A Cheatsheet of the Bash Programming Language

DONG Yuxuan


When Bash read a command from a source file or a keyboard, it splits the command into tokens by blanks. For example, rm a b means call the rm program and pass 2 arguments a and b to it. This cause rm to delete two files a and b. If what we want is to delete one single file a b it will fail us. To achieve our goal we could use one of the following commands.

\, '...', and "..." are all escaping characters in Bash. Escaping means treat a meta character like blank as a normal one.

\ escapes the meta character follows it. "..." escapes most meta characters in ... except $ and \. '...' escapes all meta characters in ....

Bash has a large number of meta characters and you can find them and their meanings here.

The important ones here are the blank and newline. The blank separates arguments (tokens) and the newline ends a command.

Since \ can even escape the newline, we often use it to write a long command as multiple lines.

% rm a b c \
	d e f \
	g h i

Multiple Commands

If we want to execute more than one program in a single command. We can combine them with ;.

% echo deleting...; rm a

This is equivalent to the following.

% echo deleting...
% rm a

We could also introduce conditions here. If we want to delete file b only if we successfully remove the file a, we could write as the following.

% rm a && rm b

Like in other programming languages, && means the logical operator AND. A command is true if and only if the exit code is 0.

There are also other logical operators. || means OR and ! mens NOT. The short-circuit evaluation exits in Bash too. Thus deleting b if we failed on deleting a can be written as the following two froms.

% rm a || rm b
% ! rm a && rm b

Parentheses are used to control the priority as usual. However, when commands are placed in a pair of parentheses it not only means they have a higher priority but also means those commands should be executed in a subshell.

Here Documents/Strings

When use Bash as a programming language, we frequently want to control what goes to the standard input of a command. We could achieve it by the here documents.

cat << EOF
Hello, world.
I hate programming in Shell.
Not just Bash.

The syntax is command << END_TOKEN. As the above example shows, it takes the following content until the end token which in the example is EOF as the input for the command.

If you just want to send a simple content to the command a here document could be too heavy. We could use a here string to replace it.

% cat <<< "HELLO, WORLD"

A here string uses <<< to replace << and doesn’t need an end token.


After splits a command line into tokens Bash expands tokens with the following rules.

The above expansions are path expansions. If no files or directories are matched they will not be expanded. This behavior could be dangerous. Luckily we could run shopt -s nullglob at the beginning of the script to forbid it.

The following are non-path expansions.


Use varname=varval to create or overwrite a Shell variable. Use $varname or ${varname} to access a variable.

% name=Yuxuan
% echo $name

By default a Shell variable is not necessarily an environment variable. Thus it can’t be accessed by subprocesses.

Use export varname to export a Shell variable as an environment variable.

export name

Useful special variables are the following:

When reference a variable, $VAR and "$VAR" are different. If the variable includes blanks, $VAR will be expanded to multiple tokens but "$VAR" will be expanded to a single toke contains blanks.


Let’s give an example to return the basename without the extension name of a path.

% path=/usr/local/test.txt
% echo ${${path##*/}%.*}

Script Arguments

To handle complex arguments, we can use getopt. See getopt(1).


We can define an array using ARRAY=(val0 val1 ... valn) and reference its elements with ${ARRAY[0]}, ${ARRAY[1]}. To reference the whole array, use ${ARRAY[@]}.

ARRAY=(1 2 3 'hello world' 4 5)

# hello world
echo "${ARRAY[3]}"


# 9
echo ${ARRAY[0]}

As introduced above, $VAR and "$VAR" are different. ${ARRAY[@]} and "${ARRAY[@]}" are different in the same way.

# countargs prints the number of arguments it received
function countargs() {
	echo $#

ARRAY=(1 2 "hello world")

# 4
countargs ${ARRAY[@]}

# 3
countargs "${ARRAY[@]}"

Read from the Standard Input

read [-options] [variable...]

read reads from stdin and stores values into variables.

% read A B
hello world
% echo $A $B
hello world

Use -a to set read to read to an array.

% read -a names
DONG Yuxuan
% echo ${names[0]} ${names[1]}
DONG Yuxuan

The $IFS variable specify the delimiter read uses.


if commands; then
[elif commands; then

Just like common if-else in other languages. If commands exits with status 0, if thinks that the condtion is true.

The ; is used to help Shell to split. If you write in multiple lines it can be omitted.

if true
	echo yes

If you write all conditions in one line you need multiple ;.

if [ $UID = 501 ]; then echo YES; else echo NO; fi

[ ... ] is just a builtin command to test a condition expression. If the expression is true, it exits with status 0; Else it exits with a non-zero status code. Thus it’s very handy to use with if.

Be careful, there must be spaces after [ and before ].

[ ... ] is just the simplified form of test .... To see its detailed usage, run man test.

Besides [ ... ] and test, Bash supports [[ ... ]]. It’s the same as [ ... ] but regular expressions are supported.


if [[ $URL =~ ^http[s]?://.+\..+(/.*)*$ ]]
	echo It\'s a valid URL.
	echo It\'s an invalid URL.

Another frequently used command for building condition expressions is (( ... )). (( ... )) executes the arithmetic express .... If the result is 0 it returns non-zero exit code. Else it returns zero exit code.

if (( 100 > 50 ))
	echo YES

An assignment can be used in (( ... )).

if (( x = 100 + 2 ))
	echo $x
	echo zero

As we can see, there’s no $ while referencing variables in (( ... )).

Besides if Bash supports case.

case expression in
	pattern )
		commands ;;
	pattern )
		commands ;;

Just a simplified form of if ... then ... elif ... then ... elif ... then ... ... else ... fi.

case $1 in
	1 )
		echo Monday ;;
	2 )
		echo Tuesday ;;
	3 )
		echo Wednesday ;;
	4 )
		echo Thursday ;;
	5 )
		echo Friday ;;
	6 )
		echo Saturday ;;
	7 )
		echo Sunday ;;
	* )
		echo Other ;;


While Loops

while condition; do

Like normal while in common programming languages. If condition is true commands will be exexcuted. Repeat the procedure util condition becomes false;

Writing condition is the same as in if, nothing new.

; and line wrapping is the same as in if, nothing new.

Until Loops

until condition; do

until is the negtive of while.

Python-like For Loops

for variable in list

It’s the Python-like loop. list can either an array variable or literal list.

# print a, b, and c, one letter per line

for i in a b c
	echo $i
# print all C source files in the current directory

for i in *.c
	echo $i

C-like For Loops

for (( expression1; expression2; expression3 )); do

It’s equivalent to the following while loop.

(( expression1 ))
while (( expression2 )); do
	(( expression3 ))


There’re two ways to define a function.

fn() {
function fn() {

After a function is defined it can be used as a command. In the function arguments are also in $1, $2, ... like in usual command. The diffrence between functions and usual commands is that a funtion runs in the same process. Thus shell variables are available.

We use the local keyword to diffrentiate from function-local variables and shell-global variables.


fn() {
	local bar



# 3 2
echo $foo $bar

Using the return command we could exit a function. The value following return will be set as the exit code of the function command. As usual commands the exit code will be used to judge if the function is successfully executed.

fn() {
	return 5


# 5
echo $?