Viewing Output

Use ls (list files) and > (redirect) to create a file named "mylist" which contains a list of your files.

There are three main methods for viewing a file from the command prompt. Try each of these on your "mylist" file to get a feel for how they work:

• cat shows the contents of the entire file at the terminal and scrolls automatically to the end.

$ cat mylist

• more shows the contents of the file, pausing when it fills the screen.

  $ more mylist

  • Note that it reads the entire file before displaying, so it could take a long time to load for large files.
  • Use the spacebar to advance one page at a time.
• less is similar to more, but with more features. It shows the contents of the file, pausing when it fills the screen.

  $ less mylist

  • Note that less is faster than more on large files because it does not read the entire input file before displaying.
  • Use the spacebar to advance one page at a time, or use the arrow keys to scroll one line at a time. Enter q to quit. Entering g or G will take you to the beginning or end of the file, respectively.
  • You can also search within a file (similar to Vim) by typing / and the word or characters you are searching for (example: /foo will search for "foo"). less will jump to the first match for the word. Move between matches by using n and ? keys.

Combining Redirection and Viewing

Now let's try an exercise where we enter the famous quote "Four score and seven years ago" from Lincoln's Gettysburg address into a file called "lincoln.txt". First, use cat to direct stdin to "lincoln.txt":

Next, enter the quote above. To end the text input, press Control-D.

Four score and seven years ago
[Control-D]

Finally, you can use cat to view the file you just created:

$ cat lincoln.txt
Four score and seven years ago

Now try adding another line of the famous quote to the existing file:

If you wish, you could try appending the rest of the speech to the file. Finally, try viewing the file in both more and less to test them out. Feel free to test navigation in both and try searching with less. If you have a longer file, try viewing that as well so you can get used to scrolling.

Creating a Simple Script

We can also redirect input to a script file that we create and then run the script. First, we will create the script file called "tryme.sh" that contains the cat command without any arguments, forcing it to read from stdin.

The first line of the script #!/bin/sh indicates which shell interpreter to use. /bin/sh is a special sort of file, called a symlink, which points at the default interpreter. You can see where it points by:

$ ls -l /bin/sh

The default is often /bin/bash, but you can also specify to use bash (or another shell) directly by replacing the line with the location of bash on your system, which is usually #!/bin/bash.

Next, we can execute the script using the source command, and redirect the "lincoln.txt" file to stdin. This will cause the script to execute the cat command with the contents of "lincoln.txt" as input, consequently printing it to the screen (via stdout):

If you omit the redirection character <, the script will try to read from stdin (keyboard input) and then immediately print it back out.