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Tutorial 10 - Shell Scripting - Part 1

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= KEY CONCEPTS =

A shell script is a '''file''' that contains '''Unix/Linux commands''' and '''reserved words''' to help '''automatic''' common tasks.

===Creating & Executing Shell Scripts===

It is recommended to '''plan''' out on a piece of paper the purpose of the shell script. Once you have planned your shell script by listing the '''sequence of steps (commands)''', you need to create a file (using a '''text editor''') that will contain your Linux commands. '''NOTE:''' Avoid using filenames of already existing Linux Commands to avoid confusion. It is recommended to include a file extension that describes the type of shell for the shell script.

'''Using a Shebang Line'''

[[Image:shebang.png|thumb|right|200px|The '''shebang line''' must appear on the '''first line''' and at the '''beginning''' of the shell script.]]Since Linux shells have evolved over a period of time, using a she-bang line '''forces''' the shell script to run in a '''specific shell''', which could prevent errors in case an older shell does not recognize newer features from more recent shells. The '''she-bang''' line is a '''special comment''' at top of your shell script to run your shell script in a specific shell.

'''NOTE:''' The '''shebang line''' must appear on the '''first line''' and at the '''beginning''' of the shell script, otherwise, it will be treated as a regular comment and ignored.

'''Setting Permissions / Running Shell Scripts'''

To run your shell script by name, you need to assign '''execute permissions''' for the user. To run the shell script, you can '''execute''' the shell script using a ''relative'', ''absolute'', or ''relative-to-home'' pathname

''Examples:'' '''chmod u+x myscript.bash ./myscript.bash /home/username/myscript.bash ~/myscript.bash

'''

===Variables / Parameters===

'''Environment Variables'''

[[Image:environment.png|thumb|right|500px|Examples of using '''Environment''' and '''User Defined''' variables.]]Shell '''environment variables''' shape the working environment whenever you are logged in Common shell. Some of these variables are displayed via Linux commands in the diagram displayed on the right-side. You can issue the pipeline command set | more to view all variables. Placing a dollar sign "$" prior to the variable name will cause the variable to expand to the value contained in the variable.

'''User Defined Variables'''

User-defined variables are variables that can be used in the shell script for '''customized''' purposes.

Data can be stored and removed within a variable using an '''equal sign''' (no spaces on either side of equal sign). The '''read''' command can be used to prompt the user to enter data into a variable. The '''readonly''' command will prevent the current value of the variable for the remainder of the execution of a shell script.

'''Positional Parameters and Special Parameters'''

[[Image:positional.png|thumb|right|220px|Examples of using '''positional''' and '''special''' parameters.]]A '''positional parameter''' is a variable within a shell program; its value is set from arguments contained in a shell script or using the '''set''' command.

Positional parameters are numbered and their values are accessed by using a preceding "'''$'''" (eg. '''$1''', '''$2''', '''$3''', etc.). The positional parameter '''$0''' refers to either the '''name of shell''' where command was issued, or '''filename of shell script''' being executed. If using '''positional parameters''' greater than '''9''', then you need to include number within braces. Examples: '''echo ${10}''', '''ls ${23}'''

The '''shift''' command can be used with positional parameters to shift positional parameters to the left by one or more positions.

There are a couple of ways to assign values as positional parameters:

:*Use the '''set''' command with the values as argument after the set command

:*Run a shell script containing arguments

There are a group of '''special parameters''' that can be used for shell scripting. A few of these special parameters and their purpose are displayed below: '''$*''' , '''“$*”''' , '''"$@"''' , '''$#''' , '''$?'''

=== Command Substitution / Math Operations ===

'''Command Substitution:'''

[[Image:for-command-substitution.png|thumb|right|300px|Example of how a '''for loop with command substitution''' works.]]

'''Command substitution''' is a facility that allows a command to be run and its output to be pasted back on the command line as arguments to another command. Reference: https://en.wikipedia.org/wiki/Command_substitution

''Usage:''

command1 $(command2) or command1 `command2`

''Examples:''

file $(ls) mail -s "message" $(cat email-list.txt) < message.txt echo "The current directory is $(pwd)" echo "The current hostname is $(hostname)" echo "The date is: $(date +'%A %B %d, %Y')"

'''Math Operations:'''

[[Image:math-op.png|thumb|right|275px|Common Math Operator Symbols.]]

In order to make math operations work, we need to convert numbers stored as '''text''' into '''binary numbers'''. We can do this by using 2 pairs of round brackets '''(( ))'''.

''Examples:''

<pre style="width:30%">num1=5;num2=10

echo “$(($num1 + $num2))”

15

echo “$((num1-num2))”

-5

((product=num1*num2))

echo “$product”

50

</pre>

===Control Flow Statements===

'''Control Flow Statements''' are used to make your shell scripts more '''flexible''' and can '''adapt''' to changing situations. In order to use control flow statements, you need to test a condition to get '''TRUE''' (zero value) or '''FALSE''' (non zero value). This can be done two ways:<ul><li>Run a command to get the exit status ($?)</li><li>Use the '''test''' command</li></ul> Refer to the diagrams on the right to see how to use the test command.

You CANNOT use the < or > symbols when using the test command since these are redirection symbols. Instead, you need to use '''options''' when performing numerical comparisons.

Refer to the diagrams to the right '''test options''' and their purposes.

'''Logic Statements'''

A '''logic statement''' is used to determine which Linux commands are executed basedon the result of a condition: '''TRUE''' (zero value) or '''FALSE''' (non-zero value).

[[Image:logic-1.png|thumb|right|210px|Example of using the '''if''' logic control-flow statement.]]

There are several logic statements, but we will just concentrate on the if statement.

if test condition

then

command(s)

fi

</pre>

Refer to the diagram to the right for using the '''if logic statement''' with the '''test''' command.

'''if-else statement:'''

[[Image:logic-2.png|thumb|right|210px|Example of how an '''if-else''' control-flow statement.]]

Unlike using an ''if'' statement, an '''if-else''' statement take '''two different sets of actions''' based on the results of the test condition.

''Example:''

if test condition

then

command(s)

else

command(s)

fi

</pre>

'''Loop Statements'''

[[Image:loop-1.png|thumb|right|210px|Example of using the '''for''' looping control-flow statement.]]

''A loop statement is a series of steps or sequence of statements executed repeatedly zero or more times satisfying the given condition is satisfied.'' Reference: https://www.chegg.com/homework-help/definitions/loop-statement-3

There are several loops, but we will look at the '''for loop''' using a '''list'''.

for item in list

do

command(s)

done

</pre>

Refer to the diagram above and to the extreme right side for an example using the '''for loop''' with a '''list'''.

= INVESTIGATION 1: CREATING A SHELL SCRIPT =

Jason Michael Carman

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Tutorial 10 - Shell Scripting - Part 1

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