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

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

Compunet Corporation 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 do echo $i done Try directly on terminal $ for i in >do >echo $i >done $

Compunet Corporation Example for Command $cat run tbl $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 $file done

Compunet Corporation Example for Command If you want to run all of the files in your current directory. for file in * do run $file done $ If the file filelist contains a list of the files that you want to run through run. files=`cat filelist` for file in $files do run $file done

Compunet Corporation The $* Variable If you found that you were using the run program to process several files at once. $ cat run for file in $* do tbl $file |nroff –mm Tlp | lp done $ $ run memo1 memo2 memo3 memo4 The $* will be replaced by the four arguments memo1, memo2, memo3, and memo4.

Compunet Corporation The $* Variable[1] The $* in for command will be replaced by shell with a b c $ cat args echo Number of argument passed is $# for arg in $* do echo $arg done $ $ args a b c Number of argument passed is 3 a b c $

Compunet Corporation The $* Variable[2] $ args 'a b' c Number of argument passed is 2 a b c $ 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.

Compunet Corporation 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 $arg done $ $ args a b c Number of argument passed is 3 a b c $

Compunet Corporation The Variable[2] $ args 'a b' c Number of argument passed is 2 a b c $ The special variable “ ” will be replaced with "$1", "$2" …, the double quotes are necessary around otherwise it will behave has $*.

Compunet Corporation The while Command The second type of looping command is while. $cat twhile i=1 While [ "$i" –le 5 ] do echo $i i=`expr $i + 1` done $twhile $

Compunet Corporation The while Command The program print each of the command-line argument one per line. $ cat prargs While [ "$#" –ne 0 ] do echo $i shift done $ $ prargs $ $ prargs a b c a b c $ prargs 'a b' c a b c $ prargs * addresses nu name phonebook stat

Compunet Corporation 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.

Compunet Corporation Example-1 until Command[1] $ cat mon if [ "$#" –ne 1 ] then echo "Usage: mon user" exit 1 fi until who | grep "^$user ">/dev/null do sleep 60 done echo " $user has logged on“ $

Compunet Corporation Example-1 until Command[2] $ mon sandy sandy has logged on $ mon sandy & 4392 $ nroff newmemodo other work … sandy has logged on

Compunet Corporation Example-2 until Command[1] $ cat mon if [ "$1" = -m ] then mailopt=TRUE shift else mailopt=FALSE fi if [ "$#" –eq 0 –o "$#" –gt 1 ] then echo "Usage: mon [-m] user" echo "-m means to be informed by mail" exit 1 fi user="$1" until who|grep "^$user ">/dev/null do sleep 60 done if [ "$mailopt" =FALSE ] then echo " $user has logged on" else echo " $user has logged on"|mail steve fi $ $ man sandy –m Usage: mon [-m] user -m means to be informed by mail

Compunet Corporation Example-2 until Command[2] $ mon –m sandy & $ mon sandy & 5435 $ nroff newmemodo other work … you have mail $ mail From steve Mon Jul 22 11:05 EDT 1985 sandy has logged on ?d $

Compunet Corporation 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.

Compunet Corporation Example break Command Both the while and for loop will be exited if error is nonnull for file do … while [ "$count" –lt 10 ] do … if [ -n "$error" ] then break 2 fi … done … done

Compunet Corporation 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.

Compunet Corporation Example continue Command Each value of file is checked to make sure that file exist. for file do if [ ! -f "$file" ] then echo "$file not found" continue fi … done

Compunet Corporation 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 $

Compunet Corporation I/O Redirection on a Loop You can also perform I/O redirection on the entire loop. $ for i in >do >echo $i >done > loopout $ cat loopout

Compunet Corporation 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 file do echo "Processing file $file" >/dev/tty … done > output echo ’s output is redirected to the terminal while the rest goes to the file output.

Compunet Corporation 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

Compunet Corporation 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 >do >echo $i >done | wc –l 4 $

Compunet Corporation Typing a Loop on One Line for i in do echo $i done becomes for i in ; do echo $i; done

Compunet Corporation 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; fi yes $ if [ 1 = 2 ]; then echo yes; else echo no; fi no Note that no semicolons appear after the then and the else.

Compunet Corporation 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

Compunet Corporation 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.

Compunet Corporation 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.

Compunet Corporation 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