One clever feature of less is that you can, at any time, press v to have your file opened inside your text editor. This defaults to Vim, but you can change that by setting the EDITOR environment variable to something else.
If you have made it this far, you can already use less better than most users. You can, at this point, justifiably skip to the next section and consider yourself proficient with less. However, if you want to be a less guru, there are two more things to learn: how to view multiple files simultaneously and how to run shell commands.
Like most other file-based commands in Linux, less can take several files as its parameters. For example:
$ less -MN 1.txt 2.txt 3.txt
That command loads all three files into less, starting at 1.txt. When viewing several files, less usually tells you in which file you are, as well as numbering them: 1.txt (file 1 of 3) should be at the bottom of the screen. That said, certain things do make that go away, so you should use -M anyway.
You can navigate between files by typing a colon and then pressing n to go to the next file or p to go to the previous file; these are referred to from now on as :n and :p. You can open another file for viewing by typing :e and providing a filename. This can be any file you have permission to read, including files outside the local directory. Use the Tab key to complete filenames. Files you open in this way are inserted one place after your current position, so if you are viewing file 1 of 4 and open a new file, the new file is numbered 2 of 5 and is opened for viewing straightaway. To close a file and remove it from the list, use :d.
Viewing multiple files simultaneously has implications for searching. By default, less searches within only one file, but it is easy to search within all files. When you type / or ? to search, follow it with a * — you should see EOF-ignore followed by a search prompt. You can now type a search and it will search in the current file; if nothing is found, it looks in subsequent files until it finds a match. You can repeat searches by pressing the Esc key and then either n or N. The lowercase option repeats the search forward across files, and the uppercase repeats it backward.
The last thing you need to know is that you can get to a shell from the less and execute commands. The simplest way to do this is just to type ! and press Enter. This launches a shell and leaves you free to type all the commands you want, as per normal. Type exit to return to less. You can also type specific commands by entering them after the exclamation mark, using the special character % for the current filename. For example, du -h % prints the size of the current file. Finally, you can use !! to repeat the previous command.
Creating Links Between Files with ln
Linux enables you to create links between files that look and work like normal files for the most part. Moreover, it allows you to make two types of links, known as hard links and symbolic links (symlinks). The difference between the two is crucial, although it might not be obvious at first!
Each filename on your system points to what is known as an inode, which is the absolute location of a file. Linux allows you to point more than one filename to a given inode, and the result is a hard link — two filenames pointing to exactly the same file. Each of these files shares the same contents and attributes. So, if you edit one, the other changes because they are both the same file.
On the other hand, a symlink — sometimes called a soft link — is a redirect to the real file. When a program tries to read from a symlink, it automatically is redirected to what the symlink is pointing at. The fact that symlinks are really just dumb pointers has two advantages: You can link to something that does not exist (and create it later if you want), and you can link to directories.
Both types of links have their uses. Creating a hard link is a great way to back up a file on the same disk. For example, if you delete the file in one location, it still exists untouched in the other location. Symlinks are popular because they allow a file to appear to be in a different location; you could store your website in /var/www/live and an under-construction holding page in /var/www/construction. Then you could have Apache point to a symlink /var/www/html that is redirected to either the live or construction directory depending on what you need.
The shred command overwrites a file's contents with random data, allowing for safe deletion. Because this directly affects a file's contents, rather than just a filename, this means that all filenames hard linked to an inode are affected.
Both types of link are created with the ln command. By default, the ln command creates hard links, but you can create symlinks by passing it the -s parameter. The syntax is ln [-s] <something> <somewhere>, for example:
$ ln -s myfile.txt mylink
That command creates the symlink mylink that points to myfile.txt. Remove the -s to create a hard link. You can verify that your link has been created by running ls -l. Your symlink should look something like this:
lrwxrwxrwx 1 paul paul 5 2007-11-12 12:39 mylink -> myfile.txt
Note how the file properties start with l (lowercase L) for link and how ls -l also prints where the link is going. Symlinks are always very small in size; the previous link is 5 bytes in size. If you created a hard link, it should look like this:
-rw-rw-r 2 paul paul 341 2007-11-12 12:39 mylink
This time the file has normal attributes, but the second number is 2 rather than 1. That number is how many hard links point to this file, which is why it is 2 now. The file size is also the same as that of the previous filename because it is the file, as opposed to just being a pointer.
Symlinks are used extensively in Linux. Programs that have been superseded, such as sh, now point to their replacements (in this case, bash), and library versioning is accomplished through symlinks. For example, applications that link against zlib load /usr/lib/libz.so. Internally, however, that is just a symlink that points to the actual zlib library: /usr/lib/libz.so.1.2.1.2. This enables multiple versions of libraries to be installed without applications needing to worry about the version name.
Finding Files from an Index with locate
When you use the find command, it searches recursively through each directory each time you request a file. This is slow, as you can imagine. Fortunately, Fedora ships with a cron job that creates an index of all the files on your system every night. Searching this index is extremely fast, which means that, if the file you are looking for has been around since the last index, this is the preferable way of searching.