unix lecture 5

LIN 6932 1

Unix Lecture 5

Unix Shell Scripts

LIN 6932 2

Command Coordination ; && ||

command1 ; command2

Interpretation:Do command 1. Then do command 2

Description:The semicolon links two commands that are executed by the shell in simple sequential order. This means that csh will run the first command in the sequence, then once that command is finished, it will run the next command. Linking commands with semicolons is analogous to executing them as separate commands. That is, the command line below:

% cd ~/public_html ; ls *.html

is equivalent to this:

% cd ~/public_html% ls *.html

LIN 6932 3


command1 && command2

Interpretation:Do command 2 only if command 1 executes successfully (and not otherwise).

DescriptionThe double-ampersand links two commands as a very simple case of a conditional. This means that there is a condition under which command2 is executed: namely only if command1 executes successfully. This is only a little more complex than the semicolon operator. For example, giving a command like this:

% clear && cal

will attempt to run the clear command first. If and only if the clear command works (i.e., returns an exit status of 0), the shell will run the cal command. If the clear command does not work (i.e., returns a non-zero exit status), then the cal command is not even attempted.

LIN 6932 4


command1 || command2

InterpretationDo command 1 or command 2 (but not both).

Description:|| means something like "OR". The shell will attempt to execute command1 first. If and only if command1 fails (i.e., returns a non-zero exit status), the shell will run command2. If command1 exits cleanly, the shell does not even attempt running command2. For example, if you type something like this:

% rm /etc/motd || echo "Sorry, can't remove file."

The shell will attempt to remove the file /etc/motd first. If that works, then nothing else happens. However, if that command doesn't work for any reason (like maybe you don't have permission), the shell will run the echo command, and print the message Sorry, can't remove the file to the screen. Give it a shot to see what we're talking about.

LIN 6932 5

Shell Script File

• any collection of csh commands may be stored in a file, a shell script file

• csh can be invoked to execute the commands in that file

• the language used in that file is called shell script language

• Like other programming languages it has variables and flow control statements: e.g. if-then-else, while, for, goto.

LIN 6932 6

Invoking Shell Scripts

DIRECT INTERPRETATION% csh filename [arg ...] invokes the program csh to interpret the script contained in the file `filename'.

INDIRECT INTERPRETATIONwe insert as the first line of the file

#! /bin/csh (by default)

Or use the information given when you execute the command

% echo $SHELL/usr/local/bin/tcsh

AND the file must be made executable using chmod

LIN 6932 7

A simple example: listfiles

#! /usr/local/bin/tcsh

echo "hello, $USER. I wish to list some files of yours"

echo "listing files in the current directory, $PWD"

ls # list files

• $USER and $PWD are standard variables defined by the csh and needn't be defined in the script

• the variables are expanded when the variable name is inside double quote: the shell sees the string $USER and replaces it with the variable's value then executes the command.

LIN 6932 8

Setting Variables

Any programming language needs variables. You define a variable as follows:set Xwill set the variable X to have an empty list as its valueand refer to it as follows:$X$X is used to denote the value of the variable X

set X = dear

LIN 6932 9

A simple example: listfiles1

#! /usr/local/bin/tcsh

set X = dear

echo "hello, $X $USER. I wish to list some files of yours"

echo "listing files in the current directory, $PWD"

ls # list files

• $USER and $PWD are standard variables defined by the csh and needn't be defined in the script

• the variables are expanded when the variable name is inside double quote: the shell sees the string $USER and replaces it with the variable's value then executes the command.

LIN 6932 10

Command ArgumentsMost commands have arguments, and these are accessible via the shell variable $argv.

The first parameter will be $argv[1] , the second $argv[2] , and so on.

The number of such arguments is $#argv.

Consider the following script file, named swap:

#! /bin/csh

set tmp = $argv[1]

cp $argv[2] $argv[1]

cp $tmp $argv[2]

If you have files x and y, and type

% swap x y

then the new contents of x would be what is in y.

