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
Command Substitution
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]/}
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
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
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 "${modules[@]}"
… only if the command supports multiple arguments and you are not interested in catching individual failures.
read builtin
Use the bash read
builtin whenever possible to avoid forking external commands
Example
fqdn='computer1.daveeddy.com' IFS=. read -r hostname domain tld <<< "$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.
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.
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
- 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.
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.
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.
- 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.