scripting by ragavapatamata

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									                                    Ksh Scripting



                                   Principle of Script

Defining the Shell Type

To make a ksh script (which is a ksh program) crate a new file with a starting line
like:
#!/usr/bin/ksh
It is important that the path to the ksh is propper and that the line doesn not have
more than 32 characters. The shell from which you are starting the script will find
this line and and hand the whole script over to to ksh. Without this line the script
would be interpreted by the same typ of shell as the one, from which it was started.
But since the syntax is different for all shells, it is necessary to define the shell with
that line.

Four Types of Lines

A script has four types of lines: The shell defining line at the top, empty lines,
commentary lines starting with a # and command lines. See the following top of a
script as an example for these types of lines:

#!/usr/bin/ksh

# Commentary......

file=/path/file
if [[ $file = $1 ]];then
   command
fi

Start and End of Script

The script starts at the first line and ends either when it encounters an "exit" or the
last line. All "#" lines are ignored.

Start and End of Command

A command starts with the first word on a line or if it's the second command on a
line with the first word after a";'.
A command ends either at the end of the line or whith a ";". So one can put several
commands onto one line:

print -n "Name: "; read name; print ""


One can continue commands over more than one line with a "\" immediately followed
by a newline sign which is made be the return key:
grep filename | sort -u | awk '{print $4}' | \
uniq -c >> /longpath/file

Name and Permissions of Script File

The script mus not have a name which is identical to a unix command: So the script
must NOT be called "test"!
After saveing the file give it the execute permissions with: chmod 700 filename.




                                      Variables

Filling in

When filling into a variable then one uses just it's name: state="US" and no blanks.
There is no difference between strings and numbers: price=50.

Using

When using a variable one needs to put a $ sign in front of it: print $state $price.

Arrays

Set and use an array like:

arrname[1]=4             To fill in
print ${arraname[1]} To print out
${arrname[*]}            Get all elements
${#arrname[*]}           Get the number of elements

Declaration

There are happily no declarations of variables needed in ksh. One cannot have
decimals only integers.




                                      Branching

if then fi

if [[ $value -eq 7 ]];then
   print "$value is 7"
fi
or:

if [[ $value -eq 7 ]]
then
   print "$value is 7"
fi
or:

if [[ $value -eq 7 ]];then print "$value is 7";fi

if then else fi

if [[ $name = "John" ]];then
   print "Your welcome, ${name}."
else
   print "Good bye, ${name}!"
fi

if then elif then else fi

if [[ $name = "John" ]];then
   print "Your welcome, ${name}."
elif [[ $name = "Hanna" ]];then
   print "Hello, ${name}, who are you?"
else
   print "Good bye, ${name}!"
fi

case esac

case $var in
  john|fred) print $invitation;;
  martin) print $declination;;
  *)      print "Wrong name...";;
esac


                                     Looping

while do done

while [[ $count -gt 0 ]];do
 print "\$count is $count"
 (( count -= 1 ))
done

until do done

until [[ $answer = "yes" ]];do
 print -n "Please enter \"yes\": "
 read answer
 print ""
done

for var in list do done
for foo in $(ls);do
  if [[ -d $foo ]];then
     print "$foo is a directory"
  else
     print "$foo is not a directory"
  fi
done

continue...break

One can skip the rest of a loop and directly go to the next iteration with: "continue".

while read line
do
 if [[ $line = *.gz ]];then
    continue
 else
    print $line
 fi
done

One can also prematurely leave a loop with: "break".

while read line;do
 if [[ $line = *!(.c) ]];then
    break
 else
    print $line
 fi
done


                             Command Line Arguments

(Officially they are called "positional parameters")

The number of command line arguments is stored in $# so one can check
for arguments with:

if [[ $# -eq 0 ]];then
   print "No Arguments"
   exit
fi

The single Arguments are stored in $1, ....$n and all are in $* as one string. The
arguments cannot
directly be modified but one can reset the hole commandline for another part of the
program.
If we need a first argument $first for the rest of the program we do:

if [[ $1 != $first ]];then
     set $first $*
fi

One can iterate over the command line arguments with the help of the shift
command. Shift indirectly removes the first argument.

until [[ $# -qe 0 ]];do
 # commands ....
 shift
done

One can also iterate with the for loop, the default with for is $*:

for arg;do
  print $arg
done

The program name is stored in $0 but it contains the path also!




                                     Comparisons

To compare strings one uses "=" for equal and "!=" for not equal.
To compare numbers one uses "-eq" for equal "-ne" for not equal as well as "-gt" for
greater than
and "-lt" for less than.

if [[ $name = "John" ]];then
   # commands....
fi
if [[ $size -eq 1000 ]];then
   # commands....
fi

With "&&" for "AND" and "||" for "OR" one can combine statements:

if [[ $price -lt 1000 || $name = "Hanna" ]];then
   # commands....
fi
if [[ $name = "Fred" && $city = "Denver" ]];then
   # commands....
fi


                               Variable Manipulations

Removing something from a variable

Variables that contain a path can very easily be stripped of it: ${name##*/} gives
you just the filename.
Or if one wants the path: ${name%/*}. % takes it away from the left and # from
the right.
%% and ## take the longest possibility while % and # just take the shortest one.

Replacing a variable if it does not yet exits

If we wanted $foo or if not set 4 then: ${foo:-4} but it still remains unset. To
change that we use:
${foo:=4}

Exiting and stating something if variable is not set

This is very important if our program relays on a certain vaiable: ${foo:?"foo not
set!"}

Just check for the variable

${foo:+1} gives one if $foo is set, otherwise nothing.




