Complete Idiot's Guide to Linux
(Publisher: Macmillan Computer Publishing)
Author(s): Manuel Ricart
ISBN: 078971826x
Publication Date: 12/22/98

Creating Links: ln

Links are created using the ln command. ln is used to create both soft and hard links. The syntax for the command is

     ln [options] source destination

To create a symbolic (soft) link, you specify the -s option, which stands for symbolic. Here’s an example for listing a directory:

     [alberto@digital alberto]$ ls -l
     total 2
     drwxrwxr-x   3 alberto  alberto      1024 Aug  1 20:41 adir/
     -rw-rw-r--   1 alberto  alberto        57 Aug  2 13:12 file

Here’s an example for creating a hard link:

     [alberto@digital alberto]$ ln file file2

Here’s one for creating a soft link:

     [alberto@digital alberto]$ ln -s file file3

Here is the syntax for listing directory again. Notice that file3 points to file:

     [alberto@digital alberto]$ ls -l
     total 3
     drwxrwxr-x   3 alberto  alberto      1024 Aug  1 20:41 adir/
     -rw-rw-r--   2 alberto  alberto        57 Aug  2 13:12 file
     -rw-rw-r--   2 alberto  alberto        57 Aug  2 13:12 file2
     lrwxrwxrwx   1 alberto  alberto         4 Aug  2 13:15 file3 -> file

Here is the syntax for deleting the original file:

     [alberto@digital alberto]$ rm file

Here is the syntax for listing again. Notice that file3 now points to a file that doesn’t exist!

     [alberto@digital alberto]$ ls -l
     total 2
     drwxrwxr-x   3 alberto  alberto      1024 Aug  1 20:41 adir/
     -rw-rw-r--   1 alberto  alberto        57 Aug  2 13:12 file2
     lrwxrwxrwx   1 alberto  alberto         4 Aug  2 13:15 file3 -> file

Links are used often in Linux to allow files to be placed in many directories at once. For example, the directory /usr/X11R6 contains files used by X Windows. Some older X Windows programs install into /usr/X11R5 or /usr/X11R4 instead. Through a series of symbolic links, Linux can make files from all three directories appear in a more general X Windows directory, /usr/X11, for easy access. This type of organizing is one of the primary uses for symbolic links.

Reading Files

Being able to move and organize files is an important skill. In the end, computers are primarily for storing and retrieving information. Here are a few commands that you can use for reading files when on a terminal:

•  cat: Copy the contents of a file to a terminal

•  less: Page the contents a screen at a time

•  head: Peek at the first few lines in a file

•  tail: Peek at the last few lines in a file

Concatenating: cat

The cat command reads one or more files and prints the result to the screen. (It concatenates all files to the screen.) cat is a generic command, and it is used very frequently when chaining multiple commands together. When we explore redirection in Chapter 14, “Putting the Shell to Work,” I’ll show you how the cat is really used.

The basic syntax of cat is

     cat [options] files

Here’s an example of using cat to view the /etc/passwd file (this is the file where information about users is kept):

     [alberto@digital alberto]$ cat /etc/passwd
     root:UGQDnB4s/nDAk:0:0:root:/root:/bin/bash
     bin:*:1:1:bin:/bin:
     daemon:*:2:2:daemon:/sbin:
     adm:*:3:4:adm:/var/adm:
     lp:*:4:7:lp:/var/spool/lpd:
     sync:*:5:0:sync:/sbin:/bin/sync
     shutdown:*:6:0:shutdown:/sbin:/sbin/shutdown
     halt:*:7:0:halt:/sbin:/sbin/halt
     mail:*:8:12:mail:/var/spool/mail:
     news:*:9:13:news:/var/spool/news:
     uucp:*:10:14:uucp:/var/spool/uucp:
     operator:*:11:0:operator:/root:
     games:*:12:100:games:/usr/games:
     gopher:*:13:30:gopher:/usr/lib/gopher-data:
     ftp:*:14:50:FTP User:/home/ftp:
     nobody:*:99:99:Nobody:/:
     postgres:!:100:101:PostreSQL Server:/var/lib/pgsql:/bin/bash
     alberto:YMMhuMSru1hME:501:501:AlbertoRicart:/home/alberto:/bin/bash

As you can see, cat opened the file /etc/passwd and printed every line to the console.

The cat command offers several options; the most useful are listed in Table 12.1.