Unix and Linux network configuration. Multiple network interfaces. Bridged NICs. High-availability network configurations.


Reviews of latest Unix and Linux software. Helpful tips for application support admins. Automating application support.


Disk partitioning, filesystems, directories, and files. Volume management, logical volumes, HA filesystems. Backups and disaster recovery.


Distributed server monitoring. Server performance and capacity planning. Monitoring applications, network status and user activity.

Commands & Shells

Cool Unix shell commands and options. Command-line tools and application. Things every Unix sysadmin needs to know.

Home » Commands & Shells, Featured

Using Variables in Bash

Submitted by on November 13, 2021 – 12:01 pm

A few quick tips on using variables in your Bash scripts. Nothing fancy here, just good practices that we often overlook.

Simple stuff

var2="twenty three"
echo "${var1},${var2}"

Assigning variables from user input

read -p "Enter username: " u
read -s -p "Enter password: " p

Reading variables that contain a backslash

read -r i <<<"$(echo "a\bc")"

Assigning multiple variables from command output

IFS='^' read var1 var2 var3 <<<"$(echo "value1^value2 with a space^value3")"

Assigning multiple variables from another variable

k="value1 value2 value3"
IFS=' ' read var1 var2 var3 <<<"${k}"

Assigning multiple variables from pipe

# Variables will be unset when subshell exists
echo 1 2 3 | { read a b c; echo $b; }


Checking Variables

If the variable not set, say something

if [ -z "${var}" ]; then echo "Uh-oh"; fi

Check if the variable is set and is not a space

i=" "
if [[ -z "${i// }" ]]; then echo "i is not set"; else echo "i is $i"; fi

If one or more variables not set, say something

" /root/movies
if [[ -z "${var1}" || -z "${var2}" || -z "${var3}" ]]; then echo "Uh-oh"; fi

If one or more variables not set, say something and do something

([ -z "${var1}" ] || [ -z "${var2}" ] || [ -z "${var3}" ]) && (echo "Uh-oh" && exit 1)

If variable not set, set it to some default value

var=${var:="default value"}

If a variable is not an integer, set it to zero

[[ "${var}" =~ ^[0-9]+$ ]] || var=0

If multiple variables are not set, assign them a default value

for i in var{1..3}; do
if [ -z "$(eval echo $(echo $`eval echo "${i}"`))" ] ; then
eval "$(echo ${i})"="$(echo "default_value")"
fi; done


Working with arrays of variables

If you really have lots and lots of variables that require different default values, you may use a table. Parse this table to populate two arrays: one containing names of the variables and the other – their default values. Let’s say your table.txt looks like this:

variable_two,some other default value
variable_whatever,nothing to see here

We will use it to build two arrays:

array_variables=($(awk -F, '{print $1}' table.txt))
array_defaults=($(awk -F, '{print $2}' table.txt))
unset IFS

Now we can read the array_variables array, check if the variables are set and, if not, assign a corresponding default value from the array_defaults array:

for ((i = 0; i < ${#array_variables[@]}; i++))
if [ -z "$(eval echo $(echo $`eval echo "${array_variables[$i]}"`))" ]
eval "$(echo "${array_variables[$i]}")"="$(echo "\"${array_defaults[$i]}\"")"

And to check that the variables have been set:

for i in $(printf '$%s\n' ${array_variables[@]}); do eval echo ${i}; done

some other default value
nothing to see here


Print Friendly, PDF & Email

Leave a Reply