LINUX BASH SHELL SCRIPT BASICS Part 2:

STDIN, STDOUT, STDERR AND REDIRECTION

Redirection in Linux is when executing a command you can change the standard input/output devices. The basic workflow of any Linux command is that it takes an input and gives an output.

* The standard input (stdin) device is the keyboard.
* The standard output (stdout) device is the screen.
* The standard error (stderr) is for error messages.

With redirection, the above standard input/output can be changed.

You can use pipes to take the input from one program and send it to another:

ls -l really-big-directory | less

You can send output to files:

ls -l really-gig-directory > ls-output.txt

You can throw output away:

ls -l really-big-directory > /dev/null 2>&1

You can send output to STDERR:

echo “Script Error: This is an error message!” >&2

VARIABLES

A variable is a character string to which we assign a value. The value assigned could be a number, text, filename, device, or any other type of data. A variable is nothing more than a pointer to the actual data. The shell enables you to create, assign, and delete variables. You can list shell variables with the env command.

Creating variables:

VAR1=data

Reading variables:

echo $VAR1
data

Removing variables:

unset VAR1

ARGUMENTS

Any input a user adds after a command is placed in special variables called arguments. Each argument is defined by a space between them. If we had a mythical command called twiddle, and we launched it with a command like this:

twiddle file1 file2 file3

What we typed would be put into these shell variables:

$0 = twiddle
$1 = file1
$2 = file2
$3 = file3

If you would type:

twiddle –help

$0 = twiddle
$1 = –help

A special shell variable called $@ represents all of the arguments in numerical order. This can be used by a script to loop through several arguments that contain information like file names.

SIMPLE TESTS

The test command looks for some string of data and returns an error in its exit code if it cannot find it. Using tests is how you can make a clever script that adapts to changes and catches errors.

test -f “.bashrc” && echo “.bashrc is here.” || echo “.bashrc is not here.”

dpkg -l | grep -qw ffmpeg || sudo apt-get install -yyq ffmpeg

IF STATEMENTS

You can use the if command to test for files and directories, the exit status of other commands and data you put into variables.

if [ test fir something ]; then
run commands if the answer is yes
else
run commands if the answer is no
fi

Using if to test for errros:

The exit code of commands that run in bash are placed in a variable called $?. Usually, a 0 means everything went alright, any other number indicates an error.

which ifconfig >/dev/null
if [ “$?” != “0” ]; then
echo “SyncAll Error: The ifconfig utility is not installed!” >&2
echo “Ubuntu and Linux Mint: Run ‘sudo apt install net-tools'” >&2
exit 1
fi

The exit 1 command will exit the running script with an error code of 1.

ADVANCED SCRIPTING WITH FUNCTIONS

So far, we have been talking about scripts that run one command after another. There is another way to write scripts that makes them more like real computer programs. Functions are sets of commands that get loaded into memory when the script is started and can be called on at any time and in any order as long as the script is running. They are most often used with options entered by the user and/or menus.

A scrips with functions looks like this:

#!/bin/bash

# This is an example script using functions.

# Declare Variables:

a=b
b=c
c=d

# Functions.

function1() {

local VAR1=1 # Declared as local variable.
Commands
Return # Optional command exits a function but not script.

}

function2() {

local VAR1=2 # Declared as local variable.
Commands
exit 1 # exits script with an error.

}

# Execution.

# Test for options.

# Coammnds to call functions.

exit # Optional coammnd to exit script with exit staus of 0.

LINUX BASH SHELL SCRIPT BASICS Part 1:

What is BASH?

BASH is a shell that runs in the Linux terminal. BASH is not the only shell that users can choose but it’s the one most commonly shipped with Linux distributions. BASH is also available on Unix, Mac OS and Windows. It is a universal way to interact with a computer. The shell serves as a Command Line Interpreter which means it takes humanly readable commands and passes them to the Linux kernel. The kernel then sends instructions directly to the hardware. BASH doesn’t just run commands and other programs, it has its own programming language too.

What is a BASH shell Script?

BASH does not care where the commands it runs come from. It will happliy accept input from a human being typing on a keyboard, another program running on the system or it can get its instructions from a special kind of text file called a BASH Shell Script. Shell scripts can be used to automate common tasks and they make accomplishing complex computing tasks possible with just one command or the click of a mouse. You can also set shell scripts to run at start up so special parameters can be set or certain programs are opened automatically. They can range from just a line or two all the way up to full-fledged programs.

Yes, You can learn how to write BASH shell scripts!

There are only a few hard and fast rules to shell scripting. Once you learn them, you can feel free to experiment and come up with new ways to do wherever it is you’re trying to do. BASH is very flexible. There are often many ways to do the same thing and you should not let anyone tell you’re doing it “wrong” if your scripts run and do what you need them to. Keep in mind, we all have to start somewhere.

THE BASIC SCRIPT

You will need to use a plain text editor to write your scripts. Every Linux distribution offers one in the menu and there are excellent text editors that run in the terminal. What you use is entirely up to you. A script starts as a plain text file. A basic script looks like this:

#!/bin/bash

# Comments

Commands

The first line is called the “sha-bang” or “hash-bang” and it tells the system that this is a script and that it needs to use the BASH shell to run the following commands. Any line that starts with a # is treated as a comment and will not be considered as executable code by the shell. You can also place a # after a command to add a comment telling what that command is doing.

MAKING A TEXT FILE RUN LIKE A PROGRAM

Once the script is written, it should be saved with a unique filename. You may elect to add .sh as an extension but it is not necessary. I usually only add the .sh to scripts that will not be permanently installed as programs on my machine. The file you saved to must now be made executable before you can test it. You do this with the chmod command in one of two ways:

chmod +x script.sh

chmod 755 script.sh

TESTING THE SCRIPT

Now that the proper permissions are set, the script can be run form a terminal. You will have to use ./ to tell the shel to run it from your working directory, be cause it hasn’t yet been moved to some place in your executable path:

./script.sh

If the script runs as you hopes it would then congratulations, you’ve just written your first script! Sometimes it doesn’t work that easily and you’ll get errors. If you can’t figure out where the errors are coming from, you can activate tracing in your script to get more detailed output telling you just what it’s doing. Open it in an editor and add -x to your sha-bang:

#!/bin/bash -x

Don’t forget to remove the -x when you’re done fixing the problem.

WHERE TO PUT YOUR SCRIPTS.

You’ll want to move your script to someplace with in your path statement so you can run it like any other installed program on your system. There are two preferred places for scripts you write, programs you compile yourself and any other electable that is not handles by your distribution’s package manager:

/home/username/bin

or

/usr/local/bin

Ubuntu and many other Linux distributions will discover that you have created a directory in your home directory called ‘bin’ and add it to the path statement at startup. Executables that you put in ~/bin will be only available to you and you cannot run them with sudo in front of them.

Any executable that you put in /usr/local/bin will be available to all users and can be run with sudo privileges.

If your system is not automatically seeing the newly created ~/bin directory simply open or create a file called .bashrc in your home directory and add these lines to it:

# set PATH so it includes user’s private bin if it exists
test -d “$HOME/bin” && PATH=”$HOME/bin:$PATH”

Restart the system to make this take effect.

Now your script should be ready to run just by typing its name into the command line.

THINGS TO REMEMBER

* BASH is very well documented. Don’t hesitate to search the web for help.
* Always use absolute paths in a script. Ex. /full/path/to/file
* Write detailed comments so you’ll know what you did later on.
* Save a copy of everything you write so you can draw from it later.

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