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version: "1.0.1" name: shell-scripting-fundamentals user-invocable: false description: Use when writing or modifying Bash/shell scripts. Covers script structure, variables, quoting, conditionals, and loops with modern best practices. allowed-tools:

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Shell Scripting Fundamentals

Core patterns and best practices for writing robust, maintainable shell scripts.

Script Structure

Always start scripts with a proper shebang and safety options:

bash

#!/usr/bin/env bash
set -euo pipefail
 
# Script description here

Safety Options Explained

set -e: Exit on any command failure
set -u: Error on undefined variables
set -o pipefail: Pipeline fails if any command fails

Variables

Declaration and Assignment

bash

# No spaces around =
name="value"
 
# readonly for constants
readonly CONFIG_DIR="/etc/myapp"
 
# local in functions
my_function() {
    local result="computed"
    echo "$result"
}

Always Quote Variables

bash

# Good - prevents word splitting and glob expansion
echo "$variable"
cp "$source" "$destination"
 
# Bad - can break on spaces or special characters
echo $variable
cp $source $destination

Default Values

bash

# Use default if unset
name="${NAME:-default}"
 
# Use default if unset or empty
name="${NAME:-}"
 
# Assign default if unset
: "${NAME:=default}"
 
# Error if unset
: "${REQUIRED_VAR:?Error: REQUIRED_VAR must be set}"

Conditionals

Test Syntax

bash

# Modern syntax - preferred
if [[ -f "$file" ]]; then
    echo "File exists"
fi
 
# String comparison
if [[ "$string" == "value" ]]; then
    echo "Match"
fi
 
# Numeric comparison
if (( count > 10 )); then
    echo "Greater than 10"
fi
 
# Regex matching
if [[ "$input" =~ ^[0-9]+$ ]]; then
    echo "Numeric input"
fi

Common Test Operators

Operator	Description
`-f`	File exists and is regular file
`-d`	Directory exists
`-e`	Path exists
`-r`	Readable
`-w`	Writable
`-x`	Executable
`-z`	String is empty
`-n`	String is not empty

Loops

For Loops

bash

# Iterate over list
for item in one two three; do
    echo "$item"
done
 
# Iterate over files (use glob, not ls)
for file in *.txt; do
    [[ -e "$file" ]] || continue  # Handle no matches
    process "$file"
done
 
# C-style for loop
for (( i = 0; i < 10; i++ )); do
    echo "$i"
done

While Loops

bash

# Read lines from file
while IFS= read -r line; do
    echo "$line"
done < "$filename"
 
# Read with process substitution
while IFS= read -r line; do
    echo "$line"
done < <(some_command)

Arrays

bash

# Declare array
declare -a files=()
 
# Add elements
files+=("file1.txt")
files+=("file2.txt")
 
# Iterate all elements
for file in "${files[@]}"; do
    echo "$file"
done
 
# Get array length
echo "${#files[@]}"
 
# Access by index
echo "${files[0]}"

Command Substitution

bash

# Modern syntax - preferred
result=$(command)
 
# Nested substitution
result=$(echo $(date))
 
# Avoid legacy backticks
result=`command`  # Don't use this

Functions

bash

# Function definition
process_file() {
    local file="$1"
    local output_dir="${2:-./output}"
 
    if [[ ! -f "$file" ]]; then
        echo "Error: File not found: $file" >&2
        return 1
    fi
 
    # Process the file
    cp "$file" "$output_dir/"
}
 
# Call with arguments
process_file "input.txt" "/tmp/output"

Best Practices Summary

Always use #!/usr/bin/env bash for portability
Enable strict mode: set -euo pipefail
Quote all variable expansions
Use [[ ]] instead of [ ] for tests
Use $(command) instead of backticks
Declare local variables in functions
Use arrays for lists of items
Check command existence before use: command -v cmd >/dev/null

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