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Monday, 21 March 2016

Executing Commands 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.

Executing Commands in Linux

A command is an instruction given by a user telling the computer to carry out an action. This could be to run a single program or a group of linked programs. Commands are typically initiated by typing them in at the command line (in a terminal) and then pressing the ENTER key, which passes them to the shell.
The Terminal
A terminal refers to a wrapper program which runs a shell. This used to mean a physical device consisting of little more than a monitor and keyboard. As Unix/Linux systems advanced the terminal concept was abstracted into software. Now we have programs such as LXTerminal (on the Raspberry Pi) which will launch a window in a Graphical User Interface (GUI) which will run a shell into which you can enter commands. Alternatively we can dispense with the GUI all together and simply start at the command line when we boot up.
The shell is a program which actually processes commands and returns output. Every Linux operating system has at least one shell, and most have several. The default shell on most Linux systems is bash.

The Command

Commands on Linux operating systems are either built-in or external commands. Built-in commands are part of the shell. External commands are either executables (programs written in a programming language and then compiled into an executable binary) or shell scripts.
A command consists of a command name usually followed by one or more sequences of characters that include options and/or arguments. Each of these strings is separated by white space. The general syntax for commands is;
commandname [options] [arguments]
The square brackets indicate that the enclosed items are optional. Commands typically have a few options and utilise arguments. However, there are some commands that do not accept arguments, and a few with no options. As an example we can run the lscommand with no options or arguments as follows;
The ls command will list the contents of a directory and in this case the command and the output would be expected to look something like the following;
pi@raspberrypi ~ $ ls
Desktop                   python_games
An option (also referred to as a switch or a flag) is a single-letter code, or sometimes a single word or set of words, that modifies the behaviour of a command. When multiple single-letter options are used, all the letters are placed adjacent to each other (not separated by spaces) and can be in any order. The set of options must usually be preceded by a single hyphen, again with no intervening space.
So again using ls if we introduce the option -l we can show the total files in the directory and subdirectories, the names of the files in the current directory, their permissions, the number of subdirectories in directories listed, the size of the file, and the date of last modification.
The command we execute therefore looks like this;
And so the command (with the -l option) and the output would look like the following;
pi@raspberrypi ~ $ ls -l
total 26
drwxr-xr-x 2 pi pi 4096 Feb 20 08:07 Desktop
drwxrwxr-x 2 pi pi 4096 Jan 27 08:34 python_games
Here we can see quite a radical change in the formatting and content of the returned information.
An argument (also called a command line argument) is a file name or other data that is provided to a command in order for the command to use it as an input.
Using ls again we can specify that we wish to list the contents of the python_games directory (which we could see when we ran ls) by using the name of the directory as the argument as follows;
The command (with the python_games argument) and the output would look like the following (actually I removed quite a few files to make it a bit more readable);
pi@raspberrypi ~ $ ls  python_games
4row_arrow.png           gem4.png          
4row_black.png           gem5.png                    pinkgirl.png
4row_board.png           gem6.png                    Plain_Block.png
4row_computerwinner.png  gem7.png                    princess.png
4row_humanwinner.png                   RedSelector.png
gem1.png                 match5.wav                  Wall_Block_Tall.png
gem2.png         Wood_Block_Tall.png
Putting it all together
And as our final example we can combine our command (ls) with both an option (-l) and an argument (python_games) as follows;
Hopefully by this stage, the output shouldn’t come as too much surprise, although again I have pruned some of the files for readabilities sake;
pi@raspberrypi ~ $ ls -l  python_games
total 1800
-rw-rw-r-- 1 pi pi   9731 Jan 27 08:34 4row_arrow.png
-rw-rw-r-- 1 pi pi   7463 Jan 27 08:34 4row_black.png
-rw-rw-r-- 1 pi pi   8666 Jan 27 08:34 4row_board.png
-rw-rw-r-- 1 pi pi  18933 Jan 27 08:34 4row_computerwinner.png
-rw-rw-r-- 1 pi pi  25412 Jan 27 08:34 4row_humanwinner.png
-rw-rw-r-- 1 pi pi   8562 Jan 27 08:34 4row_red.png
-rw-rw-r-- 1 pi pi  14661 Jan 27 08:34 tetrisc.mid
-rw-rw-r-- 1 pi pi  15759 Jan 27 08:34
-rw-rw-r-- 1 pi pi  18679 Jan 27 08:34
-rw-rw-r-- 1 pi pi   9771 Jan 27 08:34 Tree_Short.png
-rw-rw-r-- 1 pi pi  11546 Jan 27 08:34 Tree_Tall.png
-rw-rw-r-- 1 pi pi  10378 Jan 27 08:34 Tree_Ugly.png
-rw-rw-r-- 1 pi pi   8443 Jan 27 08:34 Wall_Block_Tall.png
-rw-rw-r-- 1 pi pi   6011 Jan 27 08:34 Wood_Block_Tall.png
-rw-rw-r-- 1 pi pi   8118 Jan 27 08:34

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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