Compunet Corporation Introduction to Unix (CA263) Round and Round By Tariq Ibn Aziz Dammam Community College

Compunet Corporation

Introduction to Unix (CA263)

Round and Round

By

Tariq Ibn AzizDammam Community College


Objectives

• In this lecture you will learn the following loops– the for;– the while; and– the until.


The for Command

• The for command is used to execute a set of commands a specified number of times.

• Here is a loop example that will execute 3 times

for i in 1 2 3do

echo $idone

• Try directly on terminal$ for i in 1 2 3> do> echo $i> done123$


Example for Command$ cat runtbl $1 |nroff –mm –Tlp |lp$• If you want to run the files

memo1 through memo2 you can use the for command

$ for file in memo1 memo2>do> run $file>done$

• The shell permits filename substitution in the list of words in the for command.

for file in memo[1-2]do

run $filedone


Example for Command• If you want to run all of

the files in your current directory.

for file in *do

run $filedone$

• If the file filelist contains a list of the files that you want to run through run.

files=`cat filelist`for file in $filesdo

run $filedone


The $* Variable• If you found that you were using the run program to

process several files at once.

$ cat runfor file in $*do tbl $file |nroff –mm Tlp | lpdone$ $ run memo1 memo2 memo3 memo4

• The $* will be replaced by the four arguments memo1, memo2, memo3, and memo4.


The $* Variable[1]• The $* in for command will be replaced by shell with a b c

$ cat argsecho Number of argument passed is $#for arg in $*do echo $argdone$ $ args a b cNumber of argument passed is 3abc$


The $* Variable[2]

$ args 'a b' cNumber of argument passed is 2abc$

• Even though a b was passed as a single argument to args, the $* in the for command was replaced by shell with a b c, which is three words.


The $@ Variable[1]• The special variable “$@” will be replaced with "$1", "$2" …, the

double quotes are necessary around $@, otherwise it will behave has $*.$ cat args

echo Number of argument passed is $#for arg in "$@"do echo $argdone$ $ args a b cNumber of argument passed is 3abc$


The $@ Variable[2]

$ args 'a b' cNumber of argument passed is 2a bc$

• The special variable “$@” will be replaced with "$1", "$2" …, the double quotes are necessary around $@, otherwise it will behave has $*.


The while Command

• The second type of looping command is while.

$ cat twhilei=1While [ "$i" –le 5 ]doecho $ii=`expr $i + 1`

done

$ twhile12345$


The while Command• The program print each of the

command-line argument one per line.

$ cat prargsWhile [ "$#" –ne 0 ]do

echo $ishift

done$ $ prargs$

$ prargs a b cabc$ prargs 'a b' ca bc$ prargs *addressesnunamephonebookstat


The until Command

• The while command continues execution as long as the command listed after the while returns a zero exit status.

• The until command is similar to the while, only it continues execution as long as the command follows the until returns a nonzero exit status.


Example-1 until Command[1]

$ cat monif [ "$#" –ne 1 ]thenecho "Usage: mon user"exit 1

fiuntil who | grep "^$user ">/dev/nulldosleep 60

doneecho " $user has logged on“$



$ mon sandysandy has logged on$ mon sandy &4392$ nroff newmemo do other work…sandy has logged on



$ cat monif [ "$1" = -m ]then

mailopt=TRUEshift

elsemailopt=FALSE

fiif [ "$#" –eq 0 –o "$#" –gt 1

]then

echo "Usage: mon [-m] user"echo "-m means to be informed by mail"exit 1

fiuser="$1"

until who|grep "^$user ">/dev/nulldo

sleep 60doneif [ "$mailopt" =FALSE ]then

echo " $user has logged on"else

echo " $user has logged on"|mail steve

fi$$ man sandy –mUsage: mon [-m] user-m means to be informed by mail



$ mon –m sandy &$ mon sandy &5435$ nroff newmemo do other work…you have mail$ mailFrom steve Mon Jul 22 11:05 EDT 1985sandy has logged on?d$


More on Loops

• Breaking Out of a Loop– Sometime you want to make an immediate exit

from a loop. You can use the break command.– If the break command is used in break n form,

then the n innermost loops are immediately exited.


Example break Command• Both the while and for

loop will be exited if error is nonnull

for filedo … while [ "$count" –lt 10 ] do …

if [ -n "$error" ]then

break 2fi…

done …done


Skipping the Remaining Commands in Loop

• The continue command is similar to break, only it doesn’t cause the loop to be exited, but the remaining commands in the loop to be skipped.

• Like the break, an optional number can follow the continue, so continue n causes the commands in the innermost n loops to be skipped; but execution of loop then continues as normal.


Example continue Command

• Each value of file is checked to make sure that file exist.

for filedo if [ ! -f "$file" ]thenecho "$file not found"continuefi…

done


Executing a loop in the Background

• An entire loop can be sent to the background simply by placing an ampersand after the done:

$ for file in memo[1-4]>do> run $file>done &9932$


I/O Redirection on a Loop

• You can also perform I/O redirection on the entire loop.

$ for i in 1 2 3 4

>do> echo $i>done > loopout

$ cat loopout1234


I/O Redirection on a Loop• You can override redirection of the entire loop’s

input or output by explicitly redirecting the input and/or output of commands inside the loop.

for filedoecho "Processing file $file" >/dev/tty…

done > output• echo’s output is redirected to the terminal while

the rest goes to the file output.


I/O Redirection on a Loop

• You can also redirect the standard error output from a loop, simply by tacking on a 2> file after the done:

while [ "$endofdata" –ne TRUE ]do…

done 2> errors


Piping Data Into and Out of a Loop

• A command output can be piped into a loop, and the entire output from a loop can be piped into another command in the executed manner.

$ for i in 1 2 3 4>do> echo $i>done | wc –l4$


Typing a Loop on One Line

for i in 1 2 3 4doecho $i

done

becomes

for i in 1 2 3 4; do echo $i; done


Typing condition on One Line

• The same rules apply to while and until loops. if commands can also be typed on the same line using a similar format:

$ if [ 1 = 1 ]; then echo yes; fiyes$ if [ 1 = 2 ]; then echo yes; else echo no; fino

• Note that no semicolons appear after the then and the else.


The getopts Command

• The shell provide a built-in command getopts that exist for the express purpose of processing command line argument.

• The general format of the command is:

getopts options variable


The getopts Command

• The getopts command is designed to be executed inside a loop. – Each time through the loop, getopts examines the

next command line argument and determine if it is a valid option.

– This check if the argument begins with a minus sign and is followed by any single letter contained inside option. If it does, getopts stores the matching option letter inside the specified variable and returns a zero exit status.


The getopts Command

• If the letter that follows the minus sign is not listed in options, getopts stores a question mark inside variable before returning with a zero exit status. It also writes an error message to standard error.

• If there are no more arguments left on the command line or if the next argument doesn’t begin with a minus sign, getopts return a nonzero exit status.


The getopts Command

• Suppose you want getopts to recognize the options –a, -i, and –r for a command called foo. Your getopts call might look like this:

• getopts air option

Documents

Compunet Corporation Introduction to Unix (CA263) Round and Round By Tariq Ibn Aziz Dammam Community College