Linux Shell (6)

7/31/2019 Linux Shell (6)

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The Linux ShellsCompound Commands

Operating Systems Laboratory

Amir Saman Memaripour


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caseCommandThe casecommand compares a variable against a series

of values or patterns. If the value or pattern matches,the corresponding statements are executed. Thename comes from the fact that the variable is tested on

a case-by-case basis.

Unlike elif commands that test the status code forindividual commands, case tests the value of a

variable. The case command is preferable to a long setof elifswhen testing string values.

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caseCommandEach individual case must end with a pair of semicolons.

Without the semicolons, Bash will attempt to execute thenext casedesignation and report an error.

printf %s -> 1 = delete, 2 = archive. Please choose one

read REPLY

case $REPLY in

1) rm $TEMPFILE ;;

2) mv $TEMPFILE $TEMPFILE.old ;;

*) printf %s\n $REPLY was not one of the choices ;;

esac

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caseCommandThe pattern asterisk (*) is the catch-all case; it matches all values

that were not handled by a previous case. Although this case isoptional, its good design to always include a catch-all case, evenif it only contains a null statement (:).

The pattern-matching rules follow the file globbing rules, asdiscussed in the previous chapter. For example, vertical bars canseparate multiple patterns.

The cases dont fall through as some computer languages do,

notably C. When one case is selected, only those commands areexecuted. The commands in the following cases are notexecuted.

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whileLoopThere are several commands for repeating a set of

commands.

The while command repeats the enclosed commands

while the command being tested succeeds. If thecommand fails on the first try, the enclosed commandsare never executed.

A while loop is completed with the donecommand, not

elihwas you might expect.

An infinite loopcan be created using the truecommand.Because truealways succeeds,the loop will continue

indefinitely. 5


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whileLoopprintf %s\n Enter the names of companies or type control-d

while read -p Company ? COMPANY; do

if test -f orders_$COMPANY.txt ; then

printf %s\n There is an order file from this company

else

printf %s\n There are no order files from this company

fi

done

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whileLoopA whileloop can be stopped prematurely with the break

command. On reaching a break, Bash stop executingcommands and breaks out of the loop and beginsexecuting the first command after the loop. break

can be followed by a number indicating how manyenclosing loops to break out of. For example: break 2,breaks out of the current loop as well as the loopenclosing the current loop.

The counterpart to break is continue, which causes theremainder of the enclosed statements to beignored; the loop is resumed at the top. continuecanbe followed by a number indicating how manyenclosing statements to break out of before

continuing. 7


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whileLoopprintf %s\n Enter the names of companies or type quit

while true ; do

read -p Company ? COMPANY

if [ $COMPANY = quit ] ; then

break

elif test -f orders_$COMPANY.txt ; then

printf %s\n There is an order file from this company

else

printf %s\n There are no order files from this companyfi

done

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untilLoopThe counterpart of the while loop is the until loop. The

until command is identical to the while commandexcept that it repeats the enclosed statements until thecondition is successful, essentially the same as

while!

until test -f $INVOICE_FILE ; do

printf %s\n Waiting for the invoice file to arrive...

sleep 30

done

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forLoopsThe for command reads a sequence of values into a

variable and repeats the enclosed commands once foreach value.

for FILE_PREFIX in order invoice purchase_order; do

if test -f $FILE_PREFIX_vendor1.txt ; then

printf %s\n There is a $FILE_PREFIX file from vendor 1...

fi

done

If the inpart is missing, the forcommand will execute theenclosed statements for each argument to the shell

script. 10


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Embedded letThe let command returns a status code of 1 if an

expression is zero, or 0 if the expression is a valueother than zero. In the same way that the testcommand can be expressed with a pair of square

brackets for easier reading in compound statements,the let command also has a form for easier reading:double parentheses.

The forloop, as found in other programming languages, iscreated using an embedded let.

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Embedded letfor (( COUNTER=1; COUNTER


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Grouping CommandsCommands can be grouped together using curly braces (

{...} ).

ls -1 | {

while read FILE ; do

echo $FILE

done

}

In this example, the results of the lscommand becomethe input for all the statements in the group.

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Grouping Commands$ test -f orders.txt && { ls -l orders.txt ; rm orders.txt; }

\

|| printf no such file

If the file orders.txt exists, the file will be listed anddeleted; otherwise, no such file is printed. Thesemicolon after the last braced command is requiredonly when the braces are on a single line.

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Shell FunctionsFunctions are a way of embedding small subscripts into a

Bash script without saving them in a separate file.Because they are embedded in a script, they do nothave to be loaded in order to be used. This, in turn,

makes them run faster than separate scripts. Likeseparate scripts, functions can be used to break up acomplex script into separate, named, tasks to improvereadability.

Shell functions differ from functions in other programminglanguages in that they return a status code instead ofa return value. Without a return value, they cannot beused in expressions.

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Shell FunctionsLike variables, functions must be declared before they

can be used. Instead of the declare command,functions are declared using the function command.Each function has a name and the statements

composing the function are enclosed in curly brackets.

function trash_tmp { }

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Other Issues Bash Security

Version Control

Network Programming Data structures

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Assignment (5 points)

Write a shell script to find and print all prime numbers

less than 100. In order to do that, use Linear Sieveor

other kinds of prime number generation algorithms.

Deadline: December 18th, 10:00 AM

How to deliver?!Via email to [email protected]

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mailto:[email protected]:[email protected]

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Linux Shell (6)