thebushidocollective/han/shell-scripting-fundamentals

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

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

# 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

# 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}"

# 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

# 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

# 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)

# 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]}"

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

# 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"