Shell Commands

1
Shell Commands

• Chapter 1.4

2
Shells

• What is a shell?
• Bourne shell
• Developed by Steve Bourne at ATT
• Korn shell
• Developed by David Korn at ATT
• C-shell
• Developed by Bill Joy for Berkeley Unix
• EZ-shell
• Developed by somebody at UWM.

3
How the shell works

• Shell displays a prompt.
• You type in a command.
• You press the return key.
• The shell interprets the commands you typed and
  tries to find the correct programs to run.
• The kernel runs the requested programs and
  returns the results to the shell.
• The shell displays the command prompt again.

4
Standard Input, Output and Error

• Standard input
• stdin
• The place the program normally looks for input.
• The keyboard.
• Standard output
• stdout
• The place where the program normally sends its
  output.
• The screen.

• Standard error
• stderr
• Used by programs to display error messages.
• Also the screen.

5
Redirection ,

• Redirects the standard input
• command
• The command will open the file and use its
  content as its source of input

6
Redirection ,

• Redirects the standard output.
• command file name
• The results of the command will be sent to the
  specified file.
• Will create or overwrite the destination file.
• cat june july aug summer2000

7
Redirection ,

• Also redirects the standard output.
• command file name
• The results of the command will be sent to the
  specified file.
• Will append the results of the command to the
  existing file.

8
Grouping commands

• Executing one command at at time can be tedious.
• Unix allows for grouping of commands by
  separating commands with a semi-colon ().
• pwd cal 1 2000 date

9
(pipe)

• Similar to redirection and grouping.
• Used to link commands.
• command command etc.
• The output of the first command is sent as the
  input to the second command, and so on, and so on
  
• who more

10
Wildcards

• Typing in Unix can be tedious.
• Unix supports three wild-card characters
• Asterisk () matches any string of characters
  including blanks.
• Question mark (?) matches single characters.
• Square brackest () Tells the shell to match
  any characters that appear inside the brackets.
• Quoting special characters

11
wc

• word count
• Used to display a word count of a file
• wc -c l w file name(s)
• The output you will see will be a line showing
  the number of lines, words and characters
• Limit display with the flags

12
sort

• Sorts the contents of a file
• sort -b f n r u file name(s)
• Takes the contents of a file and displays it in
  sorted order
• Flags
• -b ignores blanks
• -f folds upper- and lowercase letters together
• -n numeric sort
• -r reverse usual order
• -u prints duplicate entries only once

13
Job control

• Unix works via jobs or processes
• Every command or program is a separate
  job/process executed by a user
• Jobs are usually run in the foreground, but can
  be made to run in the background
• Jobs can be killed by the user who created them

14
Job control

• ctrl-c cancels a command/job
• ctrl-z suspends a command/job
• jobs
• Lists the jobs (programs) that you currently have
  running.

15
bg

• Forces a job to the background
• First, type a ctrl-z to suspend the job
• Then type bg and the job is forced to the
  background
• Use the jobs command to see it
• You can force a job to the background immediately
  with the

16
fg

• Brings a job to the foreground.
• Use the jobs command to see the jobs you have
  running.
• Type fg number and that job will be brought to
  the foreground.

17
kill

• Kills a job that you have running.
• Use the jobs command to see what you have
  running.
• Type kill number.
• Not the most graceful way out, but it works.