This style guide is meant to outline how to write bash scripts with a style that makes them safe and predictable. This guide is based on this wiki, specifically this page:
https://mywiki.wooledge.org/BashGuide/Practices
If anything is not mentioned explicitly in this guide, it defaults to matching whatever is outlined in the wiki.
Fork this style guide on GitHub https://github.com/bahamas10/bash-style-guide
I wrote this guide originally for a project I had worked on called Basher. The idea was to make a program like Puppet or Chef but using nothing but Bash - simple scripts that could do automation tasks instead of complex ruby scripts or whatever else is used by existing configuration management software.
Basher was fun to write, and for what it does it works pretty well. As part of writing it I also wrote this style guide to show 1. how I write bash and 2. how bash can be safe and predictable if written carefully.
This guide will try to be as objective as possible, providing reasoning for why
certain decisions were made. For choices that are purely aesthetic (and may
not be universally agreeable) they will exist in the Aesthetics
section
below.
tabs.
not to exceed 80.
You don't use semicolons on the command line (I hope), don't use them in scripts.
# wrong
name='dave';
echo "hello $name";
#right
name='dave'
echo "hello $name"
The exception to this rule is outlined in the Block Statements
section below.
Namely, semicolons should be used for control statements like if
or while
.
Don't use the function
keyword. All variables created in a function should
be made local.
# wrong
function foo {
i=foo # this is now global, wrong depending on intent
}
# right
foo() {
local i=foo # this is local, preferred
}
then
should be on the same line as if
, and do
should be on the same line
as while
.
# wrong
if true
then
...
fi
# also wrong, though admittedly looks kinda cool
true && {
...
}
# right
if true; then
...
fi
No more than 2 consecutive newline characters (ie. no more than 1 blank line in a row)
No explicit style guide for comments. Don't change someones comments for aesthetic reasons unless you are rewriting or updating them.
This style guide is for bash. This means when given the choice, always prefer
bash builtins or keywords instead of external commands or sh(1)
syntax.
Use [[ ... ]]
for conditional testing, not [ .. ]
or test ...
# wrong
test -d /etc
# also wrong
[ -d /etc ]
# correct
[[ -d /etc ]]
See https://mywiki.wooledge.org/BashFAQ/031 for more information
Use bash builtins for generating sequences
n=10
# wrong
for f in $(seq 1 5); do
...
done
# wrong
for f in $(seq 1 "$n"); do
...
done
# right
for f in {1..5}; do
...
done
# right
for ((i = 0; i < n; i++)); do
...
done
Use $(...)
for command substitution.
foo=`date` # wrong
foo=$(date) # right
Use ((...))
and $((...))
.
a=5
b=4
# wrong
if [[ $a -gt $b ]]; then
...
fi
# right
if ((a > b)); then
...
fi
Do not use the let
command.
Always prefer parameter
expansion
over external commands like echo
, sed
, awk
, etc.
name='bahamas10'
# wrong
prog=$(basename "$0")
nonumbers=$(echo "$name" | sed -e 's/[0-9]//g')
# right
prog=${0##*/}
nonumbers=${name//[0-9]/}
Do not parse ls(1), instead use bash builtin functions to loop files
# very wrong, potentially unsafe
for f in $(ls); do
...
done
# right
for f in *; do
...
done
Simply stated, you can't know this for sure. If you are trying to find out the full path of the executing program, you should rethink your software design.
See https://mywiki.wooledge.org/BashFAQ/028 for more information
For a case study on __dirname
in multiple languages see my blog post
https://daveeddy.com/2015/04/13/dirname-case-study-for-bash-and-node/
Use bash arrays instead of a string separated by spaces (or newlines, tabs, etc.) whenever possible
# wrong
modules='json httpserver jshint'
for module in $modules; do
npm install -g "$module"
done
# right
modules=(json httpserver jshint)
for module in "${modules[@]}"; do
npm install -g "$module"
done
Of course, in this example it may be better expressed as:
npm install -g "${modules[@]}"
... if the command supports multiple arguments, and you are not interested in catching individual failures.
Use the bash read
builtin whenever possible to avoid forking external
commands
Example
fqdn='computer1.daveeddy.com'
IFS=. read -r hostname domain tld <<< "$fqdn"
echo "$hostname is in $domain.$tld"
# => "computer1 is in daveeddy.com"
The whole world doesn't run on GNU or on Linux; avoid GNU specific options
when forking external commands like awk
, sed
, grep
, etc. to be as
portable as possible.
When writing bash and using all the powerful tools and builtins bash gives you, you'll find it rare that you need to fork external commands to do simple string manipulation.
Don't use cat(1)
when you don't need it. If programs support reading from
stdin, pass the data in using bash redirection.
