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Main Page

Variable

You can use variables to store data and configuration options. There are two types of variable as follows

  • System Variables
  • User Defined Variables

System Variables

Created and maintained by Linux bash shell itself. This type of variable (with the exception of auto_resume and histchars) is defined in CAPITAL LETTERS. You can configure aspects of the shell by modifying system variables such as PS1, PATH, LANG,HISTSIZE,and DISPLAY etc.

To see all system variables, type the following command at a console / terminal:

set
env
printenv

Commonly Used Shell Variables

System Variable Meaning To View Variable Value Type
BASH_VERSION Holds the version of this instance of bash. echo $BASH_VERSION
HOSTNAME The name of the your computer. echo $HOSTNAME
CDPATH The search path for the cd command. echo $CDPATH
HISTFILE The name of the file in which command history is saved. echo $HISTFILE
HISTFILESIZE The maximum number of lines contained in the history file. echo $HISTFILESIZE
HISTSIZE The number of commands to remember in the command history. The default value is 500. echo $HISTSIZE
HOME The home directory of the current user. echo $HOME
IFS The Internal Field Separator that is used for word splitting after expansion and to split lines into words with the read builtin command. The default value is echo $IFS
LANG Used to determine the locale category for any category not specifically selected with a variable starting with LC_. echo $LANG
PATH The search path for commands. It is a colon-separated list of directories in which the shell looks for commands. echo $PATH
PS1 Your prompt settings. echo $PS1
TMOUT The default timeout for the read builtin command. Also in an interactive shell, the value is interpreted as the number of seconds to wait for input after issuing the command. If not input provided it will logout user. echo $TMOUT
TERM Your login terminal type. echo $TERM ; export TERM=vt100
SHELL Set path to login shell. echo $SHELL
DISPLAY Set X display name echo $DISPLAY; export DISPLAY=:0.1
EDITOR Set name of default text editor. export EDITOR=/usr/bin/vim

How Do I Display The Value Of a Variable

Use echo command to display variable value. To display the program search path

echo $PATH
# Or
echo ${PATH}

User Defined Variables

Created and maintained by user. This type of variable defined may use any valid variable name, but it is good practice to avoid all uppercase names as many are used by the shell.

Creating and setting variables within a script is fairly simple. Use the following syntax

varName=someValue

Store current date (you can store the output of date by running the shell command):

now=$(date)

Default Shell Variables Value

You can set the default shell variable value using the following syntax.

${var:=defaultValue}
# Or
${var:-defaultValue}

Example

die(){
  local error=${1:-Undefined error}
  echo "$0: $error" 
}
die "File not found"
die

Rules For Variable Naming

  • Variable name must begin with alphanumeric character or underscore character (_), followed by one or more alphanumeric or underscore characters.
  • Do not use ?,* and other special characters, to name your variable.
  • Variables names are case-sensitive
  • Variables defined a NULL variable as follows (NULL variable is variable which has no value at the time of definition)

Valid shell variable examples:

HOME
SYSTEM_VERSION
vech
no

Do not put spaces on either side of the equal sign when assigning value to variable. For example, the following is valid variable declaration:

var=10

However, any of the following variable declaration will result into an error such as command not found:

var  =10
var=  10
var  =  10

Not initialized or NULL variable

var=
var=''

Export Variables

The export builtin automatically exports to the environment of child processes. By default all user defined variables are local. They are not exported to new processes. Use export command to export variables and functions to child processes. If no variable names or function names are given, or if the -p option is given, a list of all names that are exported in this shell is printed. An argument of -n says to remove the export property from subsequent NAMEs

export backup="/nas10/mysql"
echo "Backup dir $backup"

Unset Variable

Use unset command to delete the variables during program execution. It can remove both functions and shell variables.

var=/etc/profile
echo $var
unset var
echo $var

Arithmetic Expansion

$((expression))
$(( n1+n2 ))
$(( n1/n2 ))
$(( n1-n2 ))
Operator Description Example Evaluates To
+ Addition $(( 20 + 5 ))
- Substraction $(( 20 - 5 ))
/ Divison $(( x / 2 ))
% Modules $(( x % 10 ))
* Multipilication $(( x * y ))
++ post-increment $(( x++ ))
-- post-decrement $(( x-- ))
** Exponentiation $(( x ** y ))

Create Integer Variable

To create an integer variable use the declare command as follows

declare -i x=10
declare -i y=10
declare -i z=0
z=$(( x + y ))
echo "$x + $y = $z"

Create The Constants Variable

You can create the constants variables using the readonly command or declare command Syntax

readonly var
readonly varName=value

declare -r var
declare -r varName=value

Example

readonly DATA=/home/sales/data/feb09.dat

Bash Variable Existence Check

If the variable is not defined, you can stop executing the Bash script with the following syntax

${varName?Error varName is not defined}
${varName:?Error varName is not defined or is empty}

Example

#!/bin/bash
# varcheck.sh: Variable sanity check with :? 
path=${1:?Error command line argument not passed}
echo "Backup path is $path."
echo "I'm done if \$path is set."

Array

Declaration Array

We can explicitly create an array by using the declare command

declare -a my_array

We can create indexed arrays with a more concise syntax, by simply assign them some values

my_array=(foo bar)

In this case we assigned multiple items at once to the array, but we can also insert one value at a time, specifying its index

my_array[0]=foo

Aceess Array Item

${array[$i])

Print The Values Of An Array

echo ${my_array[@]}
# Or
echo ${my_array[*]}
# Or
for i in "${my_array[@]}"; do echo "$i"; done
# Or
for i in ${#my_array[@]}; do echo ${array[$i]); done

When using *, and the variable is quoted, instead, a single “result” will be produced, containing all the elements of the array

Print The Keys Of A Bash Array

It’s even possible to retrieve and print the keys used in an indexed or associative array, instead of their respective values. The syntax is almost identical, but relies on the use of the ! operator

my_array=(foo bar baz)
for index in "${!my_array[@]}"; do echo "$index"; done

Associative Arrays

${!variable[@]}

Example

declare -A my_array
my_array=([foo]=bar [baz]=foobar)
for key in "${!my_array[@]}"; do echo "$key"; done

Getting The Size Of An Bash Array

${#variable[@]}

Example

my_array=(foo bar baz)
echo "the array size : ${#my_array[@]}"
$ the array size : 3

Adding Elements To An Bash Array

variable+=(value)

Example

my_array=(foo bar)
my_array+=(baz)

# Or

my_array=(foo bar)
my_array+=(baz foobar)

To add elements to an associative array, we are bound to specify also their associated keys:

declare -A my_array
my_array[foo]="bar"
my_array+=([baz]=foobar [foobarbaz]=baz)

Deleting An Element From The Bash Array

To delete an element from the array we need to know it’s index or its key in the case of an associative array, and use the unset command

Example

my_array=(foo bar baz)
unset my_array[1]
echo ${my_array[@]}
foo baz

The same thing it’s valid for associative arrays:

declare -A my_array
my_array+=([foo]=bar [baz]=foobar)
unset my_array[foo]
echo ${my_array[@]}
foobar