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Linux tips: A Quick Introduction to Bash Programming (Part 1)

Posted In Linux - By Techtiplib on Tuesday, September 25th, 2012 With No Comments »

Bash programming is a topic that can be dealt with in a couple of pages or hundreds of pages. Harold Rodriguez explains Bash programming in this 2 Part tutorial. His slick and excellent style of writing has enabled him to cover all the essential features of bash programing in a few pages.

If you have never programmed in Bash before, this is the best place to begin. In case you have a little knowledge of bash, then too you could have a look.. a lot of interesting scripts have been explained by Harold. 

Introduction

Like all the shells available in Linux, the Bourne Again SHell is not only an excellent command line shell, but a scripting language in itself. Shell scripting, allows you to fully utilize the shell’s abilities and to automate a lot of tasks that would otherwise require a lot of commands to perform. A lot of programs lying around your Linux box are shell scripts. If you are interested in learning how they work, or in modifying them, it is essential that you understand the bash syntax and semantics. In addition, by understanding the bash language, you will be able to write your own programs to do things exactly the way you want them done.

Programming or Scripting?

People who are new to programming are generally confused as to what the difference is between a programming and scripting language. Programming languages are generally a lot more powerful and a lot faster than scripting languages. Examples of programming languages are C, C++, and Java. Programming languages generally start from source code (a text file containing instructions on how the final program is to be run) and are compiled (built) into an executable. This executable is not easily ported into different operating systems. For instance, if you were to write a C program in Linux, you would not be able to run that C program in a Windows 98 system. In order to do so, you would have to recompile the source code under the Windows 98 system. A scripting language also starts from source code, but is not compiled into an executable. Rather, an interpreter reads the instructions in the source file and executes each instruction. Unfortunately, because the interpreter has to read each instruction, interpreted programs are generally slower than compiled programs. The main advantage is that you can easily port the source file to any operating system and have it interpreted there right on the spot. bash is a scripting language. It is great for small programs, but if you are planning on doing major applications, a programming language might be more beneficial to you. Other examples of scripting languages are Perl, Lisp, and Tcl.

What do you need to know?

Writing your own shell scripts requires you to know the very basic everyday Linux commands. For example, you should know how to copy, move, create new files, etc. The one thing you must know how to do is to use a text editor. There are three major text editors in Linux, vi, emacs and pico. If you are not familiar with vi or emacs, go for pico or some other easy to use text editor.

Warning!!!!

Do not practice scripting as the root user! Anything could happen! I will not be held responsible if you accidentally make a mistake in the coding and screw up your system. You have been warned! Use a normal user account with no root privileges. You may even want to create a new user just for scripting practice. This way the worst thing that can happen is the user’s directory gets blown away.

Your first Bash program

Our first program will be the classical “Hello World” program. Yes, if you have programmed before, you must be sick of this by now. However, this is traditional, and who am I to change tradition? The “Hello World” program simply prints the words “Hello World” to the screen. So fire up your text editor, and type the following inside it:

#!/bin/bash
echo “Hello World”

The first line tells Linux to use the bash interpreter to run this script. In this case, bash is in the /bin directory. If bash is in a different directory on your system, make the appropriate changes to the line. Explicitly specifying the interpreter is very important, so be sure you do it as it tells Linux which interpreter to use to run the instructions in the script. The next thing to do is to save the script. We will call it hello.sh. With that done, you need to make the script executable

$ chmod 700 ./hello.sh

Refer to the manual page for chmod if you do not know how to change permissions for a file. Once this is done, you will be able to run your program just by typing its name:

$ ./hello.sh
Hello World

There it is! Your first program! Boring and useless as it is, this is how everyone starts out. Just remember the process here. Write the code, save the file, and make it executable with chmod. 

Commands, Commands and Commands

What exactly did your first program do? It printed the words “Hello World” to the screen. But how did it do that? It used commands. The only line of code you wrote in the program was echo “Hello World”. Well, which one is the command? echo. The echo program takes one argument and prints that argument to the screen.

