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Presentation on Looping Conditions andCommand Line Arguments
Conditional loop• conditional loop is a way for computer programs to repeat one or more various steps depending on conditions set either by the programmer initially or real-time by the actual program.• Basically a loop has a conditional statement or a command and body of that loop which has some list of commands or statements to be executed repeatedly.
While Loop• The while loop is used to execute a set of commands repeatedly until some condition occurs.• It is usually used when the value of a variable has to be manipulated repeatedly.
Basic syntaxWhile commanddo listdoneIt can also be written as,While command ; do list ; done
Steps to execute a while loop1. Execute command.2. If the exit status of command is nonzero, exit from the while loop3. If the exit status of command is zero, execute list.4. When list finishes execution, return to step 1.
For example, The output looks like,x=0 0while [ $x –lt 10 ] 1do 2 echo $x 3 x=`echo “$x + 1” | bc` 4done 5 6 7 8 9
Example(2)c=1 Output,while [ $c -le 5 ] Welcome 1 times do Welcome 2 times echo "Welcome $c Welcome 3 times times" Welcome 4 times (( c++ )) Welcome 5 times done
Nested while loop• It is a loop in which a while loop will be a part of the body of another while loop.• There is no restrictions for the number of nested while loops.• But it will b better to avoid more than 5 nested loops.
Syntax is,while command1 ; # this is loop1, the outer loopdo list1 while command2 ; # this is loop2, the inner loop do list2 done list3done
Example,x=0while [ "$x" -lt 10 ] ; # this is loop1 do y="$x" while [ "$y" -ge 0 ] ; # this is loop2 do echo "$y c" y=´echo "$y - 1" | bc´ done echo x=´echo "$x + 1" | bc´ done
The output will be,0102103210432105432106543210765432108765432109876543210
For loop• The for loop is used to execute a set of commands repeatedly for each item in a list.• One of its most common uses is in performing the same set of commands for a large number of files.
The common syntax is,for name in word1 word2 ... wordN do list doneIt can also be written as,for name in word1 word2 ... wordN ; do list ; done
For a simple for loop,for i in 0 1 2 3 4 5 6 7 8 9 The output is, do 0 echo $i 1 done 2 3 4 5 6 7 8 9
another example,for i in `cat 1.txt` Output is, do echo $i F done S TThe contents in 1.txt,FST
Example(3)alphabet="a b c d e" Output,count=0 afor letter in $alphabet b do count=`expr $count + 1` c echo "Letter $count is d [$letter]" e done
Command line arguments• The arguments used to pass to the shell scripts while interpreting are called command line arguments.• $0 indicates the name of the script.• $1 indicates the 1st argument of that script.• $2 indicates the 2nd argument.• $$ used to denote the process ID.• $# used to count the number of arguments.• $@ denotes all arguments.