LIN 6932 1 Unix Lecture 5 Unix Shell Scripts
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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
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Command Coordination ; && ||
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.
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Command Coordination ; && ||
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.
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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.
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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
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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 $USER and replaces it with the variable's value then executes the command.
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Setting Variables
Any programming language needs variables. You define a variable as follows:set Xwill set the variable X to have an empty list as its valueand refer to it as follows:$X$X is used to denote the value of the variable X
set X = dear
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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.
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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.
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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.
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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.
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Flow Control Statements Conditional
If … then … endif
# over100 -- checks if a number is greater than 100
if ( $1 > 100 ) then
echo ”That’s a big number!”
endif
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A note on Relational Operators
== equal to
!= not equal to
< less than
> greater than
<= less than or equal to
>= greater than or equal to
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Flow Control Statements Conditional
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
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Flow Control Statements Conditional
# 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.
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Flow Control Statements Conditional
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
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Flow Control Statements Conditional
#! /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 $2 are different, aren't they?"endif
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Flow Control Statements foreach ‘loop’
The syntax of foreach loop construct is
foreach var ( wordlist )
command(s)
end
The command(s) is executed once for each “word” (or item) in the wordlist, and each time the variable var will contain the value of that word.
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Flow Control Statements foreach ‘loop’
% vi marks16 - | - | - | - | - | - | - | - | - |Fred Dexter19 - | - | - | - | A | - | - | - | - |Waqas Younis16 A | A | A | B | - | A | A | A | B |David Gower16 - | - | - | - | A | - | - | - | - |Mickey Stewart20 A | A | A | A | A | A | A | A | A |Graham Thorpe
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Flow Control Statements foreach ‘loop’
% vi countmarks#!/bin/csh # counts the number of students associated with a number of # particular marks foreach number(0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20) set ms=`grep "^$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.
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Flow Control Statements foreach ‘loop’
% chmod +x countmarks ; countmarks
3 student(s) got 16 marks
1 student(s) got 19 marks
1 student(s) got 20 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.
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Flow Control Statements foreach ‘loop’
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.
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Flow Control Statements foreach ‘loop’
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
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Flow Control Statements foreach ‘loop’
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
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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 $name continue endsw end
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Del Script#! /bin/cshforeach name ($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 $name continue endsw end
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Del Script#! /bin/cshforeach name ($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 $name continue endsw end
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A note on File OperationsUsing the if command, filenames can be tested for the following:
-d filename true if filename is a directory -e filename true if filename exists -f filename true if filename is a text file -o filename true if you own filename -r filename true if filename is readable -w filename true if filename is writable -x filename true if filename is executable -z filename true if filename is empty
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Del Script#! /bin/cshforeach name ($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 $name continue endsw end
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Del Scriptecho -n vs. echo
The echo utility writes any specified operands, followed by a newline
\n
character
The following option is available:
-n Do not print the trailing newline character \n.
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Del Script#! /bin/cshforeach name ($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 $name continue endsw end
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Del Script#! /bin/csh
foreach name ($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 $name continue
endsw
end
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A note on SWITCH commandThe switch command takes the general form:
switch ($variable) # Starts a switch
case pattern1: # Defines a label in a switch command
action
breaksw
case pattern2:
action
breaksw
......
default :
action
endsw
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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.