Shell script template

This post is about proven practices in shell scripting with bash. Shell scripting is a powerful and one of the oldest tool for automating tasks, and bash is one of the most popular shells out there. The following practices can help you write more efficient, maintainable, and secure scripts. These are tips and tricks I gathered and assembled from various sources.

Proven practices

Use #!/usr/bin/env bash as shebang for better compatibility, it will search the default version of bash in the current environment, we don't have to define the absolute path to bash
Change to the script's directory, makes sourcing helpers and additional scripts easier cd "$(dirname "$0")"
Use if [[ ! (return 0 2> /dev/null); ]] then to check if the script was sourced, see: bash - How to detect if a script is being sourced - Stack Overflow
Use set -o errexit to exit on most errors, so bash exits rather than continuing the script execution
Use set -o nounset to disallow expansion of unset variables, this will make the script to fail when accessing an unset variable
Use "${VARNAME-} to access a variable that might have not been set
Use set -o pipefail to ensure that a pipeline command will fail even if one command in the pipeline fails
Use sudo -v to elevate privileges if needed, see check_priv() and elevate_priv() functions in the template
Use if [[ "${TRACE-0}" == "1" ]]; then set -o xtrace fi to enable debug mode, then start the script with TRACE=1 ./template.sh
Redirect error messages to to stderr, either use echo "Error" >&2 or the error "Error" function
Support long and and short options, --command and -c with [[ $cmd =~ ^-*c(ommand)?$ ]];

Helpers

Bash functions are a way to group a set of commands and statements together, which can be reused, they allow for code modularity, making it easier to maintain, update and test scripts.

A function is defined using the syntax function_name () { commands; }. Arguments can be passed to a function, and accessed within the function using the special variables $1, $2, and so on.

Some common uses for bash functions include defining helper functions for repetitive tasks, creating custom commands that combine existing commands, and modularizing complex scripts into smaller, more manageable functions.

# Use command with the -v option to check for the existence of an executable
has() {
	command -v "$1" 1>/dev/null 2>&1
}

Useful text formatting and the corresponding functions to display messages.

BOLD="$(tput bold 2>/dev/null || printf '')"
GREY="$(tput setaf 0 2>/dev/null || printf '')"
UNDERLINE="$(tput smul 2>/dev/null || printf '')"
RED="$(tput setaf 1 2>/dev/null || printf '')"
GREEN="$(tput setaf 2 2>/dev/null || printf '')"
YELLOW="$(tput setaf 3 2>/dev/null || printf '')"
BLUE="$(tput setaf 4 2>/dev/null || printf '')"
MAGENTA="$(tput setaf 5 2>/dev/null || printf '')"
NO_COLOR="$(tput sgr0 2>/dev/null || printf '')"

info() {
  printf '%s\n' "${BOLD}${GREY}>${NO_COLOR} $*"
}

warn() {
  printf '%s\n' "${YELLOW}! $*${NO_COLOR}"
}

error() {
  printf '%s\n' "${RED}x $*${NO_COLOR}" >&2
}

completed() {
  printf '%s\n' "${GREEN}✓${NO_COLOR} $*"

Check for elevated permission and elevate with sudo:

elevate_priv() {
  if ! has sudo; then
    error 'Could not find the command "sudo"'
    exit 1
    fi
    if ! sudo -v; then
      error "Superuser not granted, aborting installation"
      exit 1
    fi
}

check_priv() {
  if ! [ "$EUID" -ne 0 ]; then
    sudo=""
    warn "Running as root!"
  else
    warn "Escalated permissions are required to continue."
    elevate_priv
    sudo="sudo"
  fi
 }

Template

#!/usr/bin/env bash

if ! (return 0 2> /dev/null); then
    set -o errexit      # Immediately exit if any command has a non-zero exit status
    set -o nounset      # Causes the script to terminate when a variable is referenced that is not previously defined
    set -o pipefail     # instructs bash to use the return code of a failed command within a pipline as the return code of the whole pipeline
fi

# Enable debug mode, >TRACE=1 ./template.sh
if [[ "${TRACE-0}" == "1" ]]; then
    set -o xtrace
fi

BOLD="$(tput bold 2>/dev/null || printf '')"
GREY="$(tput setaf 0 2>/dev/null || printf '')"
UNDERLINE="$(tput smul 2>/dev/null || printf '')"
RED="$(tput setaf 1 2>/dev/null || printf '')"
GREEN="$(tput setaf 2 2>/dev/null || printf '')"
YELLOW="$(tput setaf 3 2>/dev/null || printf '')"
BLUE="$(tput setaf 4 2>/dev/null || printf '')"
MAGENTA="$(tput setaf 5 2>/dev/null || printf '')"
NO_COLOR="$(tput sgr0 2>/dev/null || printf '')"

info() {
  printf '%s\n' "${BOLD}${GREY}>${NO_COLOR} $*"
}

warn() {
  printf '%s\n' "${YELLOW}! $*${NO_COLOR}"
}

error() {
  printf '%s\n' "${RED}x $*${NO_COLOR}" >&2
}

completed() {
  printf '%s\n' "${GREEN}✓${NO_COLOR} $*"
}

has() {
  command -v "$1" 1>/dev/null 2>&1
}

elevate_priv() {
  if ! has sudo; then
    error 'Could not find the command "sudo"'
    exit 1
    fi
    if ! sudo -v; then
      error "Superuser not granted, aborting installation"
      exit 1
    fi
}

check_priv() {

  if ! [ "$EUID" -ne 0 ]; then
    sudo=""
    warn "Running as root!"
  else
    warn "Escalated permissions are required to continue."
    elevate_priv
    sudo="sudo"
  fi
}

usage() {
  echo -e "template.sh\\n\\tMinimal, functional bash script template\\n"
  echo "Usage:"
  echo "  -c, --command                           - command to execute"
  echo "  -h, --help                              - displays this help"
}

# Change the directory to the script's
cd "$(dirname "$0")"

main() {
  cmd="${1:-}"

  #Add default value to the first argument
  #cmd="${1:---command}"

  if [[ $cmd =~ ^-*h(elp)?$ ]]; then
    usage
  elif [[ $cmd =~ ^-*c(ommand)?$ ]]; then
    check_priv
    info "Command to execute"
    user="$(sudo whoami)"
    info "Running as $user"
  else
    usage
  fi
}

main "$@"