Some simple examples First, second, third(with errors) third2

echo Command Use echo command to display text or value of variable. echo [options] [string, variables...] Displays text or variables value on screen. Options -n Do not output the trailing new line. -e Enable interpretation of the following backslash escaped characters in the strings: \a alert (bell) \b backspace \c suppress trailing new line \n new line \r carriage return \t horizontal tab \\ backslash

Example $ echo -e "An apple a day keeps away \a\t\tdoctor\n"

More about echo Display colorful messages echo “Hello, world!” echo -e "\033[34m Hello Colorful World."

That echo statement uses ANSI escape sequence (\033[34m), above entire string ( i.e. "\033[34m Hello Colorful World." ) is process as follows 1) First \033, is escape character, which causes to take some action. 2) Here it set screen foreground color to Blue using [34m escape code. 3) Then it prints our normal message Hello Colorful World! in blue color.

If we want to display that echo statement in red echo -e "\033[31m Hello Colorful World.“ In general: Syntax echo -e "\033[escape-code your-message" More detail, see file cs315\Echo_sequence

Shell Arithmetic Use to perform arithmetic operations. Syntax: expr op1 math-operator op2 Examples: $ expr $ expr $ expr 10 / 2 $ expr 20 % 3 $ expr 10 \* 3 $ echo `expr 6 + 3`

Double quote, single quote, and back quote echo “Today is `date`” Double quote remove the meaning of the characters except “$” and “\”. echo ‘Today is `date`’ Enclosed in single quotes remains unchanged echo `date` To execute command

The read Statement Use to get input (data from user) from keyboard and store (data) to variable. Syntax: read variable1, variable2,...variableN

Example # #Script to read your name from key-board # echo "Your first name please:" read fname echo "Hello $fname, Lets be friend!"

More Unix Commands nohup + command will make it immune to the hanup and terminate signal. Example: nohup sleep 30& kill + pid to kill a process wait + pid will wait the process to finish. wait – without pid will wait all its child processes to finish.

env command Displays your UNIX environment variables. env enter

Variable assignment Variables are assigned in a script program as follows: DONE=no They are used in this manner: while [ $DONE = no ] in your script

looping Example: looping –week13

#! /bin/bash DONE=no ENTRIES="hello bye ls 1" while [ $DONE = no ] do echo Valid entries are: $ENTRIES read ENTRY # Read the variable ENTRY from the user case $ENTRY in 1) pwd ;; hello) echo How are you? ;; bye) echo exiting... DONE=yes ;; ls) ls -al |more ;; *) echo $ENTRY is an unrecognized command. ;; esac done

/bin and Running Shell Script To run a script, we can: 1../script_Name 2. script_Name 3. sh script_Name In fact, we can run the script with some other ways.

The difference between. Script_Name and./script_Name You can also run a script by using: bash script_Name enter The bash expects input from a file, you do not need execute permissions; the same is true for. Script_Name

Create a bin directory mkdir bin under your home directory. From your current directory, Type: cp script_name../bin to copy the script into /bin directory. Or, cp *../bin to copy all your files to the bin directory. Now, you can run them directly by type in the script_Name.

Example Run looping script under current directory cp looping../bin enter Type looping enter This time, you can type in the script name and run it directly.

chsh –Change Shell chsh enter Type in your password enter Type /bin/ksh will change your login shell to Korn shell

What Is Korn Shell? –The Korn shell (/bin/ksh) is the most advanced 'standard' UNIX shell. It extends the Bourne shell with lots of nice features, and is a lot faster.

Korn Shell It was written by David Korn. It is a powerful superset of the Bourne shell. The improvements include : 1. Job control 2. Command line editing 3. Programming features.

Korn Shell Korn is superset of Bourne. zsh claims to be an enhanced Korn shell, while bash has added some parts of it

Aliases in Korn shell The corn shell allows you to create your own commands by using the alias command. Example: $ alias d= `ls –l` in standard Unix Example in In our Linux alias l=ls [Enter] alias AA=“cal 2004”

Exception in using alias in Korn All built-in commands may be aliased except for: case, do, done, elif, else, esac, fi, for, function, if, select, then, time, until, while, {,}

Remove an alias-unalias The unalias will remove all of the specified aliases. format: unalias aliasName [Enter]

Functions in Korn shell Korn shell allows one to define functions that may be invoked as shell commands. Two basic format to define a function in Korn shell: 1. Function name { list }

Function --- cont. 2. Name ( ) { list } This format looks similar to C language format.

