Unix Introduction Part 3 & 4

            UNIX Tutorial Three

   3.1 Redirection  

Most processes initiated by UNIX commands write to the standard output (that is, they write to the terminal screen), and many take their input from the standard input (that is, they read it from the keyboard). There is also the standard error, where processes write their error messages, by default, to the terminal screen.

We have already seen one use of the cat command to write the contents of a file to the screen.

Now type cat without specifying a file to read

       % cat

Then type a few words on the keyboard and press the [Return] key.

Finally hold the [Ctrl] key down and press [d] (written as ^D for short) to end the input.

What has happened?

If you run the cat command without specifying a file to read, it reads the standard input (the keyboard), and on receiving the 'end of file' (^D), copies it to the standard output (the screen).

In UNIX, we can redirect both the input and the output of commands.

   3.2 Redirecting the Output  

We use the > symbol to redirect the output of a command. For example, to create a file called list1 containing a list of fruit, type  

       % cat > list1

Then type in the names of some fruit. Press [Return] after each one.

       pear
       banana
       apple
       ^D {this means press [Ctrl] and [d] to stop}

What happens is the cat command reads the standard input (the keyboard) and the > redirects the output, which normally goes to the screen, into a file called list1

To read the contents of the file, type

       % cat list1

      Exercise 3a

Using the above method, create another file called list2 containing the following fruit: orange, plum, mango, grapefruit. Read the contents of list2

    3.2.1 Appending to a file

The form >> appends standard output to a file. So to add more items to the file list1, type

       % cat >> list1

Then type in the names of more fruit

       peach
       grape
       orange
       ^D (Control D to stop)

To read the contents of the file, type

       % cat list1

You should now have two files. One contains six fruit, the other contains four fruit.

We will now use the cat command to join (concatenate) list1 and list2 into a new file called biglist. Type

      % cat list1 list2 > biglist

What this is doing is reading the contents of list1 and list2 in turn, then outputing the text to the file biglist

To read the contents of the new file, type

       % cat biglist

   3.3 Redirecting the Input  

We use the < symbol to redirect the input of a command.

The command sort alphabetically or numerically sorts a list. Type

       % sort

Then type in the names of some animals. Press [Return] after each one.

      dog
      cat
      bird
      ape
      ^D (control d to stop)

The output will be

       ape
       bird
       cat
       dog

Using < you can redirect the input to come from a file rather than the keyboard. For example, to sort the list of fruit, type

      % sort < biglist

and the sorted list will be output to the screen.

To output the sorted list to a file, type,

      % sort < biglist > slist

Use cat to read the contents of the file slist

    3.4 Pipes

To see who is on the system with you, type

      % who

One method to get a sorted list of names is to type, 

      % who > names.txt
      % sort < names.txt

This is a bit slow and you have to remember to remove the temporary file called names when you have finished. What you really want to do is connect the output of the who command directly to the input of the sort command. This is exactly what pipes do. The symbol for a pipe is the vertical bar |

For example, typing

      % who | sort

will give the same result as above, but quicker and cleaner.

To find out how many users are logged on, type

       % who | wc -l

     Exercise 3b

Using pipes, display all lines of list1 and list2 containing the letter 'p', and sort the result.

     Summary

Command	Meaning
command > file	redirect standard output to a file
command >> file	append standard output to a file
command < file	redirect standard input from a file
command1 | command2	pipe the output of command1 to the input of command2
cat file1 file2 > file0	concatenate file1 and file2 to file0
Sort	sort data
Who	list users currently logged in

           UNIX Tutorial Four

   4.1 Wildcards

    The * wildcard

The character * is called a wildcard, and will match against none or more character(s) in a  file (or directory) name. For example, in your unixstuff directory, type

       % ls list*

This will list all files in the current directory starting with list....

Try typing

      % ls *list

This will list all files in the current directory ending with ....list

    The ? wildcard

The character ? will match exactly one character.
So ?ouse will match files like house and mouse, but not grouse.
Try typing

      % ls ?list

   4.2 Filename conventions

We should note here that a directory is merely a special type of file. So the rules and conventions for naming files apply also to directories.

In naming files, characters with special meanings such as / * & % , should be avoided. Also, avoid using spaces within names. The safest way to name a file is to use only alphanumeric characters, that is, letters and numbers, together with _ (underscore) and . (dot).

Good filenames	Bad filenames
project.txt	project
my_big_program.c	my big program.c
fred_dave.doc	fred & dave.doc

 

File names conventionally start with a lower-case letter, and may end with a dot followed by a group of letters indicating the contents of the file. For example, all files consisting of C code may be named with the ending .c, for example, prog1.c . Then in order to list all files containing C code in your home directory, you need only type ls *.c in that directory.

   4.3 Getting Help

    On-line Manuals

There are on-line manuals which gives information about most commands. The manual pages tell you which options a particular command can take, and how each option modifies the behaviour of the command. Type man command to read the manual page for a particular command.

For example, to find out more about the wc (word count) command, type

      % man wc

Alternatively

      % whatis wc

gives a one-line description of the command, but omits any information about options etc.

    Apropos

When you are not sure of the exact name of a command,

     % apropos keyword

will give you the commands with keyword in their manual page header. For example, try typing

    % apropos copy

  Summary

Command	Meaning
*	match any number of characters
?	match one character
man command	read the online manual page for a command
whatis command	brief description of a command
apropos keyword	match commands with keyword in their man pages

TO BE CONTINUED...