Shell scripting tutorial

This is a tutorial for bash shell scripting.

Hello world

The easiest way to get your feet wet with a programming language is to start with a program that simply outputs a trivial text, the so-called hello-world-example. Here it is for bash:

echo "hello world"

Note that you can open a console and enter this command or you can put it into an executable file, say "foo.sh" and call the file in a console like this

./foo.sh

Or you can explicitely use bash to interpret the commands in this file:

bash foo.sh

In all cases the output will be just

hello world

on your console (to be more general: on stdout).

There is also a way to tell the script that it is explicitely written for bash (and no other shell using a shebang line like this in your file:

#!/bin/bash
echo "hello world"

input

To show you how to deal with variables, we will now write a script that asks for your name and greets you:

echo "what is your name? "
read name
echo "hello $name"

You see that the name is stored in a variable $name. Note the quotation marks " around "hello $name". By using these you say that you want variables to be replaced by their content. If you were to use apostrophes, the name would not be printed, but $name instead.

common mistakes

Note that the variable is called $name, however the correct statement to read it is

read name

It is a common mistake to write

read $name

which means "read a string and store it into the variable whose name is stored in $name"

return codes

Every bash script can communicate with the rest of the system by

• sending data to stdout
• sending data to stderr
• delivering a return code

The return code is 0 if everything worked well. You can query it for the most recent command using $?:

bootstick@bootstick:~$ echo "hello world"; echo $?
hello world
0
bootstick@bootstick:~$ echo "hello world">/proc/cmdline; echo $?
bash: /proc/cmdline: Permission denied
1

In bash, true is 0 and false is any value but 0. There exist two commands, true and false that deliver true or false, respectively:

bootstick@bootstick:~$ true; echo $?
0
bootstick@bootstick:~$ false; echo $?
1

conditions

The easiest form of a condition in bash is this if example:

echo "what is your name? "
read name
if [ $name = "Thorsten" ]; then echo "I know you"; fi

Now let's look closer at this, why does it work? Why is there a blank needed behind the [ sign? The answer is that [ is just an ordinary command in the shell. It delivers a return code for the expression that follows till the ] sign. To prove this we can write a script:

if true; then echo "the command following if true is being executed"; fi
if false; then echo "this will not be shown"; fi

arithmetic expressions

echo "what is your age? "
read age
if (( $age >= 21 )); then echo "Let's talk about sex."; fi

common mistakes

Common mistakes are:

• to forget the blank behind/before the [ or ] character
• to forget the blank behind/before the equal sign
• see what does "unary operator expected" mean

line feeds

Let's look at the following script:

read name
if [ $name = "Thorsten" ]; then echo "I know you"; fi

Instead of a semicolon you can write a line feed like this:

read name
if [ $name = "Thorsten" ]
  then echo "I know you"
fi

And instead of a line feed you can use a semicolon:

read name; if [ $name = "Thorsten" ]; then echo "I know you"; fi

If you want to insert a line feed where you do not need one, e.g. to make the code better readable, you must prepend it with a backslash:

read \
  name
if [ $name = "Thorsten" ]
  then \
    echo "I know you"
fi

calling commands

Calling commands in a bash script is as easy as it can be: You just write the command to be called, like this:

echo "Now calling a browser"
firefox
echo "Continuing with the script"

filling files

To create a file, probably the easiest way is to use cat:

cat >README<<EOF
This is line 1
This is line 2
This is the last line
EOF

See also

• bash
• shell
• scripting
• bash operators
• http://en.wikibooks.org/wiki/Bash_Shell_Scripting
• http://wiki.linuxquestions.org/wiki/Bash_tips
• http://wiki.linuxquestions.org/wiki/Bash