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 specifing a file to read
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 specifing 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.
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 (Control 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
Using the above method, create another file called list2 containing the following fruit: orange, plum, mango, grapefruit. Read the contents of list2
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
Redirecting the Input
We use the < symbol to redirect the input of a command. The command sort alphabetically or numerically sorts a list. Type
Then type in the names of some vegetables. Press [Return] after each one.
carrot beetroot artichoke ^D (control d to stop)
The output will be
artichoke beetroot carrot
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
To see who is on the system with you, type
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
a2ps -Phockney textfile is the command to print a postscript file to the printer hockney. Using pipes, print all lines of list1 and list2 containing the letter 'p', sort the result, and print to the printer hockney.
The characters * and ?
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....
% ls *list
This will list all files in the current directory ending with ....list.
The character ? will match exactly one character. So ls ?ouse will match files like house and mouse, but not grouse.
% ls ?list
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).
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.
Beware: some applications give the same name to all the output files they generate.
For example, some compilers, unless given the appropriate option, produce compiled files named a.out. Should you forget to use that option, you are advised to rename the compiled file immediately, otherwise the next such file will overwrite it and it will be lost.
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
% whatis wc
gives a one-line description of the command, but omits any information about options etc.
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
||redirect standard output to a file|
||append standard output to a file|
||redirect standard input from a file|
||pipe the output of command1 to the input of command2|
||concatenate file1 and file2 to file0|
||list users currently logged in|
||print text file to named printer|
||print postscript file to named printer|
||match any number of characters|
||match one character|
||read the online manual page for a command|
||brief description of a command|
||match commands with keyword in their man pages|
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