Advanced Unix Commands

1
Advanced Unix Commands

• Chapter 10 11

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 lt, gt, gtgt

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

6
Redirection lt, gt, gtgt

• gt
• Redirects the standard output
• command gt 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 gt summer2000

7
Redirection lt, gt, gtgt

• gtgt
• Also redirects the standard output
• command gtgt 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
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

9
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
• Though they are all on the same line, this is
  still 3 commands

10
Pipes Filters

• You can construct powerful Unix command lines by
  combining several Unix commands
• Unix commands alone are powerful, but when you
  combine them together, you can accomplish complex
  tasks with ease

11
(pipe)

• Similar to redirection and grouping combined
• Used to link commands together
• 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

12
Using a pipe

• A pipe sends the standard output of the command
  to the left of the pipe to the standard input of
  the command to the right of the pipe
• This is similar to the gt symbol used to redirect
  the standard output of a command to a file
• However, the pipe is different because it is used
  to pass the output of a command to another
  command, not a file

13
Using a filter

• A filter is a Unix command that does some
  manipulation of the text of a file
• Some simple filters include wc, sort more
• One of the most commonly used filters is grep

14
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

15
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

16
Here is an example

• alpha2 cat apple.txtcore worm seed
  jewelalpha2 cat apple.txt wc 2 4
  21alpha2
• After the first shell prompt, we see the contents
  of the file apple.txt
• In the next shell prompt, the cat command
  displays the contents of the applex.txt file
• The contents are displayed, not to the screen,
  but through a pipe to the wc (word count) command
• The wc command then does its job and counts the
  lines, words, and characters of its input

17
grep

• search for a string in a a file, display the line
  in which it appears
• alpha2 cat apple.txtcore worm seed
  jewelalpha2 grep jewel apple.txtseed jewel
• alpha2 cat apple.txt grep jewel seed jewel

18
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

19
Job control

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

20
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

21
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

22
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