Script make backup of all file names

#!/bin/bash
FILES="$@"
for f in $FILES
do
        # if .bak backup file exists, read next file
 if [ -f ${f}.bak ]
 then
  echo "Skiping $f file..."
  continue  # read next file and skip cp command
 fi
        # we are hear means no backup file exists, just use cp command to copy file
 /bin/cp $f $f.bak
done

How to add two floating point numbers ?

You can make you og 'bc' command to do the arithmatic operations in shell script.
Following script illustrates a sample implimentation

a 1.2
b 3.1
x=(echo $a +$b|bc)
echo $x

How would you get the character positions 10-20 from a text file?

cat filename.txt | cut -c 10-20


:)


If you have a string "one two three", Which shell command would you extract the strings?


Input="one two three"
for var in $Input
do
       i=0;
       var.$i=$var
       i=$i+1
done
echo $i
for ( i=0; i< $i; i++)
do
      echo `var.$i`
done


Its a little big solution but thats just a way of playing with script ;)

IF IN SHELL SCRIPT

$ cat > elf # #!/bin/sh # Script to test if..elif...else # if [ $1 -gt 0 ]; then   echo "$1 is positive" elif [ $1 -lt 0 ] then   echo "$1 is negative" elif [ $1 -eq 0 ] then   echo "$1 is zero" else   echo "Opps! $1 is not number, give number" fi

Try above script as follows:
$ chmod 755 elf
$ ./elf 1
$ ./elf -2
$ ./elf 0
$ ./elf aHere o/p for last sample run:
./elf: [: -gt: unary operator expected
./elf: [: -lt: unary operator expected
./elf: [: -eq: unary operator expected
Opps! a is not number, give number
Above program gives error for last run, here integer comparison is expected therefore error like "./elf: [: -gt: unary operator expected" occurs, but still our program notify this error to user by providing message "Opps! a is not number, give number".

SHELL COMPARISONS, CONDITIONS, CHECKS

For Mathematics, use following operator in Shell Script

Mathematical Operator in  Shell Script	Meaning	Normal Arithmetical/ Mathematical Statements	But in Shell
			For test statement with if command	For [ expr ] statement with if command
-eq	is equal to	5 == 6	if test 5 -eq 6	if [ 5 -eq 6 ]
-ne	is not equal to	5 != 6	if test 5 -ne 6	if [ 5 -ne 6 ]
-lt	is less than	5 < 6	if test 5 -lt 6	if [ 5 -lt 6 ]
-le	is less than or equal to	5 <= 6	if test 5 -le 6	if [ 5 -le 6 ]
-gt	is greater than	5 > 6	if test 5 -gt 6	if [ 5 -gt 6 ]
-ge	is greater than or equal to	5 >= 6	if test 5 -ge 6	if [ 5 -ge 6 ]

NOTE: == is equal, != is not equal.

For string Comparisons use

Operator	Meaning
string1 = string2	string1 is equal to string2
string1 != string2	string1 is NOT equal to string2
string1	string1 is NOT NULL or not defined
-n string1	string1 is NOT NULL and does exist
-z string1	string1 is NULL and does exist

Shell also test for file and directory types

Test	Meaning
-s file	Non empty file
-f file	Is File exist or normal file and not a directory
-d dir	Is Directory exist and not a file
-w file	Is writeable file
-r file	Is read-only file
-x file	Is file is executable

Logical Operators

Logical operators are used to combine two or more condition at a time

Operator	Meaning
! expression	Logical NOT
expression1  -a  expression2	Logical AND
expression1  -o  expression2	Logical OR

FEW LINUX COMMANDS RELATED TO PROCESS

For this purpose	Use this Command	Examples*
To see currently running process	ps	$ ps
To stop any process by PID i.e. to kill process	kill    {PID}	$ kill  1012
To stop processes by name i.e. to kill process	killall   {Process-name}	$ killall httpd
To get information about all running process	ps -ag	$ ps -ag
To stop all process except your shell	kill 0	$ kill 0
For background processing (With &, use to put particular command and program in background)	linux-command  &	$ ls / -R | wc -l &
To display the owner of the processes along with the processes	ps aux	$ ps aux
To see if a particular process is running or not. For this purpose you have to use ps command in combination with the grep command	ps ax | grep  process-U-want-to see	For e.g. you want to see whether Apache web server process is running or not then give command$ ps ax | grep httpd
To see currently running processes and other information like memory and CPU usage with real time updates.	top See the output of top command.	$ top Note that to exit from top command press q.
To display a tree of processes	pstree	$ pstree

* To run some of this command you need to be root or equivalnt user.

HOW TO READ LINES OF DATA IN A SHELL SCRIPT?

There's a very easy way to solve this:

while read myline
do
  echo $myline
done < inputfile

If the fields in a given line are separated by a known delimiter, either a tab or a comma, for example, then I suggest that you could use the cut command to extract specific values.

To demonstrate, let's pull some useful data out of the /etc/passwd file, a file that has lines of data in known fields, separated with a ":" as the deilmiter. Here's a typical line of data-

unknown:*:99:99:Unknown User:/var/empty:/usr/bin/false

The first field (remember, they're separated by colons) is the account name, the second the encrypted password (not shown because it's in a separate 'shadow' file for security), then the remaining fields are account ID, group ID, full user name, home directory and login shell.
Let's just pull out login and full name to see what that looks like:

#!/bin/sh

while read inputline
do
  login=`(echo $inputline | cut -d: -f1)`
  fulln=`(echo $inputline | cut -d: -f5)`
  echo login = $login and fullname = $fulln
done < /etc/passwd

exit 0

You can see how the cut program makes this a straightforward task, albeit one that can be done more quickly in other scripting languages like Perl. But if you want to work with shell scripts, the combination of a while read loop with the input redirected and the great cutcommand should give you all the data parsing capabilities you need.

Shell Built in Variables

Shell Built in Variables	Meaning
$#	Number of command line arguments. Useful to test no. of command line args in shell script.
$*	All arguments to shell
$@	Same as above
$-	Option supplied to shell
$$	PID of shell
$!	PID of last started background process (started with &)

$? - (1) If return value is zero (0), command is successful.
(2) If return value is nonzero, command is not successful or some sort of error executing command/shell script.
This value is know as Exit Status.

How would you get the character positions 10-20 from a text file?

How would you get the character positions 10-20 from a text file?

cut -c10-20

Or

substring ${string_variable_name:starting_position:length}

Or

cat filename.txt | cut -c 10-20

How to read arguments in shell program

How to read arguments in shell program

#!/bin/sh
for i in $*
do
echo $i
done

$# returns the number of parameters that are passed to a shell script
$? returns the exit code of the last executed command (0 : Successful, 1 or other: Failed)




On executig the above script with any number of command-line arguments it will display all the parameters.