Shell Style Guide

Shell Style Guide

Review and write shell scripts following the Google Shell Style Guide.

When Writing Shell Scripts

Use Bash Only

• Use #!/bin/bash with minimal flags
• Set shell options via set rather than shebang flags (e.g., set -o errexit, set -o nounset)
• Caution: (( )) evaluating to zero returns non-zero, which causes exit under set -e
• If a script exceeds ~100 lines or has complex control flow, recommend rewriting in Python or Go

File Conventions

• Executables: no extension (strongly preferred) or .sh
• Libraries (sourced only): .sh extension, not executable
• SUID/SGID: forbidden — use sudo instead

Error Output

Direct all error messages to STDERR:

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

Review Checklist

When reviewing shell code, check these categories in order:

1. Critical bugs — unquoted variables, missing error handling, unsafe eval
2. Formatting — 2-space indent, 80-char line limit, pipeline style
3. Naming — lowercase_with_underscores for functions/variables, UPPER_CASE for constants
4. Best practices — [[ ]] over [ ], $(cmd) over backticks, arrays over space-delimited strings
5. Structure — main function pattern, functions grouped at top, comments on non-obvious logic

For detailed rules and examples in each category, see references/style-rules.md.

Key Rules Summary

Formatting

• Indent: 2 spaces, no tabs
• Line length: max 80 characters
• Pipelines: one-liners stay on one line; multi-segment pipelines split with | on newline
• Loops/conditionals: ; then / ; do on same line as if / for / while

Quoting & Variables

• Always quote strings with variables, command substitutions, spaces, or metacharacters
• Prefer "${var}" over "$var" (exception: single-char specials like $?, $!, $@ don't need braces)
• Use "$@" over $*

Features

• [[ ... ]] over [ ... ] — avoids pathname expansion and word splitting
• $(command) over backticks — cleaner nesting
• (( ... )) for arithmetic — never let, $[ ], or expr
• Use arrays for lists — avoid space-delimited strings
• Avoid eval — use safer alternatives
• Use process substitution < <(cmd) or readarray instead of piping to while
• Use ShellCheck to catch common bugs

Naming

Structure

#!/bin/bash
# File header comment describing the script's purpose.

# Set shell options instead of using shebang flags.
# Note: set -e causes (( )) returning 0 to exit. Use with caution.

# Constants
readonly SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"

# Functions (grouped together, before main)
parse_args() { ... }
validate_input() { ... }

main() {
  parse_args "$@"
  validate_input
  # ...
}

main "$@"

Local Variables

• Declare with local in functions
• Separate declaration from assignment when capturing command output:

# Correct — exit code is checkable
local my_var
my_var="$(my_func)"

# Wrong — local always returns 0, masking errors
local my_var="$(my_func)"

Return Values & Pipelines

• Always check return values
• For pipelines, check PIPESTATUS immediately (it resets on next command):

tar -cf - ./* | gzip > archive.tar.gz
if (( PIPESTATUS[0] != 0 || PIPESTATUS[1] != 0 )); then
  err "Pipeline failed"
fi

Builtins Over External Commands

Prefer shell builtins for efficiency:

# Prefer
addition=$(( x + y ))
[[ "${input}" =~ ^[0-9]+$ ]]
"${str##*/}"   # basename

# Avoid
addition=$(expr "${x}" + "${y}")
echo "${input}" | grep -q '^[0-9]+$'
"$(basename "${str}")"