Use when writing or modifying Bash/shell scripts. Covers script structure, variables, quoting, conditionals, and loops with modern best practices.
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name: shell-scripting-fundamentals description: Use when writing or modifying Bash/shell scripts. Covers script structure, variables, quoting, conditionals, and loops with modern best practices. allowed-tools:
Core patterns and best practices for writing robust, maintainable shell scripts.
Always start scripts with a proper shebang and safety options:
#!/usr/bin/env bash
set -euo pipefail
# Script description here
set -e: Exit on any command failureset -u: Error on undefined variablesset -o pipefail: Pipeline fails if any command fails# 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
| 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 |
# 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" "$output_dir/"
}
# Call with arguments
process_file "input.txt" "/tmp/output"
#!/usr/bin/env bash for portabilityset -euo pipefail[[ ]] instead of [ ] for tests$(command) instead of backtickscommand -v cmd >/dev/null