An argument is anything that follows after you type the program name. In this case, “Hello World” was the argument that you passed to echo When you type the command ls /home/root, the argument to ls is /home/root. So what does this all mean? It means that if you have a program that takes an argument and prints it to the screen, you can use that instead of using echo. Let us assume that we have a program called foo This program will take a single argument, a string of words, and print them to the screen. We can rewrite our program as such

#!/bin/bash
foo “Hello World”

Save it and chmod it and then run it

$ ./hello
Hello World

The exact same result. Was there any unique code at all? No. Did you really write anything? Not unless you are the author of the echo program. All you did, was to embed the echo program into your shell program, with a argument already given. A real world example of an alternative to the echo command is the printf command. printf offers more control, especially if you are familiar with C programming. In fact, the exact same result could have been done without making a shell program

$ echo “Hello World”
Hello World

bash shell scripting offers a wide variety of control and is easy to learn. As you have just seen, you use Linux commands to write your shell programs with. Your shell program is a collection of other programs, specially put together to perform a task.

A More Useful Program

We will write a program that will move all files into a directory, and then delete the directory along with its contents, and then recreate the directory. This can be done with the following commands

$ mkdir trash
$ mv * trash
$ rm -rf trash
$ mkdir trash

Instead of having to type all that interactively on the shell, write a shell program instead

#!/bin/bash
mkdir trash
mv * trash
rm -rf trash
mkdir trash
echo “Deleted all files!”

Save it as clean.sh and now all you have to do is to run clean.sh and it moves all files to a directory, deletes them, recreates the directory, and even prints a message telling you that it successfully deleted all files. So remember, if you find that you are doing something that takes a while to type over and over again, consider automating it with a shell program.

Comments

Comments help to make your code more readable. They do not affect the output of your program. They are specially made for you to read. All comments in bash begin with the hash symbol: “#”, except for the first line (#!/bin/bash). The first line is not a comment. Any lines after the first line that begin with a “#” is a comment. Take the following piece of code

#!/bin/bash
# this program counts from 1 to 10:
for i in 1 2 3 4 5 6 7 8 9 10; do
    echo $i
done

Even if you do not know bash scripting, you immediately know what the above program does, because of the comment. It is good practice to make use of comments. You will find that if you need to maintain your programs in the future, having comments will make things easier.

Variables

Variables are basically “boxes” that hold values. You will want to create variables for many reasons. You will need it to hold user input, arguments, or numerical values. Take for instance the following piece of code

#!/bin/bash
x=12
echo “The value of variable x is $x”

What you have done here, is to give x the value of 12. The line echo “The value of variable x is $x” prints the current value of x. When you define a variable, it must not have any whitespace in between the assignment operator: “=”. Here is the syntax

variable_name=this_value

The values of variables can be accessed by prefixing the variable name with a dollar symbol: “$”. As in the above, we access the value of x by using echo $x.

There are two types of variables. Local variables, and environmental variables. Environmental variables are set by the system and can usually be found by using the env command. Environmental variables hold special values. For instance, if you type

$ echo $SHELL
/bin/bash

You get the name of the shell you are currently running. Environmental variables are defined in /etc/profile and ~/.bash_profile. The echo command is good for checking the current value of a variable, environmental, or local. 

Note : Setting of Environmental variables are explained in detail in Article: How to set Shell Environment Variables. The article also explains some features of the bash shell. 

If you are still having problems understanding why we need variables, here is a good example

#!/bin/bash
echo “The value of x is 12.”
echo “I have 12 pencils.”
echo “He told me that the value of x is 12.”
echo “I am 12 years old.”
echo “How come the value of x is 12?”

