Shell scripting tutorial

From Linuxintro
Revision as of 18:59, 7 January 2012 by imported>ThorstenStaerk (→‎filling files)

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 in your home directory with the following content:
#!/bin/bash
echo "Hello, world!"

cd
chmod +x hello
  • now you can execute your file like this:
# ./hello
Hello, world!
  • or like this:
# bash hello
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"

variables

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.

${}

The ${} operator stands for the variable, there is no difference if you write 

echo "$name"

or 

echo "${name}"

So what is the sense of this? Imagine you want to echo a string directly, without any blank, after the content of a variable: 

echo "if I add the syllable owa to your name it will be ${name}owa"

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

storing a command's output

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 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

not equal

To check if a variable is NOT equal to whatever, use !=: 

if [ "$LANG" != "C" ]; then echo "please set your system lanugage to C"; fi

common mistakes

Common mistakes are:

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. The following example writes text into README till a line occurs that only contains the string "EOF": 

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

Afterwards, README will contain the 3 lines below the cat command and above the line with 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 $!)

functions

To define a function in bash, use a non-keyword and append opening and closing parentheses. Here a function greet is defined and it prints "Hello, world!". Then it is called: 

greet()
{
    echo "Hello, world!"
}
greet

react on CTRL_C

The command trap allows you to trap CTRL_C keystrokes so your script will not be aborted 

#!/bin/bash

trap shelltrap INT

shelltrap()
{
    echo "You pressed CTRL_C, but I don't let you escape"
}

while true; do read line; done
Note
You can still pause your script by pressing CTRL_Z, send it to the background and kill it there. To catch CTRL_Z, replace INT by TSTP in the above example. To get an overview of all signals that you might be able to trap, open a console and enter
kill -l

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 print your distribution's name, but lsd_release outputs too much: 

# lsb_release -d
Description:    openSUSE 12.1 (x86_64)

You want to remove this string "Description" so you replace it by nothing: 

lsb_release -d | sed "s/Description:\t//"
openSUSE 12.1 (x86_64)

tr: replace linebreaks

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 character by another one, even over line breaks. For example here is how you remove all empty lines from your processor information: 

# cat /proc/cpuinfo | while read a; do ar=$(echo -n $a|tr '\

' ';'); if [ "$ar" <> ";" ]; then echo "$ar"; fi; done

See also

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