Lection 2: Bash scripting

A bash shell script can have parameters. The numbering you see in the script below continues if you have more parameters. You also have special parameters containing the number of parameters, a string of all of them, and also the process id, and the last return code. The man page of bash has a full list.

#!/bin/bash
echo The first argument is $1
echo The second argument is $2
echo The third argument is $3
echo \$ $$ PID of the script
echo \# $# count arguments
echo \? $? last return code
echo \* $* all the arguments

The shift statement can parse all parameters one by one. This is a sample script.

#!/bin/bash
echo my name is $0
if [ "$#" == "0" ] then
echo You have to give at
least one parameter.
exit 1
fi
while (( $# ))
do
echo You gave me $1
shift
done

The getopts function allows you to parse options given to a command. The following script allows for any combination of the options a, b and c

#!/bin/bash
while getopts ":abc" option;
do
case $option in
    a)
        echo received -a ;;
    b)
        echo received -b ;;
    c)
        echo received -c ;;
    *)
        echo "invalid option -$OPTARG" ;;
esac
done

You can also check for options that need an argument, as this example shows.

#!/bin/bash
while getopts ":ab:c:" option;
do
case $option in
a)
echo received -a ;;
b)
echo received -b with $OPTARG ;;
c)
echo received -c with $OPTARG ;;
:)
echo "option -$OPTARG needs an argument" ;;
*)
echo "invalid option -$OPTARG" ;;
esac
done

Shell functions can be used to group commands in a logical way.

#!/bin/bash
function greetings {
echo Hello World!
echo and hello to $USER to!
}
echo We will now call a function
greetings
echo The end

You can read any file line by line in bash by using loop. Create a file named, ‘read_file.sh’ and add the following code to read an existing file named, ‘book.txt’.

#+/bin/bash
file='book.txt'
while read line;
    do
        echo $line
    done < $file

home

We can use variables as in any programming languages. Their values are always stored as strings, but there are mathematical operators in the shell language that will convert variables to numbers for calculations. There is no needed to declare variables. Just assign a value to its reference will create it

• The shell programming language does not type-cast its variables.
  This means that a variable can hold number data or character data
• Switching the TYPE of a variable can lead to confusion for the writer of the script or someone trying to modify it, so it is recommended to use a variable for only a single TYPE of data in a script

There are two types of variables:

• Local Variables
• Environmental Variables

The backquote ` is different from the single quote ’, it is usede for command substitution: `command`

    #!/bin/bash
    list=`ls -l`
    echo $list

The let statement can be used to do mathematical functions.
An arithmetic expression can be evaluated by $[expression] or $((expression))

    let x=10+2*4
    echo $x
    echo "$((10+2*4))"
    val=$[10+2*4]
    echo "$[10+$val]"

Important:
Bash does not know how to work with floating point units

The most basic form is:

    if [ expression ];
       then
           statements
       elif [ expression ];
       then
           statements
       else
           statements
    fi

• the elif (else if) and else sections are optional • Put spaces after [ and before ], and around the operators and operands.

An expression can be:

• String comparison
• Numeric comparison
• File operators and logical operators and it is represented by [expression]