Okay, now suppose you decide that you want the value of x to be 8 instead of 12. What do you do? You have to change all the lines of code where it says that x is 12. But wait… there are other lines of code with the number 12. Should you change those too? No, because they are not associated with x. Confusing right? Now, here is the same example, only it is using variables

#!/bin/bash
x=12     # assign the value 12 to variable x
echo “The value of x is $x.”
echo “I have 12 pencils.”
echo “He told me that the value of x is $x.”
echo “I am 12 years old.” echo “How come the value of x is $x?”

Here, we see that $x will print the current value of variable x, which is 12. So now, if you wanted to change the value of x to 8, all you have to do, is to change the line x=12 to x=8, and the program will automatically change all the lines with $x to show 8, instead of 12. The other lines will be unaffected. Variables have other important uses as well, as you will see later on.

Control Structures

Control structures allow your program to make decisions and to make them more compact. More importantly as well, it allows us to check for errors. So far, all we have done is write programs that start from the top, and go all the way to the bottom until there are no more commands left in the program to run. For instance

#!/bin/bash
cp /etc/foo .
echo “Done.”

This little shell program, call it bar.sh, copies a file called /etc/foo into the current directory and prints “Done” to the screen. This program will work, under one condition. You must have a file called /etc/foo. Otherwise here is what happens

$ ./bar.sh
cp: /etc/foo: No such file or directory
Done.

So you can see, there is a problem. Not everyone who runs your program will have /etc/foo in their system. It would perhaps be better if your program checked if /etc/foo existed, and then if it did, it would proceed with the copying, otherwise, it would quit. In pseudo code, this is what it would look like

if /etc/code exists, then
    copy /etc/code to the current directory
    print “Done.” to the screen.
otherwise,
    print “This file does not exist.” to the screen
    exit

Can this be done in bash? Of course! The collection of bash control structures are, if, while, until, for and case. Each structure is paired, meaning it starts with a starting “tag” and ends with an ending “tag”. For instance, the if structure starts with if, and ends with fi. Control structures are not programs found in your system. They are a built in feature of bash. Meaning that from here on, you will be writing your own code, and not just embedding programs into your shell program.

if … else … elif … fi

One of the most common structures is the if structure. This allows your program to make decisions, like, “do this if this conditions exists, else, do something else”. To use the if structure effectively, we must make use of the test command. test checks for conditions, that is, existing files, permissions, or similarities and differences. Here is a rewrite on bar.sh

#!/bin/bash
if test -f /etc/foo
then
    # file exists, so copy and print a message.
    cp /etc/foo .
    echo “Done.”
else
    # file does NOT exist, so we print a message and exit.
    echo “This file does not exist.”
    exit
fi

Notice how we indent lines after then and else. Indenting is optional, but it makes reading the code much easier in a sense that we know which lines are executed under which condition. Now run the program. If you have /etc/foo, then it will copy the file, otherwise, it will print an error message. test checks to see if the file /etc/foo exists. The -f checks to see if the argument is a regular file. Here is a list of test’s options

Command Line Parameters for ‘ test ‘
-d
check if the file is a directory
-e
check if the file exists
-f
check if the file is a regular file
-g
check if the file has SGID permissions
-r
check if the file is readable
-s
check if the file’s size is not 0
-u
check if the file has SUID permissions
-w
check if the file is writetable
-x
check if the file is executable

else is used when you want your program to do something else if the first condition is not met. There is also the elif which can be used in place of another if within the if. Basically elif stands for “else if”. You use it when the first condition is not met, and you want to test another condition.

If you find that you are uncomfortable with the format of the if and test structure, that is

if test -f /etc/foo
then

then, you can do it like this

if [ -f /etc/foo ]; then

The square brackets form test. If you have experience in C programming, this syntax might be more comfortable. Notice that there has to be white space surrounding both square brackets. The semicolon: “;” tells the shell that this is the end of the command. Anything after the semicolon will be run as though it is on a separate line. This makes it easier to read basically, and is of course, optional. If you prefer, just put then on the next line.