                              Ksh Regular Expressions

Ksh has it's own regular expressions.
Use an * for any string. So to get all the files ending it .c use *.c.
A single character is represented with a ?. So all the files starting with any sign
followed bye 44.f can be fetched by: ?44.f.

Especially in ksh there are quantifiers for whole patterns:

?(pattern) matches zero or one times the pattern.
*(pattern) matches any time the pattern.
+(pattern) matches one or more time the pattern.
@(pattern) matches one time the pattern.
!(pattern) matches string without the pattern.

So one can question a string in a variable like: if [[ $var = fo@(?4*67).c ]];then ...




                                       Functions

Description

A function (= procedure) must be defined before it is called, because ksh is
interpreted at run time.
It knows all the variables from the calling shell except the commandline arguments.
But has it's
own command line arguments so that one can call it with different values from
different places in
the script. It has an exit status but cannot return a value like a c funcition can.
Making a Function

One can make one in either of the following two ways:

function foo {
  # commands...
}

foo(){
  # commands...
}

Calling the Function

To call it just put it's name in the script: foo. To give it arguments do: foo arg1 arg2
...
The arguments are there in the form of $1...$n and $* for all at once like in the main
code.
And the main $1 is not influenced bye the $1 of a particular function.

Return

The return statement exits the function imediately with the specified return value as
an exit status.




                                  Data Redirection

General

Data redirection is done with the follwoing signs: "> >> < <<". Every program has
at least a

standardinput, standardoutput and standarderroroutput. All of these can be
redirected.

Command Output to File

For writing into a new file or for overwriting a file do: command > file

For appending to a file do: command >> file

Standard Error Redirection

To redirect the error output of a command do: command 2> file

To discard the error alltogether do: command 2>/dev/null

To put the error to the same location as the normal output do: command 2>&1
File into Command

If a program needs a file for input over standard input do: command < file

Combine Input and Output Redirection

command < infile > outfile
command < infile > outfile 2>/dev/null

Commands into Program ( Here Document )

Every unix command can take it's commands from a text like listing with:

command <<EOF
input1
input2
input3
EOF

From eof to eof all is feeded into the above mentioned command.




                                         Pipes

For a serial processing of data from one command to the next do:
command1 | command2 | command3 ...
e.g. last | awk '{print $1}' | sort -u.




                                     Coprocesses

One can have one background process with which one can comunicate with read -
p and print -p. It is started with command |&. If one uses: ksh |& then this shell in
the background will do everything for us even telnet and so on: print -p "telnet
hostname".




                       Read Input from User and from Files

Read in a Variable

From a user we read with: read var. Then the users can type something in. One
should first print something like: print -n "Enter your favorite haircolor: ";read var;
print "". The -n suppresses the newline sign.

Read into a File Line for Line
To get each line of a file into a variable iteratively do:

{ while read myline;do
  # process $myline
done } < filename

To catch the output of a pipeline each line at a time in a variable use:

last | sort | {
while read myline;do
  # commands
done }


                                    Special Variables

$# Number of arguments on commandline.
$? Exit status of last command.
$$ Process id of current program.
$! Process id of last backgroundjob or background function.
$0 Program name including the path if started from another directory.
$1..n Commandline arguments, each at a time.
$* All commandline arguments in one string.




                  Action on Success or on Failure of a Command

If one wants to do a thing only if a command succeded then: command1 &&
command2. If the second command has to be performed only if the first one failed,
then: command1 || command2.




                                  Trivial Calculations

Simpe calculations are done with either a "let" in front of it or within (( ... )). One
can increment a variable within the (( )) without a "$": (( a+=1 )) or let a+=1.




                         Numerical Calculations using "bc"

For bigger caluculations one uses "bc" like: $result=$(print
"n=1;for(i=1;i<8;i++)n=i*n;n"|bc)




                                          "grep"
Search for the occurence of a pattern in a file: grep 'pattern' file. If one just wants to
know how often soemthing occurs in a file, then: grep -c 'pattern file. This can be
used in a script like:
if [[ $(grep -c 'pattern' file) != 0 ]];then ......;fi. The condition is fullfilled if the
pattern was found.




                                          "sed"

Sed means stream line editor. It searches like grep, but is then able to replace the
found pattern. If you want to change all occurences of "poor" with "rich", do: sed -e
's/poor/rich/g' filename. Or what is often seen in software packages, that have to be
compiled after getting a propper configuration, is a whole file stuffed with
replacements patterns like: /@foo@/s;;king;g. This file with inumerable lines like
that has to be given to sed with: sed -f sedscript filename. It then precesses each
line from file with all the sed commands in the sedscript. (Of course sed can do much
more:-))




                                         "awk"

Awk can find and process a found line with several tools: It can branch, loop, read
from files and also print out to files or to the screen, and it can do arithmetics.
For example: We have a file with lines like: Fred 300 45 70 but hundreds of them.
But some lines have a "#" as the first sign of them and we have to omit these ones
for both, processing and output. And we want to have lines as output like: 415
Fredwhere 415 is the sum of 300, 45 and 70. Then we call on awk:

awk '$1 !~ /^#/ && $0 ~ /[^ ]/ {print $2+$3+$4,"\t",$1}' filename.

This ignores lines with a "#" at the beginning of the first field and also blank lines. It
then prints the desired sum and the $1 ist only printed after a tab. This is the most
trivial use of awk only.

       Check my AWK programming introduction bye clicking on this sentence!

                                     "perl"




Perl is a much richer programming language then ksh, but still one can do perl
commands from within a ksh script. This might touch Randal, but it's true. Let's say
you want to remove all ^M from a file, then take perl for one line in your ksh script:

perl -i -ep 's/\015//g' filename.

Perl can do an infinite amount of things in many different ways. For anything bigger
use perl instead of a shell script.

								
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