What is the terminal?
-A command line, or terminal, is a text based interface to the system. You are able to enter commands by typing them on the keyboard and feedback will be given to you similarly as text.
-The command line typically presents you with a prompt. As you type, it will be displayed after the prompt. Most of the time you will be issuing commands.
Within a terminal you have what is known as a shell. This is a part of the operating system that defines how the terminal will behave and looks after running (or executing) commands for you. There are various shells available but the most common one is called bash which stands for Bourne again shell.
The Path
Whenever we refer to either a file or directory on the command line, we are in fact referring to a path. ie. A path is a means to get to a particular file or directory on the system.
There are 2 types of paths we can use, absolute and relative. Whenever we refer to a file or directory we are using one of these paths. Whenever we refer to a file or directory, we can, in fact, use either type of path (either way, the system will still be directed to the same location).
To begin with, we have to understand that the file system under linux is a hierarchical structure. At the very top of the structure is what’s called the root directory. It is denoted by a single slash ( / ). It has subdirectories, they have subdirectories and so on. Files may reside in any of these directories.
-Absolute paths specify a location (file or directory) in relation to the root directory. You can identify them easily as they always begin with a forward slash ( / )
-Relative paths specify a location (file or directory) in relation to where we currently are in the system. They will not begin with a slash.
A file extension is normally a set of 2 - 4 characters after a full stop at the end of a file, which denotes what type of file it is. The following are common extensions:
In other systems such as Windows the extension is important and the system uses it to determine what type of file it is. Under Linux the system actually ignores the extension and looks inside the file to determine what type of file it is. So for instance I could have a file myself.png which is a picture of me. I could rename the file to myself.txt or just myself and Linux would still happily treat the file as an image file. As such it can sometimes be hard to know for certain what type of file a particular file is. Luckily there is a command called file which we can use to find this out.
Cases
This is very important and a common source of problems for people new to Linux. Other systems such as Windows are case insensitive when it comes to referring to files. Linux is not like this. As such it is possible to have two or more files and directories with the same name but letters of different case.
Also be aware of case sensitivity when dealing with command line options. For instance with the command ls there are two options s and S both of which do different things. A common mistake is to see an option which is upper case but enter it as lower case and wonder why the output doesn’t match your expectation.
Spaces
Spaces in file and directory names are perfectly valid but we need to be a little careful with them. As you would remember, a space on the command line is how we seperate items. They are how we know what is the program name and can identify each command line argument.
What happens is that Holiday Photos is seen as two command line arguments. cd moves into whichever directory is specified by the first command line argument only. To get around this we need to identify to the terminal that we wish Holiday Photos to be seen as a single command line argument. There are two ways to go about this, either way is just as valid.
Quotes
The first approach involves using quotes around the entire item. You may use either single or double quotes (later on we will see that there is a subtle difference between the two but for now that difference is not a problem). Anything inside quotes is considered a single item.
Escape Characters
Another method is to use what is called an escape character, which is a backslash ( \ ). What the backslash does is escape (or nullify) the special meaning of the next character.
Hidden Files
Linux actually has a very simple and elegant mechanism for specifying that a file or directory is hidden. If the file or directory’s name begins with a . (full stop) then it is considered to be hidden. You don’t even need a special command or action to make a file hidden. Files and directories may be hidden for a variety of reasons. Configuration files for a particular user (which are normally stored in their home directory) are hidden for instance so that they don’t get in the way of the user doing their everyday tasks.
To make a file or directory hidden all you need to do is create the file or directory with it’s name beginning with a . or rename it to be as such. Likewise you may rename a hidden file to remove the . and it will become unhidden. The command ls which we have seen in the previous section will not list hidden files and directories by default. We may modify it by including the command line option -a so that it does show hidden files and directories.