When using variables with test, it is a good idea to have them surrounded with quotation marks. Example:

if [ “$name” -eq 5 ]; then 

the -eq operator will be explained later on in this article.

while … do … done

The while structure is a looping structure. Basically what it does is, “while this condition is true, do this until the condition is no longer true”. Let us look at an example

#!/bin/bash
while true; do
   echo “Press CTRL-C to quit.”
done

true is actually a program. What this program does is continuously loop over and over without stopping. Using true is considered to be slow because your shell program has to call it up first and then run it. You can use an alternative, the “:” command

#!/bin/bash
while :; do
   echo “Press CTRL-C to quit.”
done

This achieves the exact same thing, but is faster because it is a built in feature in bash. The only difference is you sacrifice readability for speed. Use whichever one you feel more comfortable with. Here is perhaps, a much more useful example, using variables

#!/bin/bash
x=0;     # initialize x to 0
while [ “$x” -le 10 ]; do
    echo “Current value of x: $x”
    # increment the value of x:
    x=$(expr $x + 1)
    sleep 1
done

As you can see, we are making use of the test (in its square bracket form) here to check the condition of the variable x. The option -le checks to see if x is less than, or equal to the value 10. In English, the code above says, “While x is less than 10 or equal to 10, print the current value of x, and then add 1 to the current value of x.”. sleep 1 is just to get the program to pause for one second. You can remove it if you want. As you can see, what we were doing here was testing for equality. Check if a variable equals a certain value, and if it does, act accordingly. Here is a list of equality tests

Checks equality between numbers
x -eq y
Check is x is equal to y
x -ne y
Check if x is not equal to y
x -gt y
Check if x is greater than y
x -lt y
Check if x is less than y

 

Checks equality between strings
x = y
Check if x is the same as y
x != y
Check if x is not the same as y
-n x
Evaluates to true if x is not null
-z x
Evaluates to true if x is null

The above looping script we wrote should not be hard to understand, except maybe for this line

x=$(expr $x + 1)

The comment above it tells us that it increments x by 1. But what does $(…) mean? Is it a variable? No. In fact, it is a way of telling the shell that you want to run the command expr $x + 1, and assign its result to x. Any command enclosed in $(…) will be run

#!/bin/bash
me=$(whoami)
echo “I am $me.”

Try it and you will understand what I mean. The above code could have been written as follows with equivalent results

#!/bin/bash
echo “I am $(whoami).”

You decide which one is easier for you to read. There is another way to run commands or to give variables the result of a command. This will be explained later on. For now, use $(…).

until … do … done

The until structure is very similar to the while structure. The only difference is that the condition is reversed. The while structure loops while the condition is true. The until structure loops until the condition is true. So basically it is “until this condition is true, do this”. Here is an example

#!/bin/bash
x=0
until [ “$x” -ge 10 ]; do
    echo “Current value of x: $x”
    x=$(expr $x + 1)
    sleep 1
done

This piece of code may look familiar. Try it out and see what it does. Basically, until will continue looping until x is either greater than, or equal to 10. When it reaches the value 10, the loop will stop. Therefore, the last value printed for x will be 9.

for … in … do … done

The for structure is used when you are looping through a range of variables. For instance, you can write up a small program that prints 10 dots each second

#!/bin/bash
echo -n “Checking system for errors”
for dots in 1 2 3 4 5 6 7 8 9 10; do
    echo -n “.”
done
echo “System clean.”

In case you do not know, the -n option to echo prevents a new line from automatically being added. Try it once with the -n option, and then once without to see what I mean. The variable dots loops through values 1 to 10, and prints a dot at each value. Try this example to see what I mean by the variable looping through the values

#!/bin/bash
for x in paper pencil pen; do
    echo “The value of variable x is: $x”
    sleep 1
done

When you run the program, you see that x will first hold the value paper, and then it will go to the next value, pencil, and then to the next value, pen. When it finds no more values, the loop ends.

