Variables and Loops

We are able to declare and use variables within the shell.

[user@host ~]$ var1='hello'
[user@host ~]$ echo $var1 world!
hello world!

When assigning a variables do not include spaces either side of the ‘=’ sign. Declared variables can be referenced using the ‘$’ sign and their name.

Quoting variables

[user@host ~]$ myname="John Smith"
[user@host ~]$ echo "Hello my name is $myname. Nice to meet you."
Hello my name is John Smith. Nice to meet you.
[user@host ~]$ echo 'Hello my name is $myname. Nice to meet you.'
Hello my name is $myname. Nice to meet you.

When using double quotes, bash will scan the contents and expand any variables.

Look at the example below:

[user@host ~]$ fruit=orange
[user@host ~]$ echo "I love eating $fruits."
I love eating .

because of the s, bash thinks the variable name is fruits.

In some cases we need to surround a variable name with braces.

[user@host ~]$ echo "I love eating ${fruit}s."
I love eating oranges.

Arithmetic with variables

[user@host ~]$ two=2
[user@host ~]$ result=$(( $two + 2 ))
[user@host ~]$ echo $result  
4
[user@host ~]$

Integer arithmetic can be done inside $(( ))

• + addition
• - subtraction
• / integer division
• * multiplication

Storing output of commands in variables

Run commands inside $( ) and assign the wrapped command to a variable.

[user@host ~]$ ls
a_directory  a_file
[user@host ~]$ dir_contents=$( ls )
[user@host ~]$ echo $dir_contents
a_directory a_file

Special environment variables

What is the output of this command?

[user@host ~]$ echo $PATH

Note the structure: <path1>:<path2>:<path3>

PATH is an environment variable which Bash uses to search for commands typed on the command line without a full path.

Use the command env to discover more environment variables.

Next activity: Reading - Finding Loops.