Some simple examples
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Transcript of Some simple examples
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:
• Syntaxecho -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 1 + 3$ expr 2 - 1$ 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 fnameecho "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 Process1 Process2 Process2
System call Timer interrupt
Deviceinterrupt
System callhandler
SchedulerInterrupt 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
Kernel Basics
• 1. Sharing the CPU and RAM between competing progresses
• 2. Processing all system calls• 3. Handling peripherals• It mostly written in C, some parts of the
kernel are written in assembly language• Users only interact with the kernel by
system calls interface
Device Driver Interface
tty tty Tape Disk
System call interface
Virtual File System
Character device files Block device filesKernel
ttydriver
Tapedriver
Disk driver
P P PP
Disk
Kernel --- cont.
Kernel Subsystems
• 1. Memory Management
• 2. Process Management
• 3. Inter-process Communication
• 4. Input/Output
• 5. File Management
Kernel, hardware, and software• .
Kernel
Peripheral Peripheral Peripheral
Process Process 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
0
1
2
3
4
Highestpriority
Lowestpriority
Hardware errors
Clock
Disk I/O
Keyboard
Traps (software interrupt)
Pointers to kernelInterruptHandlers
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.