Posted on 01/30/2019 7:42:58 AM PST by ShadowAce
Why do you need to learn the command line anyway? Well, let me tell you a story. A few years ago we had a problem where I used to work. There was a shared drive on one of our file servers that kept getting full. I won't mention that this legacy operating system did not support user quotas; that's another story. But the server kept getting full and it stopped people from working. One of our software engineers spent the better part of a day writing a C++ program that would look through all the user's directories and add up the space they were using and make a listing of the results. Since I was forced to use the legacy OS while I was on the job, I installed a Linux-like command line environment for it. When I heard about the problem, I realized I could do all the work this engineer had done with this single line:
du -s * | sort -nr > $HOME/user_space_report.txt
Graphical user interfaces (GUIs) are helpful for many tasks, but they are not good for all tasks. I have long felt that most computers today are not powered by electricity. They instead seem to be powered by the "pumping" motion of the mouse! Computers were supposed to free us from manual labor, but how many times have you performed some task you felt sure the computer should be able to do but you ended up doing the work yourself by tediously working the mouse? Pointing and clicking, pointing and clicking.
I once heard an author say that when you are a child you use a computer by looking at the pictures. When you grow up, you learn to read and write. Welcome to Computer Literacy 101. Now let's get to work.
Simply put, the shell is a program that takes commands from the keyboard and gives them to the operating system to perform. In the old days, it was the only user interface available on a Unix-like system such as Linux. Nowadays, we have graphical user interfaces (GUIs) in addition to command line interfaces (CLIs) such as the shell.
On most Linux systems a program called bash (which stands for Bourne Again SHell, an enhanced version of the original Unix shell program, sh, written by Steve Bourne) acts as the shell program. Besides bash, there are other shell programs that can be installed in a Linux system. These include: ksh, tcsh and zsh.
It's a program called a terminal emulator. This is a program that opens a window and lets you interact with the shell. There are a bunch of different terminal emulators you can use. Most Linux distributions supply several, such as: gnome-terminal, konsole, xterm, rxvt, kvt, nxterm, and eterm.
Your window manager probably has a way to launch a terminal from the menu. Look through the list of programs to see if anything looks like a terminal emulator. If you are a KDE user, the terminal program is called "konsole," in Gnome it's called "gnome-terminal." You can start up as many of these as you want and play with them. While there are a number of different terminal emulators, they all do the same thing. They give you access to a shell session. You will probably develop a preference for one, based on the different bells and whistles each one provides.
OK, let's try some typing. Bring up a terminal window. You should see a shell prompt that contains your user name and the name of the machine followed by a dollar sign. Something like this:
[me@linuxbox me]$
Excellent! Now type some nonsense characters and press the enter key.
[me@linuxbox me]$ kdkjflajfks
If all went well, you should have gotten an error message complaining that it cannot understand you:
[me@linuxbox me]$ kdkjflajfks
bash: kdkjflajfks: command not found
Wonderful! Now press the up-arrow key. Watch how our previous command "kdkjflajfks" returns. Yes, we have command history. Press the down-arrow and we get the blank line again.
Recall the "kdkjflajfks" command using the up-arrow key if needed. Now, try the left and right-arrow keys. You can position the text cursor anywhere in the command line. This allows you to easily correct mistakes.
If the last character of your shell prompt is # rather than $, you are operating as the superuser. This means that you have administrative privileges. This can be potentially dangerous, since you are able to delete or overwrite any file on the system. Unless you absolutely need administrative privileges, do not operate as the superuser.
Even though the shell is a command line interface, the mouse is still handy.
Besides using the mouse to scroll the contents of the terminal window, you can copy text with the mouse. Drag your mouse over some text (for example, "kdkjflajfks" right here on the browser window) while holding down the left button. The text should highlight. Release the left button and move your mouse pointer to the terminal window and press the middle mouse button (alternately, you can press both the left and right buttons at the same time if you are working on a touch pad). The text you highlighted in the browser window should be copied into the command line.
When you installed your Linux system and its window manager (most likely Gnome or KDE), it was configured to behave in some ways like that legacy operating system.
