Coding Shell Scripts

Coding Shell Scripts

Overview

This skill provides templates and patterns for writing bash scripts. Choose the appropriate template based on your needs:

• Simple Script Structure: For straightforward utilities and single-purpose tools
• Full Script Template: For complex scripts needing logging, cleanup handlers, and robust error handling

Simple Script Structure

For straightforward scripts (utilities, single-purpose tools):

#!/usr/bin/env bash

main() {
  # Script logic here
  local arg="$1"

  if [ -z "$arg" ]; then
    echo "Usage: $(basename "$0") <argument>" >&2
    exit 1
  fi

  # Do the work
  echo "Processing: $arg"
}

main "$@"

Important: Standalone scripts MUST use exit (not return) for error codes, since they run in their own process.

Full Script Template

For more complex scripts with logging, cleanup, and robust error handling:

#!/usr/bin/env bash
set -euo pipefail

IFS=$'\n\t'

# Normalize TMPDIR (strip trailing slash for consistent path construction)
TMPDIR="${TMPDIR:-/tmp}"
TMPDIR="${TMPDIR%/}"

DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" >/dev/null 2>&1 && pwd )"

#/ Usage:
#/ Description:
#/ Examples:
#/ Options:
#/   --help: Display this help message
usage() { grep '^#/' "$0" | cut -c4- ; exit 0 ; }
expr "$*" : ".*--help" > /dev/null && usage

readonly LOG_FILE="${TMPDIR}/$(basename "$0").log"
info()    { echo "[INFO]    $@" | tee -a "$LOG_FILE" >&2 ; }
warning() { echo "[WARNING] $@" | tee -a "$LOG_FILE" >&2 ; }
error()   { echo "[ERROR]   $@" | tee -a "$LOG_FILE" >&2 ; }
fatal()   { echo "[FATAL]   $@" | tee -a "$LOG_FILE" >&2 ; exit 1 ; }

cleanup() {
  # Remove temporary files
  # Restart services
  # ...
}

if [[ "${BASH_SOURCE[0]}" = "$0" ]]; then
  trap cleanup EXIT
  # Script goes here
  # ...
fi

Script Validation

CRITICAL: Always validate shell scripts after creating or editing them.

Required Validation Steps

After creating or editing any shell script, you MUST run both validation tools:

1. Syntax Check with bash -n

   bash -n script.sh
   

   Catches basic syntax errors:

   • Missing closing quotes
   • Unmatched brackets or braces
   • Invalid command syntax

2. Static Analysis with shellcheck

   shellcheck script.sh
   

   Catches deeper issues:

   • Quoting problems (word splitting, glob expansion)
   • Deprecated or unsafe syntax
   • Common bugs and anti-patterns
   • Unused or misspelled variables
   • Portability issues

Validation Pattern

Run both checks together:

bash -n script.sh && shellcheck script.sh

Only consider the script complete after both tools pass without errors.

Handling Shellcheck Warnings

When shellcheck warnings are intentional, disable them with directives:

# Disable specific warning for one line
# shellcheck disable=SC2086
echo $unquoted_on_purpose

# Disable multiple codes for entire file (at top)
# shellcheck disable=SC2086,SC2181

# Document why you're disabling
# We want word splitting here for argument passing
# shellcheck disable=SC2086
command $args

Common codes you might need to disable:

• SC2086: Word splitting/globbing (when intentional for argument passing)
• SC2181: Testing $? explicitly (when needed for clarity)
• SC1090: Dynamic source paths (shellcheck can't follow them)
• SC2034: Unused variables (e.g., when reading into multiple vars)

Important: Only disable shellcheck warnings when you understand why they're triggered and have a valid reason to ignore them. Document your reasoning in comments.

Bash Constructs

for

for i in ( ls some-directory ); do
  echo $i
done

SomeArray=("foo"  "bar"  "baz")
for item in ${SomeArray[*]}; do
  echo $item
done

case

case "$variable" in
  abc)  echo "\$variable = abc" ;;
  xyz)  echo "\$variable = xyz" ;;
esac

ln

ln -s <real_file> <future_link>

select

select result in Yes No Cancel
do
  echo $result
done

tput

tput cuu1 # Move cursor up by one line
tput el   # Clear the line

Common Patterns

Ternary expressions

<expression> && <on-true expression> || <on-false expression>
# See below example for file/stdin to variable.

File or stdin assignment

Assign either a file or stdin (piped input) to a variable, with fallbacks:

# Use a filepath from args when available, or use stdin
[ $# -ge 1 -a -f "$1" ] && input="$1" || input="-"
content=$(cat $input)

# Use stdin if pipe is occupied, otherwise use a file
(test -s /dev/stdin) && input="-" || input="./config.json"
content=(cat $input)

Arithmetic Operations

Use (()) for arithmetic:

echo $(( 5 - 1 ))

CLI Arguments

More involved: https://medium.com/@Drew_Stokes/bash-argument-parsing-54f3b81a6a8f

# Simple double dash arg
arg_no_bump="false"
expr "$*" : ".*--no-bump" > /dev/null && arg_no_bump="true"

Reference Links

http://tldp.org/LDP/abs/html/index.html
https://stackoverflow.com/documentation/bash/topics
https://dev.to/thiht/shell-scripts-matter