Example f ( ) { echo parameter l = $1 echo parameter list = $* } # main program f l f cat dog goat (week13 func6)

Enhanced job control Command : jobs --- Display current jobs bg %job# --- resumes the specified job as a background process. fg %job# --- resumes the specified job as the foreground process. kill

TILDE “~” substitution ~ --- $HOME ~user --- home directory of user ~/pathname --- $HOME pathname ~+ --- $PWD (current working directory) ~- --- $OLDPWD (previous working directory)

More let x=y+z written as ( ( x = y + z )) select - allows use of simple menus functions can be written with parameters f( ) { echo $1 $2 $3 } called by f cat dog cow

Kermel 1. The kernel itself is not a process but a process manager. 2. System calls are some specific program constructs that perform the kernel service required by processes. 3. Each system call sets up the group of parameters that identifies the process request and then executes the hardware- dependent CPU instructions to switch from User Mode to Kernel Mode.

Kernel thread---Privileged processes Besides user processes, Unix systems include a few privileged processes ---Kernel thread. 1. They run Kernel Mode in the kernel address space. 2. They do not interact with users, and thus do not require terminal devices. 3. They usually created during startup and remain alive until the system is shut down.

Transition between User Mode and Kernel Mode User Mode Kernel Mode. Process1 Process2 System callTimer interrupt Device interrupt System call handler Scheduler Interrupt handler

Process Implementation To let the kernel manage processes, each process is represented by a process descriptor that includes information about the current state of the process. When kernel stops the execution of a process, it saves the current contents of several processor registers in the process descriptor. These include: PC, SP, General-purpose registers, floating point registers, processor control registers, and the memory management registers.

Device Drivers The kernel interacts with I/O devices by means of device drivers and each driver interacts with the remaining part of the kernel (even with other drivers) through a specific interface. The advantages are: 1. Device-specific code can be encapsulated in a specific module. 2. Vendors can add new devices without knowing the kernel source code: only the interface specifications must be know. 3. The kernel deals with all devices in a uniform way and accesses them through the same interface. 4. Dynamically load and unload device drivers to minimize the kernel image stored in RAM.

Kernel

Device Driver Interface tty TapeDisk System call interface Virtual File System Character device filesBlock device files Kernel tty driver Tape driver Disk driver P P PP Disk

Kernel --- cont.

Kernel, hardware, and software. Kernel Peripheral Process System calls Hardware interrupt

User Mode and Kernel Mode Normally, when a user process is running, it operates in a special machine mode called user mode. The only way for a user process to enter kernel mode is to execute a system call.

System calls. Open a file open Close a file close Perform I/O read/write Send a signal kill Create a pipe pipe Create a socket socket Duplicate a process fork Terminate a process exit

Hardware interrupt. Interrupt vector table Highest priority Lowest priority Hardware errors Clock Disk I/O Keyboard Traps (software interrupt) Pointers to kernel Interrupt Handlers

Interrupt can be interrupted! Because of the priority, a lower priority interrupt can be interrupted by a higher priority interrupt.

Screen saver example # name: lock # function: lock the screen until a correct password is entered # clear echo -e "\n\nEnter your password>" read pword_1 clear echo -e "\n\n This system is locked..... " pword_2= until [ "$pword_1" = "$pword_2" ] do read pword_2 done clear echo "The password you typed in is correct! You can use the computer now!“ week14 lock1

Problems with the lock1 1. Password being displayed while enter it. 2. There are some ways one can enter the system without password. Such as ctrol+c … The reason is it can cause an interrupt to the kernel and the kernel knows about the device. The kernel interrupt has higher priority than your process.

Some of the shell signals Signal # Name Meaning 1 hang up Terminal connection lost 2 interrupt One of the interrupt key 3 quit One of the quit key 9 kill The kill –9 has been issued 15 terminator The kill command has been issued.

trap command We can use the trap command to disable the signals created by the kernel. Basically, it change the process default action to whatever you specify. Example: trap “optional commands” signal numbers Trap “echo I refuse to die!” 15 display the message instead of terminate the process. (week14 lock3 use the trap command)

Trap cont… If you write trap “ “ [signal] numbers The system just ignore the signals.