User System Interaction Done via O'S' Commands

1
User System Interaction (Done via
O.S. Commands)

• File System Commands .
• Editor Invocation Commands.
• Process Creation Manipulation Commands.
• Control Flow Commands / Control Constructs.
• Derived Commands.
• User Utility Commands.

2
The Unix/ Linux Shell - 1 ( An Introduction)

• As soon as any UNIX/LINUX O.S. is
  installed the following things are also installed
  automatically
• a) The System Library (Helps to
  implement System Calls).
• b) At least one command interpreter
  /shell that helps any user to interact with the
  system. There are several types of shells
  normally in use Cshell (the oldest) , bash
  (Bourne Again Shell) which is the most powerful.
  We shall use bash shell.
• c) Some screen Editor like vi, emacs.

3
The Unix/ Linux Shell - 2 ( An Introduction)

• Shell ? Command Interpreter A Programming
  Language
• A Command Programming Language
• Provides terminal mode Interface between any
  user and the underlying LINUX/UNIX Operating
  System. This includes the following
• a) Helps user to issue any System
  Commands like ls, cat etc.
• b) Enables an user to make use of System
  Calls like fork().
• Enables an user to create of new tailor-made
  commands by suitably sequencing the existing
  commands and function calls through the use of
  some in built command language (The Shell
  Scripts).

4
The Unix/ Linux Shell ( The facilities)

• System Commands and their Interpretation along
  with Command Line Parameters.
• Two way communication is possible between the
  shell and commands.
• Following control flow constructs are available
  (to be illustrated later)
• a) Two Way branching like if-then-else.
• b) Multi Way branching like case switch.
• c) Looping constructs like for , while.
  
• String Valued Parameters like file names or
  flags can be passed to a command.
• A RETURN CODE is set by commands that may be used
  to determine
• future / subsequent control-flow.
• The standard output from a command may be used
  as Input to another shell command.
• Enables use of Variables String substitution.

5
The Unix/ Linux Shell ( The facilities) - 2

• The shell can modify the environment in which
  commands run.
• Input and Output can be redirected to files.
• Processes that communicates through pipes can be
  invoked.
• Commands are found by searching directories in
  the file system in a sequence that can be defined
  by the user.
• Commands can be read either from the terminal or
  from a user created file which allows command
  procedures to be stored for later use.
• Shell scripts can be executed with only the O.S.
  Installed.
• Shell commands / scripts are Interpreted i.e. as
  soon as a command is recognized , it gets
  executed . THEY ARE NOT COMPILED ? ( Why)?

6
A Few Simple Commands

• It consists of one or more words separated by
  blanks.
• Format
• lt Commandgt ltblankgtltArgument1gtltblankgtltArgument2gt
  .
• Display the current user who
• Display the current Directory pwd
• Display the contents of the current Directory
• ls ( Short Display)
• ls -l (Detailed Display)
• N.B Each of these commands creates a new process
  and waits for it to finish before any other
  command is taken up.

7
Back ground Commands

• Commands may be run without waiting for such
  background commands to finish . This actually
  forces sharing of the CPU time between the
  foreground and the background commands.
• e.g. cc myfile.c ? Background Directive
• This implies Compile the C Program Source myfile
  in the background. The Process Number of this
  created process is reported .
• The List of running processes can be obtained
  using the ps command.

8
Input Output Redirection

• ls l gt outfile ? Output the details Directory
  content to the existing file outfile after
  ERASING its current content OR create the file
  outfile and then write the Directory content
  there.
• ls l gtgt outfile ? The details of the current
  Directory is appended to the existing file
  outfile.
• wc -l lt infile ? The command wc l (Line
  counter) reads its standard input from the
  redirected input file infile.

9
Pipelines / Pipes

• ls l gt outfile wc -l lt outfile ?
• a) Output the details Directory content to
  the existing file outfile after ERASING its
  current content OR create the file outfile and
  then write the Directory content there.
• b) Then use the file outfile as a parameter
  to the line counter command wc to count the
  number of lines generated by the ls l command.
• Same effect can be achieved by the following
  combination of commands through pipes
• ls l gt wc -l lt outfile (?)
• Here is the pipe command that causes the
  output generated by ls l to be fed as input to
  the next command wc -l .
• N.B 1) Any number of processes/commands can be
  piped in this way. Each process / command will
  run in parallel. When the Pipe becomes empty then
  ALL the subsequent successor commands will halt.
  Consequently at any point of time whenever the
  Pipe becomes full the feeder processes are halted
  automatically.

10
Filters

• It is a command that reads its standard Input,
  transforms it in some way and prints the result
  as output.
• e.g. ls grep old ? Here grep is the filter
  that selects from its input (here the output
  generated by the ls command) those lines that
  contain the specified parameter string old
• who sort ? The filter sort here sorts its
  input here the logged in users.

11
Wild Cards File Name Generation

• ls l main.c ? Prints details about the file
  main.c
• ls l .c ? Prints details about ALL the C
  source files in the current directory.

12
Quoting Meta Characters ( Characters with a
special meaning)

• Any character preceded by a \ is quoted and loses
  its special meaning , if any. The \ is elided so
  that
• echo \? Will echo a single ?
• Quoting a string is achieved by enclosing within
  single quotes .
• Double quotes are also employed sometimes.

13
Changing the Prompt

• Default first prompt . Can be changed using
  PS1ltNew Promptgt
• Default Subsequent prompt after pressing newline
  (when more inputs are needed) happens to be gt
  which can be changed using PS2 ltNew Promptgt
• e.g. PS1CAOS2K7 will cause all the Subsequent
  prompts to appear as
• CAOS2K7

14
Shell and login

• After login if the user directory contains the
  file .profile then it is assumed to contain
  commands and is read by the shell before reading
  any commands from the terminal (?).

15
Shell Procedures(Reading Executing Commands
stored in a File)

• sh ltFile Name gt ltArgument Listgt
• The file myfile contains
• who grep 1
• Then it can be executed by
• sh myfile fred ? who grep fred
• Alternately one can make this file executable by
  using the command chmod x myfile
• Subsequently ./myfile fred ? sh myfile fred

16
Hello World!!!

• My first shell program hello.sh
• echo Hello world
• OR
• echo Hello world
• allows you to put spaces between the words
• To run the shell program
• sh hello.sh
• OR
• chmod x hello.sh
• ./hello.sh
• Note for printing quotes use the escape
  sequence \

17
Using Variables in Shell

• shell program using variables
• var.sh
• stringNice to meet you
• num5
• numexpr num 1 use back quotes for
  executing commands
• echo string num
• Variables have no type and need not be
  declared.
• Variables only exist within the scope of the
  program, unless specified otherwise.

18
Special Variables

• - number of parameters in the command line
• 1, .., 9 Nine command line parameters
• _at_ - all command line parameters
• shell program using command line parameters
  add.sh
• echo expr 1 2

19
If then.

• shell program showing If then structure if.sh
• if test 1 gt 2
• then
• echo 1
• else
• fi
• echo 2
• Common operators eq, -ne, -lt, -le, -gt, and
  ge

20
For loops

• shell program for loops for.sh
• for i in hello I am PSDEY
• do
• echo i done
• OR
• for i in
• do
• echo i done

21
While loops

• shell program while loop while.sh
• str
• while str ! quit
• do
• echo Please type in a string (quit to exit)
  
• read str
• echo Input String str
• done

22
While loops

• shell program for printing a file line by line
  line.sh
• file1
• while read f do
• echo f done lt file