fortypoundhead.com

Interactive Use of the Unix Shell

Primary Category = Tutorial

Posted On 2005-11-1 by FortyPoundHead
Keywords: Unix Tutorial Part 13
Tags: Tutorial Tip Linux
Views: 1430
Rating: / 5.00

  • 1
  • 2
  • 3
  • 4
  • 5

 



Section 13: Interactive Use of the Shell



This section discusses tips and tricks to make your use of the shell
more efficient.



File name completion



Both ksh and csh will perform file name completion for you. You can
type in a partial file name, and press the ESCAPE key (once for csh,
twice for ksh). The shell will then complete the name of the file for
you. If no file exists that begins with the characters you typed, the
shell will beep at you, and will not complete the name. If more than
one file begins with the characters you typed, the shell will complete
the name up to the point where the names differ. Then you can type
additional letters to specify the file name you want, reusing the
ESCAPE key if desired.

Command name aliasing



Both csh and ksh provide command name aliasing, to allow you to rename
commands. Aliasing can save a lot of keystrokes if you must
frequently issue the same lengthy command. The alias command requires
two pieces of information: The command you wish to alias, and the
alias you wish to use to refer to it.



EXAMPLE: To alias the "history" command to "hi" you could use
the following command in the Korn shell:
alias
hi='history'
After entering that alias, you could type the
command "hi" and the shell would substitute "hi" with the string
"history" before executing it. The same command could be accomplished
in the C shell with the syntax:
alias hi history




EXERCISE: Create an alias in the Korn shell called "clean" that
would remove any files from your home directory that have the
extension .gif or .jpg.



EXPLANATION: The command
alias clean='rm ~/*.gif; rm
~/*.jpg'
would work.



Command aliasing can be tricky. Surround the alias string with single
quotes (') to prevent the shell from interpreting special characters.
If you use command history substitution in an alias, use the backslash
character (\) to escape characters that you don't want the shell to
interpret.




EXAMPLE: This example, written for the C shell, creates an
alias for the cd command, so that it stores the current location in a
shell variable called old before it changes to the new location. It
also creates a new command alias called back that allows us to go back
to the previous location:
alias cd 'set old=$cwd; chdir
\!*; pwd'
alias back 'set foo=$old; cd $foo; unset foo'




There are several things to note in the above example. The alias for
cd has three parts: The first reads the current working directory from
the shell variable cwd, and saves it in a shell variable called old.
The second part uses history substitution and chdir to change the
current location. The use of chdir prevents an "aliasing loop," where
the cd command calls itself. The third part executes the pwd command
to print the new location on the screen.



The alias for back also has three parts: The first part reads the
previous location from the shell variable old, and stores it in a
shell variable called foo. That is necessary because the new cd alias
will change the value of old when we call it in the second part of the
back alias. The third part cleans up our mess by unsetting the
variable foo, removing it from the environment.



You can remove an alias using the unalias command. To remove the
"clean" alias you created in a previous exercise, enter the command:
unalias clean




Command history substitution



The C shell and Korn shell will keep an ordered list of the commands
you have issued, and allow you to retrieve commands from the list.
That facility, called command history substitution, makes it
possible to reuse all or part of your previously issued commands.
Each command on the list is given a command number, according to the
order it was issued. You can view the command history list by issuing
the command:
history
The exact mechanism of
retrieving commands from the command history list depends on the shell
you're using, and how you have customized your shell.




ksh




When using the Korn shell, the number of commands remembered by the
shell is controlled by the HISTSIZE environment variable. Use the
command
HISTSIZE=50;export HISTSIZE
to set the
length of the history list to fifty. By default, the history size is
set to 128 lines.



The shell command "set -o" is used to specify the editing mode for the
command line, either emacs or vi. Since an earlier section of this
workshop dealt with emacs, we will confine our discussion to the emacs
editing style. To use the emacs editing mode, enter the command
set -o emacs




In emacs editing mode, recall previous commands with the emacs command
for "previous line," or Control-P. Repeated use of Control-P will
recall earlier commands. You can also use the emacs command for "next
line," or Control-N, to go forward through your command history,
toward more recently-issued commands. You can only use Control-N
after you have used Control-P at least once.



csh


The C shell allows you to recall previous commands in whole or in
part. In the C shell, the history shell variable is used to specify
the number of lines the shell will remember. The statement
set history=60
will cause the C shell to
remember sixty commands.



To recall previous commands from the history list, the C shell uses
the exclamation point (!) character, sometimes referred to in computer
jargon as "bang." The bang character can be used in combination with
history line numbers, and text patterns. Here are some examples of
how to use history substitution in the C shell:




Recall the last command:
!!

Recall the third most recent command:
!-3

Recall command number ten from the history list:
!10

Recall the last command that began with the letters "ls":
!ls





You can also recall specific pieces of previous commands, and use them
to create new commands. The colon character is used to select
specific words from a command. Each word in the command is referred
to by position. The command name itself is item number zero. Here
are some examples:



Recall the third word from the last command:
!:2

Perform an "ls" on the second word from command number 8:
ls !8:1

Use more to view the last item from command number ten:
more !10:$



Editing the command line



The Korn shell (ksh) provides the ability to edit the command history
list almost as if your were in an editor program like vi, or emacs.
The current command is always the last line in the history list, and
begins blank. You can type in a new command, or recall an earlier
command from the list. You can also modify the text on the current
line using basic text editor commands.



The mechanism for setting the editor style in ksh is the "set -o"
command. To edit in emacs mode, issue the command:
set -o
emacs




Manipulating command line text in emacs mode is done in much the same
way as text editing with emacs. When you finish editing the command
line, press the return key to issue the command to the shell. You
might want to go back and refresh your memory on emacs by reviewing
section ten of this tutorial, titled "Text Editing with Emacs."




About the Author

FortyPoundHead has posted a total of 1975 articles.

You can find more information from FortyPoundHead by visiting .

Comments On This Post

No comments on this post yet!

 

Do you have a thought relating to this post? You can post your comment here. If you have an unrelated question, you can use the Q&A section to ask it.

Or you can drop a note to the administrators if you're not sure where you should post.


Your IP address is:54.163.159.27

Before you can post, you need to prove you are human. If you log in, this test goes away.