Define a Variable

To define a variable, simply use variable-name=value without and space.

$ a=5 # set a to 5
$ b = 5 # Opps, error
$ c=3+2 # Opps, does not work, 
        # which will be explained soon.

If you are interesting in the value stored in c in the third case above, you can try echo

$ echo $c
3+2

Strangely enough, it would output 3+2 instead of 5, far from our ideal behaviour.
The correct command to assign 5 to c would be discussed soon. As shown above, $ is uesd to refer to a variable.

$ echo a
a
$ echo $a
5
$ vim ${a}.cpp # equivelant to vim 5.cpp
               # {} can be ignored

Tree Kinds of Quote

Single Quote

A single quote will make anything inside it a plain text.

$ echo 'I am $USER, and today is `date`'
I am $USER, and today is `date`

Nothing will be translated.

Double Quote

Similar to single quote, double quote makes it a string. However, double quote allow $ to access to the variable and back quote to execute a command.

$ echo "I am $USER, and today is `date`"
I am yb, and today is 2017年 12月 01日 星期五 18:30:17 CST

Back Quote

Whatever inside the back quote(not a single quote!) would be treated as a command to run.

$ expr 4 + 2
6
$ a=`expr 4 + 2`
$ echo $a
6

run a shell

put your bar.sh to the path below
~/.local/bin and your script can be executed globally by current user.