Redirection, combining commands optional, reference material

Redirection and combining commands are relatively simple in Linux, and fabulously enhance its flexibility. These would very aptly be called the `little things' in Linux, because these are very compact expressions which are however indispensible, like the the little things such as `politeness' in life. I'll just list them here for reference:

• $<,>,>>$: redirection
• $\vert$, piping
• ;, a simple way to combine commands
• *, a wildcard

The simplest way to `combine' commands is to use the wildcard, *. This allows you to apply the command to files of similar types. For example
% grep start *.dat
searches for the string start in all data files.

$\circ$ Exercise 13: Look again at the exercise with grep using the word `planet'. Use the wildcard, *, to find the word `planet' in all .txt files in the directory docs.

If you would like to send output or take input from nonstandard directions, like for instance storing which output in a file, use one of the redirectors, $<$, $>$ or $>>$, which get input from a file, send output to a file, or append to a file, respectively. An example is the cat $>$ file command we've already seen, which redirects input from the keyboard into a file. Likewise, cat $>>$ redirects input from keyboard to the end of an existing file.

Another way to combine commands uses the semicolon `;', allowing you to combine several commands on one line.

For example, from the directory tutorial,
% cd texts;pwd;ls -al
changes directories to texts, checks the directory name, and lists all the files in it.

Finally, inserting a `$\vert$' between commands is yet another way to combine them. The syntax command1 $\vert$ command2 means send the output of command1 into the input of command2. For example:
% grep turn test.txt
And returned on the previous night.
%,
which gives the line containing the string turn, can be reproduced in the following way:
% cat test.txt | grep turn
And returned on the previous night.
% .
In the second case, we've used two commands instead of one. The input to grep was the entire file test.txt. grep then proceeded to search for the string turn in that input. In this example, you see that `$\vert$' served very much like a `pipeline' between two commands! This pipeline is what makes `$\vert$' different from `;', in that the pipe allows the commands to share information. On the other hand, `;' merely separates the commands: there is no information passed through the `;'.