The Shell and some useful administrative Unix Commands

1
The Shell and some useful administrative Unix
Commands

• How Unix works along with some additional, useful
  administrative Unix commands you might need to
  know.

2
UNIX Consistency

• The version of the UNIX kernel that is used on a
  machine is determined by the architecture of the
  hardware.
• Many of the same shells are found across all the
  versions of UNIX.
• All the versions of UNIX seem to be the same
  due to the identical interface.

3
Shells

• Bourne shell (sh)
• Only shell guaranteed to be on every version of
  UNIX
• Probably the fastest shell due to its lack of
  fanciness
• C - shell (csh)
• Uses C programming like syntax, but lacks a
  strong I/O component which makes scripting hard.
• Korn shell (ksh)
• Combines functionality of C-shell with the
  scripting strength of Bourne shell.
• T Shell (tcsh)
• Much better interface than C shell, but not
  uniformly available across all UNIX. Is on
  miller / grid

4
Shell Commands

• Command line UNIX is the most powerful interface,
  but the most difficult to master.
• Commands follow a general format
• command -options arguments
• command this is the actual command that you
  are sending to the shell to be executed.
• -options Options are also referred to as
  flags. They are usually preceded with a minus
  sign (-) Options modify the functionality of a
  command.
• arguments Used to tell the command what to
  work with or what to work upon.
• The spaces allow for the shell to parse the
  command.

5
How the shell works

• Shell displays a prompt.
• Example for miller miller.cs
• Example for alpha ( alpha ) n
• where n is the number of commands so far.
• 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.

6
Changing The Shell

• You can switch between which shell your using
  relatively easy.
• On miller / grid Simply type the shell that you
  want to use. Example miller.cs tcsh
• On alpha use the chsh command
• Example (alpha) 1 chsh
• Changing shell for SampleUser.
• Old shell /usr/local/bin/tcsh
• New shell csh

7
The 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.

8
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.
• eg. wc lt input.txt

9
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.
• eg cal 2007 gt calendar.file

10
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.
• eg. cal 2008 gtgt calendar.file

11
Grouping commands

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

12
(pipe)

• 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
  
• Example miller.cs who more

13
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 brackets () Tells the shell to match
  any characters that appear inside the brackets.

14
Job control

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

15
Job control

• ctrl-c cancels a command/job
• Good for breaking out of infinite loops!
• ctrl-z suspends a command/job
• ps displays the status of current processes
• Example miller.cs ps
• PID TT S TIME COMMAND
• 3979 pts/131 S 000 -csh
• 5629 pts/131 S 000 tcsh

16
bg

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

17
fg

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

18
kill

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