CSC128, : Introduction to UNIX

I/O Redirection

Standard Input/Output

When a UNIX command-line program (like ls, cat, head, tail , etc.) is executed, by default it reads input from the standard input, stdin, which is usually the terminal keyboard.  It writes its output by default to the standard output, stdout, which is usually the terminal screen.

For example, if we type the command cat by itself, the program (cat ) will try to read its input from the terminal keyboard, until it reaches the end-of-file (EOF) character, which is Ctrl-D.  As usual, it will copy its input (stdin ) to its output (stdout ).  When you specify file(s) to cat , it reads from them instead of stdin .

So what do you do when you would like cat to write to a file instead of stdout ?

Redirecting Output

You can redirect the command's output from stdout to a file by using > or >>. Instead of the command's output being sent to the terminal screen, it will be sent to the file named after the > or >>. To use the command cat to type words directly to a file, you can redirect the output from stdout to the file like this:

cat > test.txt

Since no filename is specified to the cat command, it reads input from the terminal keyboard (stdin ) until it reaches and EOF character (Ctrl-D).  The > redirects output from the terminal screen (stdout ) to the file specified, test.txt .

This will create the file test.txt or overwrite it if it exists.  To append to the end of a file instead of overwriting it, use >> instead.

Redirecting Input

Redirecting input is similar in form to redirecting output.  An example:

cat < test.txt

This will redirect the command's input to read from the file named test.txt   instead of reading from stdin .   

Another example:

smauney@sol:~ $ ls
public projects stuff example_file

smauney@sol:~ $ rm example_file
/bin/rm: remove `example_file'? n

smauney@sol:~ $ cat y.txt
y [Enter]
[Ctrl-D]

smauney@sol:~ $ rm example_file < y.txt
/bin/rm: remove `example_file'?

smauney@sol:~ $ ls
public projects stuff

smauney@sol:~ $

When stdin is redirected from y.txt, you don't even get the chance to say no. The rm command looks to stdin , sees a "y" and a newline from having pressed the enter key, and dutifully removes the file. Since stdin is normally the terminal keyboard, rm assumes that the response is being interactively typed at the keyboard, although it isn't

Pipes

A pipe (|) connects the standard output of one command directly to the standard input of another.  An example:

smauney@sol:~/public $ ls
harold  harry  lab1  poem  richard  stuff  thomas  verse

smauney@sol:~/public $ ls | sort
harold
harry
lab1
poem
richard
stuff
thomas
verse

smauney@sol:~/public $

The ls command lists the files in the directory to stdout .  The pipe (|) redirects the stdout of ls to the stdin of sort .  Since the output of sort isn't redirected, it is sent to stdout , which is the terminal screen. Another example:

>ls | sort | grep ha
harold
harry
richard

After the second escape character \ there is also a space. The \ tell the command to use the next character as a regular character The grep command only sends lines that contain the letter sequence "ha" to stdout .  The output of ls is redirected to the input of sort.  sort 's output is redirected to the input of grep .  grep sends its output to stdout , the terminal screen.

Filters

A filter is a command that processes an input stream of data and produces an output stream of data.  In the above example, sort and grep are examples of commands that are filters.  

Filters are used often in UNIX.  You can string together many filters with pipes to produce the desired output.

Tee

tee is a special filter that takes the data it gets from stdin and both redirects it to a file and its stdout .  An example:

>ls | sort | tee list | grep ha
harold
harry
richard

smauney@sol:~/public $ cat list
harold
harry
lab1
poem
richard
stuff
thomas
verse

smauney@sol:~/public $

The file list was created by tee before the grep command had a chance to modify the data stream.