Setting priority on new processesAt this point you are probably wondering how you can set your own priority levels on processes. To changethe prioritywhen issuing a new command you do nice -n [nice value] [command]:nice -n 10 apt-get upgradeThis will increment the default nice value by a positive 10 for the command, apt-get upgrade This is often useful for times when you want to upgrade apps but dont want the extra process burden at the given time. Remember a positive number is gives less priority for a process.Setting Priority on Existing ProcessesObviously at some point you are going to want to alter the nice value of something that is already running. Just the other day I was doing an upgrade to Ubuntu Jaunty and Firefox started to become unusably slow. A quick renice command rescheduled the upgrade to a lower priority and Iwas back to surfng in no time.To change the priority of an existing process just do renice [nice value] -p [process id]:renice 10 -p 21827This will increment the priority of the process with an id of 21827 to 10.Note: Only root can apply negative nice values.Shell scripting: The shell provides you with an interface to the UNIX system. It gathers input from you and executes programs based on that input. A shell is an environment in which we can run our commands, programs, and shell scripts. There are diferent favors of shells.Shell prompt : The prompt, $, which is called command prompt, is issued by the shell. While the prompt is displayed, you can type a command. For eg: $ dateCase -esac programinshell scripting :Syntax :case word inpattern1) Statement(s) to be executed if pattern1 matches ;;pattern2) Statement(s) to be executed if pattern2 matches;;pattern3) Statement(s) to be executed if pattern3 matches;;EsacExample 1 :echo Enter any value between 1 to 5read numcase $num in1)echo ONE ;;2)echo TWO;;3)echo THREE;;4)echo FOUR;;5)echo FIVE;;*)echo INVALID VALUE;;esacNote:* indicates default value.If else :Comparisons: -eqequal to-nenot equal to-ltless than-leless than or equal to-gtgreater than-gegreater than or equal toIf -elseprogram :Syntax :if [ expression ]then Statement(s) to be executed if expression is trueelse Statement(s) to be executed if expression is not truefExample 2 :valid_password=welcomeEcho Enter your passwordreadPSWDif [ $PSWD = $valid_password ]thenecho Valid passwordelseecho Access deniedfExample 3 :Echo Enter the numberRead numIf test$num le 30Thenecho Number is less than equal to 30elseecho Number is greater than 30fif then elif :Syntax :if [ expression 1 ]then Statement(s) to be executed if expression 1 is trueelif [ expression 2 ]then Statement(s) to be executed if expression 2 is trueelif [ expression 3 ]then Statement(s) to be executed if expression 3 is trueelse Statement(s) to be executed if no expression is truefExample 4 :echo enter the value of aread aecho enter the value of bread bif [ $a = $b ]thenecho a is equal to belif [ $a gt $b ]thenecho a is greater than belif [ $a lt $b ]thenecho a is less than belseecho INVALID VALUEfWhileloop :Syntax :while commanddo Statement(s) doneExample 5 :echo Enter the value of aread awhile [ $a -lt 5 ]do a=`expr $a + 1` echo $adoneForloop :Syntax :for (( EXP1; EXP2; EXP3 ))docommand1command2command3doneExample 6 :for((c=1;cReturns True, if both condition1 and condition2 are True (Like AND operator)|| -------- >Returns True, If any one condition is True (Like OR operator)Boolean Operators:There are following boolean operators supported by BourneShell.Assume variable a holds 10 and variable b holds 20 then:Operator Description Example!This is logical negation. This inverts a true condition into false and vice versa.[ ! false ] is true. -oThis is logical OR. If one of the operands is true then condition would be true.[ $a -lt 20 -o $b -gt 100 ] is true. -a This is logical AND. If both [ $a -lt 20 -a $b -gt the operands are true then condition would be true otherwise it would be false.100 ] is false. Example 11: (For || operator)echo Enter the value of aread aecho Enter the value of bread bif (( a == 1 )) | | (( b == 8 ));thenecho Enable!!!elseecho DisablefFILETESTOPERATORS :The diferent fle test operators are listed below:a : True if the fle exists.b : True if the fle exists and is a block special fle.c : True if the fle exists and is a character special fle.d : True if the fle exists and is a directory.e : True if the fle exists.f : True if the fle exists and is a regular fle.g : True if the fle exists and its SGID bit is set.h : True if the fle exists and is a symbolic link.k : True if the fle exists and its sticky bit is set.p : True if the fle exists and is a named pipe (FIFO).r : True if the fle exists and is readable.s : True if the fle exists and has a size greater than zero.t : True if fle descriptor is open and refers to a terminal.u : True if the fle exists and its SUID (set user ID) bit is set.w : True if the fle exists and is writable.x : True if the fle exists and is executable.O : True if the fle exists and is owned by the efective user ID.G : True if the fle exists and is owned by the efective groupID.L : True if the fle exists and is a symbolic link.N : True if the fle exists and has been modifed since it waslast read.S : True if the fle exists and is a socket.Syntax : (mostly used in if-loop)if [ -option flename ]thendo somethingelsedo somethingfExample 12 :(bud.sh is an already existing fle)fle=bud.shIf [-r $fle ] (similarly, for write if [ -w $fle ] and for execute if [ -x $fle ] )ThenEcho Read accessElseEcho Read permission deniedfExample13 :File=directory1If [ -d $fle ]ThenEcho Directory is presentElseEcho Directory is not presentfDebugging Shell Scripts:-x option to debug a shell scriptRun a shell script with -x option.$ Bash -x script-name$ bash -x domains.shUse of set built in commandBash shell ofers debugging options which can be turn on or of using set command.Set -x:Display commands and their arguments as they are executed.Set -v: Display shell input lines as they are read.How to execute:Bash x flename.shBash v flename.sh