08-04 — Shell Scripting: Functions & Best Practices¶

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Functions¶

Functions let you group commands and reuse them. Define once, call many times.

#!/bin/bash

# Define a function
greet() {
    local NAME=$1     # $1 is the first argument passed to the function
    echo "Hello, $NAME!"
}

# Call the function
greet "Jahid"
greet "Alice"

Output:

Hello, Jahid!
Hello, Alice!

Structure: - functionname() { — defines the function - } — ends the function - The function does nothing until you call it by name - Arguments work the same way as script arguments: $1, $2, $#, etc. — but they refer to what was passed to the function, not the script

`local` Variables¶

local makes a variable private to the function — it only exists inside it and doesn't interfere with the rest of the script.

#!/bin/bash

GREETING="Good morning"    # global variable

say_hello() {
    local GREETING="Hi"    # local — only exists inside this function
    echo "$GREETING, $1!"
}

say_hello "Jahid"
echo "$GREETING"           # still "Good morning" — not affected by the function

Output:

Hi, Jahid!
Good morning

Always use local for variables inside functions. Without it, the function variable would overwrite the global one, which is rarely what you want.

Returning Values from Functions¶

return can only return a number (0–255), used as an exit code. To pass a string back from a function, echo it and capture it with $():

add() {
    echo $(( $1 + $2 ))
}

RESULT=$(add 5 3)
echo "5 + 3 = $RESULT"

Output: 5 + 3 = 8

Returning a status (success/failure)¶

is_even() {
    if [ $(( $1 % 2 )) -eq 0 ]; then
        return 0    # success = even
    else
        return 1    # failure = odd
    fi
}

if is_even 4; then
    echo "4 is even"
fi

if ! is_even 7; then
    echo "7 is odd"
fi

By convention: return 0 = success/true, return 1 (or any non-zero) = failure/false.

Exit Codes¶

Every command exits with a number called an exit code: - 0 = success - Any non-zero = failure (different numbers mean different errors)

You can check it with $?:

ls /nonexistent
echo "Exit code: $?"    # prints a non-zero number (usually 2)

ls /home
echo "Exit code: $?"    # prints 0 (success)

In scripts, you should check exit codes to detect failures:

cp important.txt /backup/
if [ $? -ne 0 ]; then
    echo "ERROR: Backup failed!"
    exit 1
fi
echo "Backup successful."

Set your own exit code with exit N at the end of a script. By convention, exit 0 = success, exit 1 = general error.

`set -euo pipefail` — Safe Script Mode¶

Add this near the top of every script:

set -euo pipefail

What each option does:

Option	What it does
`-e`	Exit the script immediately if any command fails (non-zero exit code)
`-u`	Treat unset variables as an error — catches typos like `$USERNME` instead of `$USERNAME`
`-o pipefail`	If any command in a pipeline (`cmd1 \\| cmd2`) fails, the whole pipeline is considered failed

Without -e, your script keeps running even after errors, which can cause serious problems (e.g., a failed backup command followed by deletion of the source).

Practical Example — Backup Script¶

Here's a complete, real-world script that puts everything together:

#!/bin/bash
# backup.sh — Back up a directory with a timestamp

set -euo pipefail

# ── Functions ─────────────────────────────────────────────────────────────────

log() {
    echo "[$(date +%H:%M:%S)] $1"
}

die() {
    echo "ERROR: $1" >&2
    exit 1
}

# ── Main ──────────────────────────────────────────────────────────────────────

SOURCE=${1:-"/home/jahid"}      # use argument, or default to /home/jahid
DEST=${2:-"/backup"}
TIMESTAMP=$(date +%Y%m%d_%H%M%S)
ARCHIVE="$DEST/backup_$TIMESTAMP.tar.gz"

[ -d "$SOURCE" ] || die "'$SOURCE' is not a directory."

mkdir -p "$DEST"

log "Backing up: $SOURCE"
log "Saving to:  $ARCHIVE"

tar -czf "$ARCHIVE" "$SOURCE"

log "Done! Archive size: $(du -sh "$ARCHIVE" | cut -f1)"

Run it:

chmod +x backup.sh
./backup.sh                           # backs up /home/jahid to /backup
./backup.sh /var/www /backups/web     # custom source and destination

What this script demonstrates: - set -euo pipefail — safe mode at the top - log() function for timestamped output - die() function that prints to stderr (>&2) and exits with code 1 - ${1:-"/home/jahid"} — default value if argument is missing - [ -d "$SOURCE" ] || die "..." — validate input before proceeding