Bash Script Arguments Explained
Bash arguments are values you pass to a script when you run it. They let one script work with different files, services, directories or domains without editing the script each time.
What are Bash script arguments?
Arguments are extra values you add after the script name.
./check-file.sh /etc/passwdInside the script, /etc/passwd becomes $1, because it is the first argument.
#!/usr/bin/env bash
echo "First argument: $1"First argument: /etc/passwd$0, $1, $2, $#, $@ explained
| Variable | Meaning | Example |
|---|---|---|
$0 | The script name | ./show-args.sh |
$1 | The first argument | file.txt |
$2 | The second argument | nginx |
$# | The number of arguments | 2 |
$@ | All arguments | "$@" preserves each argument safely |
#!/usr/bin/env bash
echo "Script name: $0"
echo "First argument: $1"
echo "Second argument: $2"
echo "Number of arguments: $#"Script name: ./show-args.sh
First argument: apple
Second argument: banana
Number of arguments: 2Store arguments in named variables
$1 and $2 are useful, but named variables make scripts easier to read.
FILE="$1"
SERVICE="$2"
echo "File: $FILE"
echo "Service: $SERVICE"This is easier to understand later than using $1 and $2 everywhere.
Check if an argument was provided
Good scripts check their input before doing anything important.
FILE="${1:-}"
if [ -z "$FILE" ]; then
echo "Usage: $0 file"
exit 1
fi${1:-} means “use the first argument if it exists, otherwise use an empty value”. This avoids errors if you later use set -u.
Usage: ./check-file.sh fileLoop through all arguments with "$@"
Use quoted "$@" when you want to process every argument safely.
#!/usr/bin/env bash
for item in "$@"; do
echo "Argument: $item"
doneArgument: one
Argument: two
Argument: three words"$@", not plain $@. The quoted version preserves arguments with spaces.Handling arguments with spaces
If an argument contains spaces, quote it when running the script and quote it inside the script.
./check-file.sh "my file.txt"FILE="$1"
if [ -f "$FILE" ]; then
echo "File exists: $FILE"
fiWithout quotes, Bash may treat one filename as multiple separate words.
Example script: check if a file exists
#!/usr/bin/env bash
FILE="${1:-}"
if [ -z "$FILE" ]; then
echo "Usage: $0 file"
exit 1
fi
if [ -f "$FILE" ]; then
echo "[OK] File exists: $FILE"
exit 0
else
echo "[WARN] File not found: $FILE"
exit 1
fi[OK] File exists: /etc/passwdExample script: check a service name passed as an argument
#!/usr/bin/env bash
SERVICE="${1:-}"
if [ -z "$SERVICE" ]; then
echo "Usage: $0 service-name"
exit 1
fi
if systemctl is-active --quiet "$SERVICE"; then
echo "[OK] $SERVICE is running"
else
echo "[WARN] $SERVICE is not running"
exit 1
fi[OK] sshd is runningExample script: dry-run backup using two arguments
#!/usr/bin/env bash
SOURCE="${1:-}"
DEST="${2:-}"
if [ -z "$SOURCE" ] || [ -z "$DEST" ]; then
echo "Usage: $0 source-dir destination-dir"
exit 1
fi
if [ ! -d "$SOURCE" ]; then
echo "[ERROR] Source directory not found: $SOURCE"
exit 1
fi
mkdir -p "$DEST"
rsync -av --dry-run "$SOURCE"/ "$DEST"/sending incremental file list
index.html
styles.css
images/logo.png
sent 219 bytes received 27 bytes 492.00 bytes/sec
total size is 14,122 speedup is 57.41
Dry run complete. Remove --dry-run only after checking the output.For more backup ideas, see Automate Linux Backups Using rsync and systemd.
Download the example scripts
Common Bash argument mistakes
| Mistake | Problem | Better |
|---|---|---|
FILE=$1 | Can break with spaces or empty values. | FILE="${1:-}" |
for item in $@ | Splits arguments with spaces. | for item in "$@" |
| No usage check | The script fails unclearly when input is missing. | Print Usage: $0 file and exit. |
Using $1 everywhere | Harder to read and maintain. | Use named variables like FILE="$1". |
Keep learning Bash
A solid argument-checking pattern
Most admin scripts should validate arguments before doing any work. This avoids half-running a script with the wrong target.
#!/usr/bin/env bash
usage() {
echo "Usage: $0 DOMAIN LOGFILE"
echo "Example: $0 example.com /var/log/httpd/access_log"
}
if [ "$#" -ne 2 ]; then
usage
exit 1
fi
domain="$1"
logfile="$2"
if [ ! -f "$logfile" ]; then
echo "ERROR: logfile not found: $logfile"
exit 2
fi
echo "Checking $domain in $logfile"$ ./check-domain.sh
Usage: ./check-domain.sh DOMAIN LOGFILE
Example: ./check-domain.sh example.com /var/log/httpd/access_log
$ ./check-domain.sh example.com missing.log
ERROR: logfile not found: missing.logFrequently Asked Questions
What does $1 mean in Bash?
$1 is the first positional argument passed to the script or function.
What does $# mean in Bash?
$# is the number of positional arguments passed to the script or function.
What is the difference between $@ and $*?
$@ preserves each argument as a separate value when quoted, which is usually safer for loops and forwarding arguments.
How do I check if a Bash argument is missing?
Check the argument count with $# or test a specific variable with -z, for example [ -z "$1" ].