LIN 6932 11

Command ArgumentsMost commands have arguments, and these are accessible via the shell variable $argv.

The first parameter will be $argv[1] , the second $argv[2] , and so on.

The number of such arguments is $#argv.

Consider the following script file, named swap:#! /usr/local/bin/tcsh

set tmp = $argv[1]

cp $argv[2] $argv[1]

cp $tmp $argv[2]

If you have files x and y, and type

% swap x y

then the new contents of x would be what is in y.

LIN 6932 12

Flow Control Statements Conditional

If … then … endifConditionals are commands which are executed only if a certain condition is true. Syntax of the if conditional:

if ( expression ) then command

endifIf expression is true, then command is executed. If expression is false, command is not executed.

LIN 6932 13


If … then … endif

# over100 -- checks if a number is greater than 100

if ( $1 > 100 ) then

echo ”That’s a big number!”

endif

LIN 6932 14

A note on Relational Operators

== equal to

!= not equal to

< less than

> greater than

<= less than or equal to

>= greater than or equal to

LIN 6932 15


If expression is true, more than one command can be executed

if ( expression ) then

command1 # these commands are executed if expression is true

command2

...

endif

LIN 6932 16


# over100add -- checks if a number is greater than 100, adds 1 to # it and then subtracts 2 from it.

if ( $1 > 100 ) then

echo ”That’s a big number!”

echo "$1 is a big number."

echo ”But `expr $1 + 1` is even bigger.”

echo `expr $1 - 2`

endif

Recall: The backquotes ` around a command signify COMMAND SUBSTITUTION: The output from the backquoted command is included within the command line for another command.

LIN 6932 17


an if statement can also have a set of commands which run if expression is false, using the else command:

if ( expression ) then

command1 # these commands are executed if expression is true

command2

...

else

command11 # these commands are executed if expression is false

command12

...

endif

LIN 6932 18


#! /bin/csh

# string_check -- checks two strings

# first, make sure the user typed in two arguments following the commandif ( $# != 2 ) then echo "This script needs exactly two (2) arguments." echo "Exiting...(annoyed)" exit 666endif

# now, compare themif ( $1 == $2 ) then echo "$1 and $2 are the same, aren't they?"else echo "$1 and $2 are different, aren't they?"endif

LIN 6932 19

Flow Control Statements foreach ‘loop’

The syntax of foreach loop construct is

foreach var ( wordlist )

command(s)

end

The command(s) is executed once for each “word” (or item) in the wordlist, and each time the variable var will contain the value of that word.

LIN 6932 20


% vi marks16 - | - | - | - | - | - | - | - | - |Fred Dexter19 - | - | - | - | A | - | - | - | - |Waqas Younis16 A | A | A | B | - | A | A | A | B |David Gower16 - | - | - | - | A | - | - | - | - |Mickey Stewart20 A | A | A | A | A | A | A | A | A |Graham Thorpe

LIN 6932 21


% vi countmarks#!/bin/csh # counts the number of students associated with a number of # particular marks foreach number(0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20) set ms=`grep "^$number " marks | wc -l` if ($ms != 0) then echo “$ms ' student(s) got ' $number marks” endifend

Note: set ms= will set the variable ms to have the value that corresponds to the output of

the grep and wc command line utilities.

LIN 6932 22


% chmod +x countmarks ; countmarks

3 student(s) got 16 marks



Recall: The semicolon ; links two commands that are executed by the shell in simple sequential order; csh will run the first command in the sequence, then once that command is finished, it will run the next command.

LIN 6932 23


Example: % mkdir city; cd city; mkdir sf ny la; touch sf/a ny/b la/c

Note: touch is a standard Unix program used to create a blank file; it is also used to change a file's access and modification timestamps.