In particular, it probably has its focus policy set to "click to focus." This means that in order for a window to gain focus (become active) you have to click in the window. This is contrary to traditional X Window behavior. You should consider setting the focus policy to "focus follows mouse". You may find it strange at first that windows don't raise to the front when they get focus (you have to click on the window to do that), but you will enjoy being able to work on more than one window at once without having the active window obscuring the the other. Try it and give it a fair trial; I think you will like it. You can find this setting in the configuration tools for your window manager.
In this lesson, I will introduce your first three commands: pwd (print working directory), cd (change directory), and ls (list files and directories).
If you have not worked with a command line interface before, you will need to pay close attention to this lesson, since the concepts will take some getting used to.
Like that legacy operating system, the files on a Linux system are arranged in what is called a hierarchical directory structure. This means that they are organized in a tree-like pattern of directories (called folders in other systems), which may contain files and other directories. The first directory in the file system is called the root directory. The root directory contains files and subdirectories, which contain more files and subdirectories and so on and so on.
Most graphical environments today include a file manager program to view and manipulate the contents of the file system. Often you will see the file system represented like this:
One important difference between the legacy operating system and Unix-like operating systems such as Linux is that Linux does not employ the concept of drive letters. While drive letters split the file system into a series of different trees (one for each drive), Linux always has a single tree. Different storage devices may contain different branches of the tree, but there is always a single tree.
Since a command line interface cannot provide graphic pictures of the file system structure, it must have a different way of representing it. Think of the file system tree as a maze, and you are standing in it. At any given moment, you are located in a single directory. Inside that directory, you can see its files and the pathway to its parent directory and the pathways to the subdirectories of the directory in which you are standing.
The directory you are standing in is called the working directory. To find the name of the working directory, use the pwd command.
[me@linuxbox me]$ pwd
/home/me
When you first log on to a Linux system, the working directory is set to your home directory. This is where you put your files. On most systems, your home directory will be called /home/your_user_name, but it can be anything according to the whims of the system administrator.
To list the files in the working directory, use the ls command.
[me@linuxbox me]$ ls Desktop Xrootenv.0 linuxcmd GNUstep bin nedit.rpm GUILG00.GZ hitni123.jpg nsmail
I will come back to ls in the next lesson. There are a lot of fun things you can do with it, but I have to talk about pathnames and directories a bit first.
To change your working directory (where you are standing in the maze) you use the cd command. To do this, type cd followed by the pathname of the desired working directory. A pathname is the route you take along the branches of the tree to get to the directory you want. Pathnames can be specified in one of two different ways; absolute pathnames or relative pathnames. Let's look with absolute pathnames first.
An absolute pathname begins with the root directory and follows the tree branch by branch until the path to the desired directory or file is completed. For example, there is a directory on your system in which most programs are installed. The pathname of the directory is /usr/bin. This means from the root directory (represented by the leading slash in the pathname) there is a directory called "usr" which contains a directory called "bin".
Let's try this out:
[me@linuxbox me]$ cd /usr/bin [me@linuxbox bin]$ pwd /usr/bin [me@linuxbox bin]$ ls [ lwp-request 2to3 lwp-rget 2to3-2.6 lxterm a2p lz aalib-config lzcat aconnect lzma acpi_fakekey lzmadec acpi_listen lzmainfo add-apt-repository m17n-db addpart magnifier
and many more...
Now we can see that we have changed the current working directory to /usr/bin and that it is full of files. Notice how your prompt has changed? As a convenience, it is usually set up to display the name of the working directory.
Where an absolute pathname starts from the root directory and leads to its destination, a relative pathname starts from the working directory. To do this, it uses a couple of special notations to represent relative positions in the file system tree. These special notations are "." (dot) and ".." (dot dot).
The "." notation refers to the working directory itself and the ".." notation refers to the working directory's parent directory. Here is how it works. Let's change the working directory to /usr/bin again:
[me@linuxbox me]$ cd /usr/bin
[me@linuxbox bin]$ pwd
/usr/bin
O.K., now let's say that we wanted to change the working directory to the parent of /usr/bin which is /usr. We could do that two different ways. First, with an absolute pathname:
[me@linuxbox bin]$ cd /usr
[me@linuxbox usr]$ pwd
/usr
Or, with a relative pathname:
[me@linuxbox bin]$ cd ..
