Bash Style Guide

This guide outlines how to write bash scripts with a style that makes them safe and predictable. This guide is written by Dave Eddy as part of the YSAP (You Suck at Programming) series ysap.sh and is the working document for how I approach bash scripting when it comes to style, design, and best-practices.

Preface

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.

Though good style alone won't ensure that your scripts are free from error, it can certainly help narrow the scope for bugs to exist. This guide attempts to explicitly state my style choices instead of implicitly relying on a sense or a "vibe" of how code should be written.

Aesthetics

Tabs / Spaces

Tabs.

Columns

Not to exceed 80.

Semicolons

Avoid using semicolons in scripts unless required in control statements (e.g., if, while).

# 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.

Functions

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
}

Block Statements

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

Spacing

No more than 2 consecutive newline characters (ie. no more than 1 blank line in a row).

Comments

No explicit style guide for comments. Don't change someones comments for aesthetic reasons unless you are rewriting or updating them.

---

Bashisms

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.

test(1)

Use [[ ... ]] for conditional testing, not [ .. ] or test ...

# wrong
test -d /etc

# also wrong
[ -d /etc ]

# correct
[[ -d /etc ]]

See BashFAQ031 for more information about these.

Sequences

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

• YSAP052
• YSAP053

Command Substitution

Use $(...) for command substitution.

foo=`date`  # wrong
foo=$(date) # right

• YSAP022

Math / Integer Manipulation

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.

Parameter Expansion

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]/}

• YSAP026
• YSAP056

Listing Files

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

• YSAP001

Determining path of the executable (__dirname)

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 BashFAQ028 for more information

For a case study on __dirname in multiple languages see my blog post

Dirname Case Study

Arrays and lists

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[@]}"

... only if the command supports multiple arguments and you are not interested in catching individual failures.

• YSAP020
• Arrays explained in 7 minutes

read builtin

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"

---

External Commands

GNU userland tools

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.

• YSAP029

Useless Use of Cat Award

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.

• UUOC

---

Style

Quoting

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

1. The only exception to this rule is if the code or bash controls the variable for the duration of its lifetime. For example code like this:

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; quote all expansions.

• YSAP021

Variable Declaration

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

shebang

Bash is not always located at /bin/bash, so use this line:

#!/usr/bin/env bash

Unless you’re intentionally targeting a specific environment (e.g. /bin/bash on Linux servers with restricted PATHs).

Error Checking

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

Using set -e

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 controversial 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.

• BashFAQ105

Using eval

Never.

It opens your code to code injection and makes static analysis impossible. Almost every use-case can be solved more safely with arrays, indirect expansion, or proper quoting.

---

Common Mistakes

Using {} instead of quotes.

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.

Abusing for-loops when while would work better

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.

1. This reads all usernames into memory, instead of processing them in a streaming fashion.
2. If the first field of that file contained spaces or tabs, the for loop would break on that as well as newlines.
3. This only works because $users is unquoted in the for 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.

• YSAP038

---

References

• YSAP
• BashGuide
• BashPitFalls
• Bash Practices

Get This Guide

• curl style.ysap.sh - View this guide in your terminal.
• curl style.ysap.sh/plain - View this guide without color in your terminal.
• curl style.ysap.sh/md - Get the raw markdown.
• Website - Dedicated website for this guide.
• GitHub - View the source.

License

MIT License