Bash Scripting Tutorial: Variables, Loops, Functions, and Error Handling

Overview

Bash scripts automate repetitive tasks on Linux and macOS. This tutorial walks through the building blocks, from variables and conditionals to functions and safe error handling, with a complete example at the end.

Your First Script

#!/usr/bin/env bash
set -euo pipefail

echo "Hello, world!"

chmod +x hello.sh
./hello.sh

set -euo pipefail is a safety line worth putting at the top of every script: -e exits on error, -u treats unset variables as errors, and pipefail makes a pipeline fail if any stage fails.

Variables

name="Alice"
count=3

echo "$name has $count messages"
echo "${name}s"          # braces disambiguate the variable name

readonly MAX=100
export PATH="$HOME/bin:$PATH"

Do not put spaces around = when assigning.

Command Substitution and Arithmetic

today=$(date +%F)
files=$(ls -1 | wc -l)

sum=$((3 + 4))
((count++))

Arguments and Exit Codes

Variable	Meaning
`$0`	Script name
`$1`, `$2`...	Positional arguments
`$#`	Number of arguments
`$@`	All arguments as separate words
`$?`	Exit status of the last command
`$$`	Current process ID

if [[ $# -lt 2 ]]; then
  echo "Usage: $0 source destination" >&2
  exit 1
fi

Conditionals

if [[ -f "$file" ]]; then
  echo "File exists"
elif [[ -d "$file" ]]; then
  echo "It is a directory"
else
  echo "Not found"
fi

Test	Meaning
`-f`	Regular file exists
`-d`	Directory exists
`-z`	String is empty
`-n`	String is non-empty
`-eq`, `-ne`	Numeric equal / not equal
`==`, `!=`	String equal / not equal

Loops

# Iterate over a list
for fruit in apple banana cherry; do
  echo "$fruit"
done

# Iterate over files
for file in *.log; do
  gzip "$file"
done

# C-style loop
for ((i = 1; i <= 5; i++)); do
  echo "Line $i"
done

# While loop
while read -r line; do
  echo "$line"
done < input.txt

Functions

log() {
  local level="$1"
  shift
  echo "[$(date +%T)] [$level] $*" >&2
}

log INFO "Starting backup"
log ERROR "Disk full"

Use local to keep variables scoped to the function. Return values with echo and capture them via command substitution.

Error Handling

set -euo pipefail

cleanup() {
  rm -f "$tmpfile"
}
trap cleanup EXIT

tmpfile=$(mktemp)
echo "working..." > "$tmpfile"

The trap ... EXIT pattern guarantees cleanup even if the script fails.

Complete Example: Log Rotation

#!/usr/bin/env bash
set -euo pipefail

LOG_DIR="${1:-/var/log/myapp}"
KEEP_DAYS="${2:-7}"

if [[ ! -d "$LOG_DIR" ]]; then
  echo "Directory not found: $LOG_DIR" >&2
  exit 1
fi

archived=0
while IFS= read -r -d '' file; do
  gzip "$file"
  ((archived++))
done < <(find "$LOG_DIR" -type f -name "*.log" -mtime +"$KEEP_DAYS" -print0)

find "$LOG_DIR" -type f -name "*.gz" -mtime +$((KEEP_DAYS * 4)) -delete

echo "Archived $archived file(s), removed old archives."

Best Practices

Always start with #!/usr/bin/env bash and set -euo pipefail.
Quote variables: "$var", not $var.
Use [[ ]] instead of [ ] for conditionals.
Prefer $(...) over backticks.
Run scripts through ShellCheck before committing.