[me@linuxbox usr]$ pwd
/usr
Two different methods with identical results. Which one should you use? The one that requires the least typing!
Likewise, we can change the working directory from /usr to /usr/bin in two different ways. First using an absolute pathname:
[me@linuxbox usr]$ cd /usr/bin
[me@linuxbox bin]$ pwd
/usr/bin
Or, with a relative pathname:
[me@linuxbox usr]$ cd ./bin
[me@linuxbox bin]$ pwd
/usr/bin
Now, there is something important that I must point out here. In almost all cases, you can omit the "./". It is implied. Typing:
[me@linuxbox usr]$ cd bin
would do the same thing. In general, if you do not specify a pathname to something, the working directory will be assumed. There is one important exception to this, but we won't get to that for a while.
If you type cd followed by nothing, cd will change the working directory to your home directory.
A related shortcut is to type cd ~user_name. In this case, cd will change the working directory to the home directory of the specified user.
Typing cd - changes the working directory to the previous one.
Now that you know how to move from working directory to working directory, we're going to take a tour of your Linux system and, along the way, learn some things about what makes it tick. But before we begin, I have to teach you some tools that will come in handy during our adventure. These are:
The ls command is used to list the contents of a directory. It is probably the most commonly used Linux command. It can be used in a number of different ways. Here are some examples:
|
| ||
| Command |
|
Result |
|---|---|---|
|
|
||
| ls |
|
List the files in the working directory |
|
|
||
| ls /bin |
|
List the files in the /bin directory (or any other directory you care to specify) |
|
|
||
| ls -l |
|
List the files in the working directory in long format |
|
|
||
| ls -l /etc /bin |
|
List the files in the /bin directory and the /etc directory in long format |
|
|
||
| ls -la .. |
|
List all files (even ones with names beginning with a period character, which are normally hidden) in the parent of the working directory in long format |
These examples also point out an important concept about commands. Most commands operate like this:
command -options arguments
where command is the name of the command, -options is one or more adjustments to the command's behavior, and arguments is one or more "things" upon which the command operates.
In the case of ls, we see that ls is the name of the command, and that it can have one or more options, such as -a and -l, and it can operate on one or more files or directories.
If you use the -l option with ls, you will get a file listing that contains a wealth of information about the files being listed. Here's an example:
-rw------- 1 bshotts bshotts 576 Apr 17 1998 weather.txt drwxr-xr-x 6 bshotts bshotts 1024 Oct 9 1999 web_page -rw-rw-r-- 1 bshotts bshotts 276480 Feb 11 20:41 web_site.tar -rw------- 1 bshotts bshotts 5743 Dec 16 1998 xmas_file.txt
---------- ------- ------- -------- ------------ ------------- | | | | | | | | | | | File Name | | | | | | | | | +--- Modification Time | | | | | | | +------------- Size (in bytes) | | | | | +----------------------- Group | | | +-------------------------------- Owner | +---------------------------------------------- File Permissions
less is a program that lets you view text files. This is very handy since many of the files used to control and configure Linux are human readable.
There are many ways to represent information on a computer. All methods involve defining a relationship between the information and some numbers that will be used to represent it. Computers, after all, only understand numbers and all data is converted to numeric representation.
Some of these representation systems are very complex (such as compressed multimedia files), while others are rather simple. One of the earliest and simplest is called ASCII text. ASCII (pronounced "As-Key") is short for American Standard Code for Information Interchange. This is a simple encoding scheme that was first used on Teletype machines to map keyboard characters to numbers.
Text is a simple one-to-one mapping of characters to numbers. It is very compact. Fifty characters of text translates to fifty bytes of data. Throughout a Linux system, many files are stored in text format and there are many Linux tools that work with text files. Even the legacy operating systems recognize the importance of this format. The well-known NOTEPAD.EXE program is an editor for plain ASCII text files.
