Shell scripting tutorial

This is a shell scripting tutorial. We use the bash shell as it is standard in most Linux distributions.

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:

• create a file named hello.sh in your home directory with the following content:

#!/bin/bash
echo "hello world"

• open a console, enter

cd
chmod 777 hello.sh

• now you can execute your file like this:

# ./hello.sh 
hello world

• or like this:

# bash hello.sh 
hello world

You see - the output of your shell program is the same as if you had entered the commands into a console.

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

• 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

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

reading a command's output stream

To read a command's output into a variable use $(), backticks or piping.

$()

arch=$(uname -m)
echo "Your computer is a $arch computer."

backticks

arch=`uname -m`
echo "Your computer is a $arch computer."

piping

uname -m | while read arch; do echo "Your computer is a $arch computer."; done

comparison

The advantage of using backticks over $() is that backticks also work in the sh shell. The advantage of using $() over backticks is that you can cascade them. In the example below we use this possibility to get a list of all files installed with rpm on the system:

filelist=$(rpm -ql $(rpm -qa))

You can use the piping approach if you need to cascade in sh, but this is not focus of this bash tutorial.

common mistakes

Usually unexperienced programmers try something like

uname -m | read arch

which just does not work. You must embed the read into a while loop.

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

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

sending a process to the background

To send a process to the background, use the ampersand sign (&):

firefox & echo "Firefox has been started"

You see a newline is not needed after the &

forking a process

(find -iname "helloworld.txt") & (sleep 5; echo "Timeout exceeded, killing process"; kill $!)

awk: read a specific column

awk is a program that is installed on almost all Linux distributions. It is a good helper for text stream processing. It can extract columns from a text. Let's imagine you want to use the program vmstat to find out how high the CPU user load was. Here is the output from vmstat:

procs -----------memory---------- ---swap-- -----io---- -system-- -----cpu------
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
 1  0      0 1304736 190232 885840    0    0    20     8   11   17  1  0 99  0  0

We see the user load is in colum 13, and we only want to print this column. We do it with the following command:

vmstat 5 | awk '{print $13}'

This will print a line from vmstat all 5 seconds and only write the column 13. It looks like this:

# vmstat 5 | awk '{print $13}'

us
1
1
0
1

To store the CPU user load into a variable we use

load=$(vmstat 1 2 | tail -n 1 | awk '{print $13}')

What happens here? First vmstat outputs some data in its first line. The data about CPU load can only be rubbish because it did not have any time to measure it. So we let it output 2 lines and wait 1 second between them ( => vmstat 1 2 ). From this command we only read the last line ( => tail -n 1 ). From this line we only print column 13 ( => awk '{print $13}' ). This output is stored into the variable $load ( => load=$(...) ).

grep: search a string

grep is a program that is installed on almost all Linux distributions. It is a good helper for text stream processing. It can extract lines that contain a string or match a regex pattern. Let's imagine you want all external links from www.linuxintro.org's main page:

wget -O linuxintro.txt http://www.linuxintro.org
grep "http:" linuxintro.txt 

sed: replace a string

sed is a program that is installed on almost all Linux distributions. It is a good helper for text stream processing. It can replace a string by another one. Let's imagine you want to copy a file foo to a file bar, but replace all occurrences of "copyright 1990" by "copyright 1990-2012":

cat foo | sed "s/copyright 1990/copyright 1990-2012/g" | cat >bar

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
• http://linuxconfig.org/Bash_scripting_Tutorial
• http://steve-parker.org/sh/intro.shtml

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