D3 Tips and Tricks v4

Friday, 1 April 2016

Absolute and Relative Path Name Addressing in Linux

The following post is a section of the book 'Just Enough Linux'.  The entire book can be downloaded in pdf format for free from Leanpub or you can read it online here.
Since this post is a snapshot in time. I recommend that you download a copy of the book which is updated frequently to improve and expand the content.
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A strength of working with files from the command line in Linux is the flexibility and variety of options available to us as users. Some may claim that too many choices just makes it more difficult to know what is the right or wrong way to perform an action, but others (myself included) will propose that often there is no perfect way to carry out a task, there are only perfect ways.

What should be remembered is that the way that we choose to carry out a task should suit the task and the context. In that light there are plenty of options available in Linux and this is especially true when it comes to the process of specifying file locations.

Pathnames

A pathname represents a test string that acts as a pointer to a file or directory. The following pathname points to the file foo.txt which is inside the directory pi which is itself inside the directory home
The only ‘special’ or ‘reserved’ character when using a pathname is the forward slash /. This character is used solely to separate directories and file names as appropriate.
Individual directories and file names are typically restricted to 255 characters with a total length of a path restricted to 4096 characters. For the sake of context, there are approximately 1500 characters up until this point in this chapter.

Absolute Path Name Addressing

An absolute pathname will always start at the root directory (/). It’s an interesting quirk that people often identify the root directory as the forward slash, when it actually refers to a directory which has no name which exists on the left hand side of the / mark (remember that there can be no use of the / in directory or file names).
The following is an absolute pathname to the file foo.txt which is inside the directory pi which is itself inside the directory home which is in the root directory.

Relative Path Name Addressing

A relative pathname is one where the location of the file or directory is taken relative to the current directory the user is in.
For example, if we were in the directory home/pi (the ‘pi’ users home directory) and we wanted to list (ls) the file foo.txt we could use absolute addressing to reach the file by typing;
However, since we are in the directory /home/pi we could use relative addressing and simply type in;
To use relative addressing effectively we need to be able to let the computer know when we want to move up the directory hierarchy (moving down is done by simply typing the name of the directory). Moving up to a parent directory is accomplished by using the .. (dot dot) characters. If we imagine ourselves in the /home/pi directory again and want to list the file bar.txt which is in the /home/raspberry directory, we can type the following;
Here the .. characters first jumped up into the home directory and then went into the raspberry directory.

The ‘home’ short-cut

The tilde (~) character is used as a short cut to designate the home directory for a user. Therefore assuming that the ‘pi’ user has the home directory of /home/pi, but the directory that we (as the ‘pi’ user) are in is the /tmp directory, we can list the foo.txt file using the following relative path and the home shortcut character;


The post above (and heaps of other stuff) is in the book 'Just Enough Linux' that can be downloaded for free (or donate if you really want to :-)).

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