Shell-Scripting-CheatSheet

Table of Contents

• First Script
• Basic Printing
• Variables
• Loops
• Tests
• Special character usage: $, *, and etc.

First Script

Since there are various kinds of shells in a Unix system, we usually define the type of shell we want to use in our shell script before we actually get into the main content. In the first line of your script, type

#!./bin/sh

Usually the shell is located in the ./bin directory, the !# is followed by the path to the shell we wish to use. In this case, we are using the Bourne shell. We can also use bash, which is short for Bourne-Again shell, by typing

#!./bin/bash

After we have configured the shell to run our script, we can start writing our first shell script. To print something one the terminal, we use the command echo. In your script, type

echo Hello World

To run the shell script, simply type ./<name of the script> in your terminal. In my case, I named by shell script first_script.sh.

$ ./first_script.sh

In the terminal, you should see

$ ./first_script.sh
Hello World
$

You have now written your first shell script!

Basic Printing

As seen in the example above, the echo command is to print something on the terminal. It is essentially the same as print in Python (since Python is also a scripting language, it is obvious to make this analogy). The content following echo, or any other shell commands, are called arguments.

$ echo Hello World
Hello World
$ echo Hello     World
Hello World

Before the shell executes a line, it first parses the line. The first token in the line is the command, and the following tokens, seperated by whitespaces, are the arguments. In both examples above, echo is the command (print something), and the arguments are Hello and World. echo prints the arguments seperated by a space, hence the result.

To preserve the whitespaces between the two words, we enclose the string we wish to print with ".

$ echo "Hello     World"
Hello     World

echo now treats "Hello World" as a single argument.

Variables

Like any other programming language, we can also assign values to variables in shell scripts. Consider the following command

$ var=10

The value 10 is assigned to the variable var. Note that no spaces should appear around =. For the line

$ var = 10

The shell would treat var as a command and =, 10 as arguments. Apparently there are no shell commands named var, hence you would recieve an error message

$ var = 10
var: no command found

Printing the value of the variable is similar to printing a string, except we add a $ in front of the variable.

$ echo "The value of var is $var"
The value of var is 10

The command read assigns user input to a variable. Type the following in your shell script, then run the script on the terminal.

echo "What is your name?"
read USERNAME
echo "Hello $USERNAME

You should see the following

$ ./test.sh
What is your name? 

Enter your name and see

$ ./test.sh
What is your name? Brian
Hello Brian

Variable Scopes

Loops

Most shells support the while and for loop, just like any other programming language.

for loops

for loops itereate through a list of values. Try to run the following script on your terminal

for NUMBER in 1 2 3 4 5
do 
  echo "NUMBER is $NUMBER"
done

You should see

$ ./test.sh
NUMBER is 1
NUMBER is 2
NUMBER is 3
NUMBER is 4
NUMBER is 5

In each iteration, the variable (should be defined between for and in) will be assigned to one of the arguments in the argument list (located after in). It is recommended to indent the content between do and done for clarity.

while loops

The syntax of a basic while loop is as follows

while [ test ]
do
  ...
done

[test] is the condition that the loop checks every iteration. Tests will be introduced in the next chapter. For now, we try to implement an infinite loop, where the condition is replaced by a *, indicating true for every iteration.

while *
do 
  echo "This is an infinite loop!"
done

Tests [ ]

Conditional statements are essential elements in any programming language. In shell scripts, conditional statements are enclosed by two middle brackets [ ]; for example:

var=100 # var is assigned to 100
if [ var = 100 ] # checks if var equals to 100
then 
  echo "var is 100"
else
  echo "var is not 100"
fi

Note that in test statements, brackets must be surrounded by spaces. That is,

if' '[' 'var' '=' '100' ']

is the only correct format for test statements. Note that = here refers to equality, and needs to be surrounded by spaces too, which is different from assignment.