  local DEST="$2"
  tar -czf "$DEST/$(basename "$SRC")-$(date +%F).tar.gz" "$SRC" && log "Backed up $SRC"
}

backup_folder /var/www/site /home/deploy/backups

Writing safer scripts

Terminal

#!/bin/bash
set -euo pipefail

Option	Effect
`-e`	Stop on the first failing command
`-u`	Error on undefined variables (catches typos like `$DESTT`)
`-o pipefail`	A pipeline fails if any command in it fails

Other habits:

Quote variables: rm -r "$DIR", not rm -r $DIR. An empty or space-containing variable without quotes can delete the wrong things.
Check required variables: : "${BACKUP_DIR:?BACKUP_DIR not set}".
Run shellcheck (sudo apt install shellcheck, then shellcheck script.sh); it finds common bugs.
Never store passwords in scripts; read them from a protected file or environment variable.

Real script 1: website and database backup

Terminal

#!/bin/bash
set -euo pipefail
DATE=$(date +%F)
SITE=/var/www/site
DEST=/home/deploy/backups
mkdir -p "$DEST"

tar -czf "$DEST/site-$DATE.tar.gz" -C "$(dirname "$SITE")" "$(basename "$SITE")"
# credentials come from ~/.my.cnf (chmod 600), not from this script
mysqldump --single-transaction shopdb | gzip > "$DEST/db-$DATE.sql.gz"

find "$DEST" -type f -mtime +14 -delete      # keep 14 days
echo "Backup done: $DATE"

Copy backups off the server too (rsync to another machine or cloud storage): a backup on the same disk dies with the disk.

Real script 2: bulk rename photos

Terminal

#!/bin/bash
# rename IMG_1234.jpg -> wedding-001.jpg, wedding-002.jpg ...
n=1
for f in IMG_*.jpg; do
  new=$(printf "wedding-%03d.jpg" "$n")
  mv -i -- "$f" "$new"
  n=$((n + 1))
done

Real script 3: create accounts for a class

Terminal

#!/bin/bash
set -euo pipefail
# students.txt has one username per line
while IFS= read -r user; do
  [ -z "$user" ] && continue
  if id "$user" &>/dev/null; then
    echo "$user exists, skipping"
  else
    sudo useradd -m -s /bin/bash "$user"
    echo "$user:ChangeMe-$RANDOM" | sudo chpasswd
    sudo chage -d 0 "$user"          # force a password change at first login
    echo "Created $user"
  fi
done < students.txt

Real script 4: top visitors from a web log

Terminal

#!/bin/bash
LOG=${1:-/var/log/nginx/access.log}
echo "Top 10 IP addresses:"
awk '{print $1}' "$LOG" | sort | uniq -c | sort -rn | head -10
echo "Top 10 pages:"
awk '{print $7}' "$LOG" | sort | uniq -c | sort -rn | head -10

Scheduling with cron

Terminal

crontab -e

# min hour day month weekday  command
30 2 * * * /home/deploy/backup.sh >> /home/deploy/backup.log 2>&1
*/15 * * * * /home/deploy/disk-check.sh

The first runs the backup at 02:30 every day; the second checks disk space every 15 minutes. Use full paths in cron jobs, because cron runs with a minimal environment. The services/cron lesson covers cron in depth.

Think about it: Your script has rm -rf $BUILD_DIR/* and someone runs it where BUILD_DIR isn't set. What happens, and how do you prevent it?Show answer

The command becomes rm -rf /*, which tries to delete the whole system. Prevent it with set -u (error on unset variables), quoting, and a guard: rm -rf "${BUILD_DIR:?not set}"/*, which stops the script if the variable is empty.

Summary

Scripts start with #!/bin/bash, need chmod +x, and run with ./script.sh.
Variables (NAME=value), quotes, $(command) and $((math)) are the basics; $1, $#, "$@" read arguments.
Exit code 0 = success; &&/|| chain on success/failure.
if [ ... ], case, for, while, until and functions structure logic.
Write safe scripts: set -euo pipefail, quote variables, use shellcheck, keep secrets out; schedule with cron.

Check yourself

What is the first line #!/bin/bash called?
Show answer
shebang
Which exit code means success?
Show answer
0
Which variable holds the number of arguments?
Show answer
$#
Which test checks that a directory exists?
Show answer
-d
Which keyword keeps a variable inside a function?
Show answer
local
Which tool checks shell scripts for common bugs?
Show answer
shellcheck

Lesson 11 of 12 in Linux command line · Printable course notes