# wrong
cat file | grep foo
# right
grep foo < file
# also right
grep foo file
Prefer using a command line tools builtin method of reading a file instead of passing in stdin. This is where we make the inference that, if a program says it can read a file passed by name, it's probably more performant to do that.
Use double quotes for strings that require variable expansion or command substitution interpolation, and single quotes for all others.
# right
foo='Hello World'
bar="You are $USER"
# wrong
foo="hello world"
# possibly wrong, depending on intent
bar='You are $USER'
All variables that will undergo word-splitting must be quoted (1). If no splitting will happen, the variable may remain unquoted.
foo='hello world'
if [[ -n $foo ]]; then # no quotes needed:
# [[ ... ]] won't word-split variable expansions
echo "$foo" # quotes needed
fi
bar=$foo # no quotes needed - variable assignment doesn't word-split
- The only exception to this rule is if the code or bash controls the variable for the duration of its lifetime. For instance, basher has code like:
printf_date_supported=false
if printf '%()T' &>/dev/null; then
printf_date_supported=true
fi
if $printf_date_supported; then
...
fi
Even though $printf_date_supported
undergoes word-splitting in the if
statement in that example, quotes are not used because the contents of that
variable are controlled explicitly by the programmer and not taken from a user
or command.
Also, variables like $$
, $?
, $#
, etc. don't required quotes because they
will never contain spaces, tabs, or newlines.
When in doubt however, quote all expansions.
Avoid uppercase variable names unless there's a good reason to use them.
Don't use let
or readonly
to create variables. declare
should only
be used for associative arrays. local
should always be used in functions.
# wrong
declare -i foo=5
let foo++
readonly bar='something'
FOOBAR=baz
# right
i=5
((i++))
bar='something'
foobar=baz
Bash is not always located at /bin/bash
, so use this line:
#!/usr/bin/env bash
Unless you have a reason to use something else.
cd
, for example, doesn't always work. Make sure to check for any possible
errors for cd
(or commands like it) and exit or break if they are present.
# wrong
cd /some/path # this could fail
rm file # if cd fails where am I? what am I deleting?
# right
cd /some/path || exit
rm file
Don't set errexit
. Like in C, sometimes you want an error, or you expect
something to fail, and that doesn't necessarily mean you want the program
to exit.
This is a contreversial opinion that I have on the surface, but the link below
will show situations where set -e
can do more harm than good because of its
implications.
https://mywiki.wooledge.org/BashFAQ/105
Never.
Using ${f}
is potentially different than "$f"
because of how word-splitting
is performed. For example.
for f in '1 space' '2 spaces' '3 spaces'; do
echo ${f}
done
yields
1 space
2 spaces
3 spaces
Notice that it loses the amount of spaces. This is due to the fact that the variable is expanded and undergoes word-splitting because it is unquoted. This loop results in the 3 following commands being executed:
echo 1 space
echo 2 spaces
echo 3 spaces
The extra spaces are effectively ignored here and only 2 arguments are passed
to the echo
command in all 3 invocations.
If the variable was quoted instead:
for f in '1 space' '2 spaces' '3 spaces'; do
echo "$f"
done
yields
1 space
2 spaces
3 spaces
The variable $f
is expanded but doesn't get split at all by bash, so it is
passed as a single string (with spaces) to the echo
command in all 3
invocations.
Note that, for the most part $f
is the same as ${f}
and "$f"
is the same
as "${f}"
. The curly braces should only be used to ensure the variable name
is expanded properly. For example:
$ echo "$HOME is $USERs home directory"
/home/dave is home directory
$ echo "$HOME is ${USER}s home directory"
/home/dave is daves home directory
The braces in this example were the difference of $USER
vs $USERs
being
expanded.
for
loops are great for iteration over arguments, or arrays. Newline
separated data is best left to a while read -r ...
loop.
users=$(awk -F: '{print $1}' /etc/passwd)
for user in $users; do
echo "user is $user"
done
This example reads the entire /etc/passwd
file to extract the usernames into
a variable separated by newlines. The for
loop is then used to iterate over
each entry.
This approach has a lot of issues if used on other files with data that may contain spaces or tabs.
- This reads all usernames into memory, instead of processing them in a streaming fashion.
- If the first field of that file contained spaces or tabs, the for loop would break on that as well as newlines
- This only works because
$users
is unquoted in thefor
loop - if variable expansion only works for your purposes while unquoted this is a good sign that something isn't implemented correctly.
To rewrite this:
while IFS=: read -r user _; do
echo "$user is user"
done < /etc/passwd
This will read the file in a streaming fashion, not pulling it all into memory,
and will break on colons extracting the first field and discarding (storing as
the variable _
) the rest - using nothing but bash builtin commands.
MIT License