Here is a much more useful example. The following program adds a .html extension to all files in the current directory

#!/bin/bash
for file in *; do
    echo “Adding .html extension to $file…”
    mv $file $file.html
    sleep 1
done

If you do not know, * is a wild card character. It means, “everything in the current directory”, which is in this case, all the files in the current directory. All files in the current directory are then given a .html extension. Recall that variable file will loop through all the values, in this case, the files in the current directory. mv is then used to rename the value of variable file with a .html extension.

case … in … esac

The case structure is very similar to the if structure. Basically it is great for times where there are a lot of conditions to be checked, and you do not want to have to use if over and over again. Take the following piece of code

#!/bin/bash
x=5     # initialize x to 5
# now check the value of x:
case $x in
   0) echo “Value of x is 0.”
      ;;
   5) echo “Value of x is 5.”
      ;;
   9) echo “Value of x is 9.”
      ;;
   *) echo “Unrecognized value.”
esac

The case structure will check the value of x against 3 possibilities. In this case, it will first check if x has the value of 0, and then check if the value is 5, and then check if the value is 9. Finally, if all the checks fail, it will produce a message, “Unrecognized value.”. Remember that “*” means “everything”, and in this case, “any other value other than what was specified”. If x holds any other value other than 0, 5, or 9, then this value falls into the *’s category. When using case, each condition must be ended with two semicolons. Why bother using case when you can use if? Here is the equivalent program, written with if. See which one is faster to write, and easier to read

#!/bin/bash
x=5     # initialize x to 5
if [ “$x” -eq 0 ]; then
    echo “Value of x is 0.”
elif [ “$x” -eq 5 ]; then
    echo “Value of x is 5.”
elif [ “$x” -eq 9 ]; then
    echo “Value of x is 9.”
else
    echo “Unrecognized value.”
fi

 

Quotation Marks

Quotation marks play a big part in shell scripting. There are three types of quotation marks. They are the double quote: “, the forward quote: ‘, and the back quote: `. Does each of them mean something? Yes.

Note : Article: Wildcards, Quotes, Back Quotes, Apostrophes in shell commands ( * ? [] ” ` ‘) deals with special characters in great detail. Please refer to that article in case you are not familiar with the use of these special characters in shell commands. Below is a quick explanation of some of those features

The double quote is used mainly to hold a string of words and preserve whitespace. For instance, “This string contains whitespace.”. A string enclosed in double quotes is treated as one argument. For instance, take the following examples

$ mkdir hello world
$ ls -F
hello/     world/

Here we created two directories. mkdir took the strings hello and world as two arguments, and thus created two directories. Now, what happens when you do this:

$ mkdir “hello world”
$ ls -F
hello/     hello world/     world/

It created a directory with two words. The quotation marks made two words, into one argument. Without the quotation marks, mkdir would think that hello was the first argument, and world, the second.

Forward quotes are used primarily to deal with variables. If a variable is enclosed in double quotes, its value will be evaluated. If it is enclosed in forward quotes, its value will not be evaluated. To make this clearer, try the following example

#!/bin/bash
x=5     # initialize x to 5
# use double quotes
echo “Using double quotes, the value of x is: $x”
# use forward quotes
echo ‘Using forward quotes, the value of x is: $x’

See the difference? You can use double quotes if you do not plan on using variables for the string you wish to enclose. In case you are wondering, yes, forward quotes can be used to preserve whitespace just like double quotes

$ mkdir ‘hello world’
$ ls -F
hello world/

Back quotes are completely different from double and forward quotes. They are not used to preserve whitespace. If you recall, earlier on, we used this line

x=$(expr $x + 1)

As you already know, the result of the command expr $x + 1 is assigned to variable x. The exact same result can be achieved with back quotes:

x=`expr $x + 1`

Which one should you use? Whichever one you prefer. You will find the back quote used more often than the $(…). However, I find $(…) easier to read, especially if you have something like this

$!/bin/bash
echo “I am `whoami`”

This was just the beginning. You will learn lots of more concepts in the concluding part of this article. Till then happy shell scripting..

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