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.
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 as follows. Press [Return] after each one. Terminate with the end-of-file marker control-d (^D).
pear banana apple ^D
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
To read the contents of the file, type
% cat list1
You should now have two files. One contains six fruit names, the other contains four fruits. 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
Then type in the names of some vegetables. Press [Return] after each one and terminate with ^D.
carrot beetroot artichoke ^D
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 | which, on a US keyboard, is above Enter on the right, with the backslash. 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, use wc (word count) with the option -l (ell) for number of lines only:
% who | wc -l
enscript -p myfile.ps myfile.txt is the command to format file myfile.txt to a PostScript file myfile.ps. Using pipes, print all lines of list1 and list2 containing the letter 'p', sort the result, and print to a file sorted_plist.ps. Hint: check the manpage for grep to find an option to output only the line values. Answer available here.
|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|
|who||list users currently logged in|
|enscript -p psfile textfile||print text file to named file|
M.Stonebank@surrey.ac.uk, © October 2001.