Shell scripting tutorial
This is a shell scripting tutorial. We use the bash shell as it is standard in most Linux distributions.
Contents
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"
calling commands
In your shell script you can call every command that you can call when opening a console:
echo "This is a directory listing, latest modified files at the bottom:" ls -ltr echo "Now calling a browser" firefox echo "Continuing with the script"
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"
parameters
echo "Here are all parameters you called this script with: $@"
echo "Here is parameter 1: $1"
echo "Which parameter do you want to be shown? "
read number
args=("$@")
echo ${args[$number-1]}
return codes
Every bash script can communicate with the rest of the system by
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
Redirections
To redirect the output of a command to a file you have to consider that there are two output streams in UNIX, stdout and stderr.
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
loops
for loops
Here is an example for a for-loop:
for i in $(seq 1 1 3); do echo $i; done
while loops
$ while true; do read line; done
