compunet corporation introduction to unix (ca263) round and round by tariq ibn aziz dammam community...

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Introduction to Unix (CA263)

Round and Round

By

Tariq Ibn AzizDammam Community College

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Objectives

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

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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 $i> done123$

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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

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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

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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.

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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$

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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.

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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$

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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 $*.

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The while Command

• The second type of looping command is while.

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

done

$ twhile12345$

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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

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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.

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Example-1 until Command[1]

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

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

doneecho " $user has logged on“$

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Example-1 until Command[2]

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

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Example-2 until Command[1]

$ 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

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Example-2 until Command[2]

$ 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$

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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.

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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

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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.

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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

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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 $file>done &9932$

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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

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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.

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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

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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 $i>done | wc –l4$

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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

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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.

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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

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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.

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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.

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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