Question

Please explain the following for making bash scripts:

Statements to keep in mind

                if-then-fi; if-then-else-fi and if-then-elif-then-else-fi

                                the evaluation of a single/compound condition

                while-do-done and until-do-done

                                the evaluation of a single/compound condition

                for-done

                                for every item in a list

another usage of for will need the evaluation of a single/compound condition

                case-in-esac

Answer 1

Answer:-

if-then-fi; if-then-else-fi and if-then-elif-then-else-fi:-

The if statement allows you to specify courses of action to be taken in a shell script, depending on the success or failure of some command. It is a conditional statement that allows a test before performing another statement. The syntax for the simplest form is:

if [ condition ] 
then
        block_of_statements
fi

Here,

• The condition in the if statement often involves a numerical or string test comparison, but it can also be any command that returns a status of 0 when it succeeds and some nonzero status when it fails.
• The statements that follow the then statement can be any valid UNIX command, any executable user program, any executable shell script, or any shell statement with the exception of fi.
• End every if statement with the fi statement.

while-do-done and until-do-done:-

The while loop is used to performs a given set of commands an unknown number of times as long as the given condition evaluates to true.

The Bash while loop takes the following form:

while [CONDITION]
do
  [COMMANDS]
done

Copy

The condition is evaluated before executing the commands. If the condition evaluates to true, commands are executed. Otherwise if the condition evaluates to false the loop will be terminated and the program control will be passed to the command

for-done:-

Numeric ranges for syntax is as follows:

for VARIABLE in 1 2 3 4 5 .. N do command1 command2 commandN done

OR

for VARIABLE in file1 file2 file3 do command1 on $VARIABLE command2 commandN done

OR

for OUTPUT in $(Linux-Or-Unix-Command-Here) do command1 on $OUTPUT command2 on $OUTPUT commandN done

Examples

This type of for loop is characterized by counting. The range is specified by a beginning (#1) and ending number (#5). The for loop executes a sequence of commands for each member in a list of items.