LIN 6932 24


Example: % mkdir Try; cd Try; mkdir d1 d2 d3; touch d1/a d2/b d3/c

% vi check#!/bin/csh set start=`pwd`set command=$0foreach name (`ls`) if (-d $name) then (cd $name ; $start/$command) ; echo "end of directory $name" else echo "$name is a file" endifend

LIN 6932 25


Example: % chmod +x check ; check

check is a file

c is a file

end of directory la

b is a file

end of directory ny

a is a file

end of directory sf

LIN 6932 26

Del Script

#! /bin/csh# usage: del *foreach name ($argv) if ( -f $name ) then echo -n "delete the file '${name}' (y/n/q)?" else echo -n "delete the entire directory '${name}' (y/n/q)? " endif set ans = $< switch ($ans) case n: continue case q: exit case y: rm -r $name continue endsw end

LIN 6932 27

Del Script#! /bin/cshforeach name ($argv) # $argv shell variable for an argument of a command if ( -f $name ) then echo -n "delete the file '${name}' (y/n/q)?" else echo -n "delete the entire directory '${name}' (y/n/q)? " endif set ans = $< switch ($ans) case n: continue case q: exit case y: rm -r $name continue endsw end

LIN 6932 28

Del Script#! /bin/cshforeach name ($argv) if ( -f $name ) then #tests to see if the file whose name is in $name is an ordinary file, as opposed to a directory file. echo -n "delete the file '${name}' (y/n/q)?" else echo -n "delete the entire directory '${name}' (y/n/q)? " endif set ans = $< switch ($ans) case n: continue case q: exit case y: rm -r $name continue endsw end

LIN 6932 29

A note on File OperationsUsing the if command, filenames can be tested for the following:

-d filename true if filename is a directory -e filename true if filename exists -f filename true if filename is a text file -o filename true if you own filename -r filename true if filename is readable -w filename true if filename is writable -x filename true if filename is executable -z filename true if filename is empty

LIN 6932 30

Del Script#! /bin/cshforeach name ($argv) if ( -f $name ) then echo -n "delete the file '${name}' (y/n/q)?” #The echo -n prevents a skip to a new line; the -n option of echo tells the shell not to print the newline# character, so that our answer, y/n/q, will be on the same line.

else echo -n "delete the entire directory '${name}' (y/n/q)? " endif set ans = $< switch ($ans) case n: continue case q: exit case y: rm -r $name continue endsw end

LIN 6932 31

Del Scriptecho -n vs. echo

The echo utility writes any specified operands, followed by a newline

\n

character

The following option is available:

-n Do not print the trailing newline character \n.

LIN 6932 32

Del Script#! /bin/cshforeach name ($argv) if ( -f $name ) then echo -n "delete the file '${name}' (y/n/q)?” else echo -n "delete the entire directory '${name}' (y/n/q)? " endif

set ans = $< #the symbol $< means the input from the keyboard switch ($ans) case n: continue case q: exit case y: rm -r $name continue endsw end

LIN 6932 33

Del Script#! /bin/csh

foreach name ($argv)

if ( -f $name ) then

echo -n "delete the file '${name}' (y/n/q)?"

else

echo -n "delete the entire directory '${name}' (y/n/q)?"

endif

set ans = $<

switch ($ans) #Control flow is switched to where the first match occurs case n:

continue #go to the top of the enclosing loop

case q: exit

case y:

rm -r $name continue

endsw

end

LIN 6932 34

A note on SWITCH commandThe switch command takes the general form:

switch ($variable) # Starts a switch

case pattern1: # Defines a label in a switch command

action

breaksw

case pattern2:

action

breaksw

......

default :

action

endsw

LIN 6932 35

Del Scriptcase y:rm -r $name

# rm stands for "remove" and the -r option means recursively, if an argument of # rm -r is a directory, the rm -r command will remove that directory, and all the # files (and subdirectories, etc.) within it.# You can also use *, a wildcard for everything; rm will go through the current # working directory and remove all the files in it, and also through every subdirectory # and remove all the files in them. In short, # rm -r * deletes everything in the current directory and below.

unix lecture 5

Documents