The less program is invoked by simply typing:
less text_file
This will display the file.
Once started, less will display the text file one page at a time. You may use the Page Up and Page Down keys to move through the text file. To exit less, type "q". Here are some commands that less will accept:
|
| ||
| Command |
|
Action |
|---|---|---|
|
|
||
|
Page Up or b |
|
Scroll back one page |
|
|
||
|
Page Down or space |
|
Scroll forward one page |
|
|
||
|
G |
|
Go to the end of the text file |
|
|
||
|
1G |
|
Go to the beginning of the text file |
|
|
||
|
/characters |
|
Search forward in the text file for an occurrence of the specified characters |
|
|
||
|
n |
|
Repeat the previous search |
|
|
||
|
h |
|
Display a complete list less commands and options |
|
|
||
|
q |
|
Quit |
As you wander around your Linux system, it is helpful to determine what kind of data a file contains before you try to view it. This is where the file command comes in. file will examine a file and tell you what kind of file it is.
To use the file program, just type:
file name_of_file
The file program can recognize most types of files, such as:
|
| ||||
| File Type |
|
Description |
|
Viewable as text? |
|---|---|---|---|---|
|
|
||||
|
ASCII text |
|
The name says it all |
|
yes |
|
|
||||
|
Bourne-Again shell script text |
|
A bash script |
|
yes |
|
|
||||
|
ELF 32-bit LSB core file |
|
A core dump file (a program will create this when it crashes) |
|
no |
|
|
||||
|
ELF 32-bit LSB executable |
|
An executable binary program |
|
no |
|
|
||||
|
ELF 32-bit LSB shared object |
|
A shared library |
|
no |
|
|
||||
|
GNU tar archive |
|
A tape archive file. A common way of storing groups of files. |
|
no, use tar tvf to view listing. |
|
|
||||
|
gzip compressed data |
|
An archive compressed with gzip |
|
no |
|
|
||||
|
HTML document text |
|
A web page |
|
yes |
|
|
||||
|
JPEG image data |
|
A compressed JPEG image |
|
no |
|
|
||||
|
PostScript document text |
|
A PostScript file |
|
yes |
|
|
||||
|
RPM |
|
A Red Hat Package Manager archive |
|
no, use rpm -q to examine contents. |
|
|
||||
|
Zip archive data |
|
An archive compressed with zip |
|
no |
While it may seem that most files cannot be viewed as text, you will be surprised how many can. This is especially true of the important configuration files. You will also notice during our adventure that many features of the operating system are controlled by shell scripts. In Linux, there are no secrets!
Since macOS is a Unix derivative, many of these tricks will also work in macOS.
I asked if I should post some sort of CLI tutorial here and received positive feedback.
I’d say “learn to code” but I understand that is considered hate speech these days.
Good Hunting... from Varmint Al
Bump for future reference
cp -p gets used a lot as does
sh script.sh
and
if [ -f file ] then
in a script
The Unix Programming Environment by Brian-w-Kernighan and Rob-Pike
No book that I know of is more appropriate for an introduction to unix and CS that lead to advanced concepts in a more concise and precise manner that can be used by beginners and experts alike. IMNSHO - lurkedforadecad
Something much more useful to the ‘regular’ folks would be a thread devoid of a disinterested Propellerhead’s complaints being added to the thread.
I would assert that most people do not understand that there is a profound difference between the Graphical User Interface that they interact with daily and "an Operating System". I would further assert that without a fundamental understanding of the basic principles of an Operating System and the command line level interface and the application programmers operating system interface level, one cannot properly go beyond a mere computer user and develop much of use to an enterprise.
A fundamental understanding of the POSIX API is useful on every Operating System that most people will encounter. Without an understanding at the OS level anything you develop when you "Learn to Code" will inevitably be inefficient and full of errors.
POSIX
Why am I singled out for elimination for an interest I an many others seem to have?
Hey, it’s not my forum.
Have at it.
I just found it strange to find a learn-Linux thread in a political discussions forum.
Carry on...
Thanks for the tip.
Political battles are know being fought on the internet. Knowledge is our friend!
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