Google Shell Style Guide

Google Shell Style Guide

Comprehensive guidance for writing shell scripts following Google's Shell Style Guide.

Core Principles

1. Consistency - Follow established patterns within each script
2. Safety - Quote variables, use [[ ]], check return values
3. Readability - 2-space indents, 80-char lines, clear naming

Quick Reference

Formatting Rules

Indentation

• Use 2 spaces (never tabs)
• Align case patterns with esac

Line Length

• Maximum 80 characters
• Break long pipelines before |
• Use backslash for line continuation

Control Structures

# if/then on same line
if [[ condition ]]; then
  command
elif [[ other ]]; then
  other_command
else
  fallback
fi

# while/do on same line
while [[ condition ]]; do
  command
done

# case alignment
case "${var}" in
  pattern1)
    command
    ;;
  pattern2)
    other
    ;;
esac

Variable Handling

Always Quote

# Correct
echo "${var}"
rm "${file}"

# Wrong - word splitting risk
echo $var
rm $file

Use Braces

# Correct - clear boundaries
echo "${prefix}_suffix"

# Wrong - ambiguous
echo "$prefix_suffix"

Arrays for Commands

# Correct - handles spaces safely
local -a args=(--flag "value with spaces")
command "${args[@]}"

# Wrong - breaks on spaces
local args="--flag value with spaces"
command $args

Function Guidelines

Naming and Declaration

# lowercase_with_underscores
process_file() {
  local input_file="$1"
  # ...
}

Local Variables

my_function() {
  local result=""
  local -r CONSTANT="value"  # readonly local
  # ...
}

Return Values

• Use return for status codes (0-255)
• Use echo or global variable for data
• Always check return values

if ! process_file "${input}"; then
  err "Processing failed"
  return 1
fi

Error Handling

Standard Error Function

err() {
  echo "[$(date +'%Y-%m-%dT%H:%M:%S%z')]: $*" >&2
}

Strict Mode

#!/bin/bash
set -euo pipefail

Cleanup with Traps

cleanup() {
  rm -f "${tmp_file:-}"
}
trap cleanup EXIT

Issue Severity Classification

When reviewing shell scripts, classify issues by severity:

Anti-Patterns to Avoid

1. Never use eval - Command injection risk
2. Never use aliases in scripts - Use functions instead
3. Avoid piping to while - Creates subshell, loses variable changes
4. Avoid mapfile/readarray - Not available on macOS

Additional Resources

For detailed patterns, edge cases, and comprehensive examples:

• references/detailed-guide.md - Complete style guide with all rules
• references/anti-patterns.md - Comprehensive anti-pattern catalog with fixes