Bash Boilerplate
A collection of Bash starter scripts for easily creating safe and useful command line programs.
Scripts
bash-simple
A simple bash script with some basic strictness checks and help features. Useful for simple programs that don't have many features and don't take options other than help.
Notable Features
- Strict Mode,
- Help template, printable with
-h
or--help
.
bash-simple-plus
A simple bash script with some basic strictness checks, option parsing, help features, easy debug printing. Useful for regular scripts.
Notable Features
- Strict Mode,
- Help template, printable with
-h
or--help
, debug
printing with--debug
flag,die
command with error message printing and exiting,- Option parsing.
bash-commands
An example of a bash program with commands. This contains lots of features and should be usable for creating bash programs that do multiple related tasks.
Notable Features
- Strict Mode,
- Help template, printable with
-h
or--help
, debug
printing with--debug
flag,die
command with error message printing and exiting,- Option parsing,
- Automatic arbitrary command loading,
- An nice, clean pattern for specifying per-command help,
- Built-in commands for help, version, and command listing,
- Conventions for distinguishing between functions and program commands,
- Useful utility functions.
functions.bash
Shell function examples and boilerplate. The functions in this file are
intended to be included in the interactive shell, which can be done by
defining them in a shell init file like ~/.bashrc
.
helpers.bash
Helper functions. These functions are primarily intended to be used within scripts, but can be adapted for use as shell functions.
Notes
ShellCheck
Use it. It's super useful.
ShellCheck is a static analysis and linting tool for sh/bash scripts. It's mainly focused on handling typical beginner and intermediate level syntax errors and pitfalls where the shell just gives a cryptic error message or strange behavior, but it also reports on a few more advanced issues where corner cases can cause delayed failures.
Links
- http://www.shellcheck.net/
- http://www.shellcheck.net/about.html
- https://github.com/koalaman/shellcheck
It can be used with Vim via Syntastic, Emacs via Flycheck, Sublime Text 3 via SublimeLinter, and Atom via linter, atom-lint, or linter-shellcheck.
Bash "Strict Mode"
These boilerplate scripts use some common settings for enforcing strictness in Bash scripts, thereby preventing some errors.
For some additional background, see Aaron Maxwell's "Unofficial Bash Strict Mode" (redsymbol.net) post.
Simple 'Strict Mode' TL;DR
Add this to the top of every script (note: an extended version of this is already included in the boilerplate scripts):
# Bash 'Strict Mode'
# http://redsymbol.net/articles/unofficial-bash-strict-mode
# https://github.com/alphabetum/bash-boilerplate#bash-strict-mode
set -o nounset
set -o errexit
set -o pipefail
IFS=$'\n\t'
set -o nounset
/ set -u
Treat unset variables and parameters other than the special parameters @
or
*
as an error when performing parameter expansion. An 'unbound variable'
error message will be written to the standard error, and a non-interactive
shell will exit.
Parameter Expansion
This requires using parameter expansion to test for unset variables.
http://www.gnu.org/software/bash/manual/bashref.html#Shell-Parameter-Expansion
The two approaches that are probably the most appropriate are:
${parameter:-word}
If parameter is unset or null, the expansion of word is substituted.
Otherwise, the value of parameter is substituted. In other words, "word"
acts as a default value when the value of "$parameter" is blank. If "word"
is not present, then the default is blank (essentially an empty string).
${parameter:?word}
If parameter is null or unset, the expansion of word (or a message to that
effect if word is not present) is written to the standard error and the
shell, if it is not interactive, exits. Otherwise, the value of parameter
is substituted.
Parameter Expansion Examples
Arrays:
${some_array[@]:-} # blank default value
${some_array[*]:-} # blank default value
${some_array[0]:-} # blank default value
${some_array[0]:-default_value} # default value: the string 'default_value'
Positional variables:
${1:-alternative} # default value: the string 'alternative'
${2:-} # blank default value
With an error message:
${1:?'error message'} # exit with 'error message' if variable is unbound
Usage
Short form:
set -u
Long form:
set -o nounset
set -o errexit
/ set -e
Exit immediately if a pipeline returns non-zero.
This is super useful for avoiding scary errors, but there are some
things to watch out for. When using read -rd ''
with a heredoc, the
exit status is non-zero, even though there isn't an error, and this
setting then causes the script to exit. read -rd ''
is synonymous
to read -d $'\0'
, which means read until it finds a NUL byte, but
it reaches the end of the heredoc without finding one and exits with
a 1
status. Therefore, when reading from heredocs with set -e
,
there are three potential solutions:
Solution 1. set +e
/ set -e
again:
set +e
read -rd '' variable <<HEREDOC
Example text.
HEREDOC
set -e
Solution 2. <<HEREDOC || true
:
read -rd '' variable <<HEREDOC || true
Example text.
HEREDOC
Solution 3. Don't use set -e
or set -o errexit
at all.
More information:
'builtin "read -d" behaves differently after "set -e"' (lists.gnu.org)
Usage
Short form:
set -e
Long form:
set -o errexit
set -o pipefail
Return value of a pipeline is the value of the last (rightmost) command to exit with a non-zero status, or zero if all commands in the pipeline exit successfully.
Usage
Long form (no short form available):
set -o pipefail
$IFS
Set IFS to just newline and tab.
IFS=$'\n\t'
For some background, see Filenames and Pathnames in Shell: How to do it Correctly (dwheeler.com)
Misc Notes
Explicitness and clarity are generally preferable, especially since bash can be difficult to read. This leads to noisier, longer code, but should be easier to maintain. As a result, some general design preferences:
- Use leading underscores on internal variable and function names in order
to avoid name collisions. For unintentionally global variables defined
without
local
, such as those defined outside of a function or automatically through afor
loop, prefix with double underscores. - Always use braces when referencing variables, preferring
${NAME}
instead of$NAME
. Braces are only required for variable references in some cases, but the cognitive overhead involved in keeping track of which cases require braces can be reduced by simply always using them. - Prefer
printf
overecho
. For more information, see: http://unix.stackexchange.com/a/65819 - Prefer
$_explicit_variable_name
over names like$var
. - Use the
#!/usr/bin/env bash
shebang in order to run the preferred Bash version rather than hard-coding abash
executable path. - Prefer splitting statements across multiple lines rather than writing one-liners.
- Group related code into sections with large, easily scannable headers.
- Describe behavior in comments as much as possible, assuming the reader is a programmer familiar with the shell, but not necessarily experienced writing shell scripts.
Resources
- About ShellCheck (shellcheck.net)
- Bash Reference Manual (gnu.org)
- POSIX: Shell Command Language (pubs.opengroup.org)
- The Art of Command Line (github.com)
- "Unofficial Bash Strict Mode" (redsymbol.net)
- Filenames and Pathnames in Shell: How to do it Correctly (dwheeler.com)
- Understanding Exit Codes and how to use them in bash scripts (bencane.com)
- 'builtin "read -d" behaves differently after "set -e"' (lists.gnu.org)
- Bash Pitfalls (mywiki.wooledge.org)
- Rich’s sh (POSIX shell) tricks (etalabs.net)
- Writing Robust Bash Shell Scripts (davidpashley.com)
- Bash Strict Mode (github.com/tests-always-included/wick)
- pure bash bible - A collection of pure bash alternatives to external processes.
Related Projects
- oxyc/bash-boilerplate
- e36freak/templates
- connermcd/bash-boilerplate
- chrisopedia/bash-boilerplate
- ShaneKilkelly/manuel
- kvz/bash3boilerplate
Examples
Scripts based on this project.
Copyright (c) 2015 William Melody • hi@williammelody.com