Basic Commands

Locating Applications

Depending on the specific distribution, programs and software packages can be installed in various directories. In general, executable programs should live in the following directories

/bin
/usr/bin
/sbin
/usr/sbin
/opt.

One way to locate programs is to employ the which utility. For example, to find out exactly where the diff program resides on the filesystem:

$ which diff
/usr/bin/diff

If which does not find the program, whereis is a good alternative because it looks for packages in a broader range of system directories:

$ whereis diff
diff: /usr/bin/diff /usr/share/man/man1/diff.1.gz

Accessing Directories

The following commands are useful for directory navigation:

Command	Result
cd	Change to your home directory
cd ..	Change to parent directory
cd –	Change to previous directory
cd /	Changes your current directory to the root (/) directory

Exploring the Filesystem

The tree command is a good way to get a bird’s-eye view of the filesystem tree. The following commands can help in exploring the filesystem:

Command	Result
ls	List the contents of the present working directory
ls –a	List all files including hidden files and directories
tree	Displays a tree view of the filesystem
tree -d	Just list the directories and suppress listing file names

Hard and Symbolic Links

The ln command can be used to create hard links and or soft links, also known as symbolic links or symlinks. These two kinds of links are very common in UNIX-based operating systems.

Suppose that file1.txt already exists. A hard link, called file2.txt, is created with the command:

# ln file1.txt file2.txt

Note that two files now appear to exist. However, a closer inspection of the file listing shows that this is not quite true.

# ls -l file*
-rw-r--r--. 2 root root 604 Feb 16 11:49 file1.txt
-rw-r--r--. 2 root root 604 Feb 16 11:49 file2.txt
# ls -li file*
134415251 -rw-r--r--. 2 root root 604 Feb 16 11:49 file1.txt
134415251 -rw-r--r--. 2 root root 604 Feb 16 11:49 file2.txt

The -i option prints out in the first column the i-node number, which is a unique quantity for each file object. This field is the same for both of the two files; what is really going on here is that it is only one file but it has more than one name associated with it, as is indicated by the 2 that appears in the output.

# ln file1.txt file3.txt
# ls -li file*
134415251 -rw-r--r--. 3 root root 604 Feb 16 11:49 file1.txt
134415251 -rw-r--r--. 3 root root 604 Feb 16 11:49 file2.txt
134415251 -rw-r--r--. 3 root root 604 Feb 16 11:49 file3.txt

Changing the file3.txt means change the same object as named as file1.txt, file2.txt and file3.txt.

Symbolic or Soft links are created with the -s option as in:

# ln -s file1.txt file4.txt
# ls -li file*
134415251 -rw-r--r--. 3 root root 644 Feb 16 11:59 file1.txt
134415251 -rw-r--r--. 3 root root 644 Feb 16 11:59 file2.txt
134415251 -rw-r--r--. 3 root root 644 Feb 16 11:59 file3.txt
134415252 lrwxrwxrwx. 1 root root   9 Feb 16 11:59 file4.txt -> file1.txt

Notice file4.txt no longer appears to be a regular file, and it clearly points to file1 and has a different inode number. Symbolic links take no extra space on the filesystem. They are extremely convenient as they can easily be modified to point to different places. An easy way to create a shortcut from your home directory to long pathnames is to create a symbolic link.

Unlike hard links, soft links can point to objects even on different filesystems (or partitions) which may or may not be currently available or even exist. In the case where the link does not point to a currently available or existing object, you obtain a dangling link.

Hard links are very useful and they save space, but you have to be careful with their use, sometimes in subtle ways. For one thing if you remove either file1.txt or file2.txt in the example, the inode object will remain, which might be undesirable as it may lead to subtle errors later if you recreate a file of that name. If you edit one of the files, exactly what happens depends on your editor; most editors including vi and gedit will retain the link by default but it is possible that modifying one of the names may break the link and result in the creation of two objects.