Shell Scripting

Hash shell supports POSIX-compatible shell scripting for automation and reusable scripts.

Running Scripts

Shebang Line

Create a script with the hash shebang:

#!/usr/local/bin/hash-shell

echo "Hello from hash!"

Make executable and run:

chmod +x script.sh
./script.sh

Direct Execution

hash-shell script.sh

Command String

hash-shell -c 'echo "Hello"; echo "World"'

From Standard Input

echo 'echo "Hello"' | hash-shell -s

Script Arguments

Positional Parameters

#!/usr/local/bin/hash-shell

echo "Script name: $0"
echo "First arg: $1"
echo "Second arg: $2"
echo "All args: $@"
echo "Arg count: $#"

Run:

./script.sh hello world
# Output:
# Script name: ./script.sh
# First arg: hello
# Second arg: world
# All args: hello world
# Arg count: 2

Special Variables

Functions

greet() {
    echo "Hello, $1!"
}

greet "World"

With return value:

is_even() {
    if [ $(($1 % 2)) -eq 0 ]; then
        return 0  # true
    else
        return 1  # false
    fi
}

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

Variables

Assignment

NAME="value"        # No spaces around =
export NAME="value" # Export to environment

Expansion

echo $NAME          # Simple
echo ${NAME}        # Braced
echo "${NAME}"      # Quoted (preserves whitespace)

Exit Codes

#!/usr/local/bin/hash-shell

if [ ! -f "$1" ]; then
    echo "Error: File not found" >&2
    exit 1
fi

# Process file...
exit 0

Common codes:

• 0 - Success
• 1 - General error
• 2 - Misuse of command

Comments

# This is a comment
echo "Hello"  # Inline comment

Best Practices

1. Always quote variables: "$var" prevents word splitting
2. Use meaningful names: FILE_COUNT not fc
3. Check for errors: Test return codes
4. Add comments: Explain complex logic
5. Use exit codes: Return 0 for success
6. Validate input: Check arguments first

Practical Example

#!/usr/local/bin/hash-shell

# Backup script
BACKUP_DIR="/backup"
DATE=$(date +%Y%m%d)

if [ ! -d "$BACKUP_DIR" ]; then
    mkdir -p "$BACKUP_DIR"
fi

for dir in /home/*; do
    if [ -d "$dir" ]; then
        user=$(basename "$dir")
        tar -czf "$BACKUP_DIR/${user}_${DATE}.tar.gz" "$dir"
        echo "Backed up: $user"
    fi
done

echo "Backup complete!"

See Also

• Variables & Expansion
• Control Structures
• Test Command