Programming/Linux - Revision history

Admin: /* Basics */

2020-12-28T13:55:57Z

Basics

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	For example, <tt>/var/log/apache2/error.log</tt> is an absolute path to Apache's error log file. Assuming that the current directory is <tt>/var/log</tt> (another absolute path), the relative path to the same log file is given by <tt>apache2/error.log</tt>.		For example, <tt>/var/log/apache2/error.log</tt> is an absolute path to Apache's error log file. Assuming that the current directory is <tt>/var/log</tt> (another absolute path), the relative path to the same log file is given by <tt>apache2/error.log</tt>.

	<tt>.</tt> and <tt>..</tt> have a special meaning in paths. <tt>.</tt> refers to the current directory~~, whereas~~ <tt>..</tt> refers to its parent.		<tt>.</tt> and <tt>..</tt> have a special meaning in paths. <tt>.</tt> refers to the current directory; <tt>..</tt> refers to its parent.

	== What is the current directory? ==		== What is the current directory? ==

Admin: /* Basics */

2020-12-28T13:55:26Z

Basics

← Older revision		Revision as of 13:55, 28 December 2020
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	For example, <tt>/var/log/apache2/error.log</tt> is an absolute path to Apache's error log file. Assuming that the current directory is <tt>/var/log</tt> (another absolute path), the relative path to the same log file is given by <tt>apache2/error.log</tt>.		For example, <tt>/var/log/apache2/error.log</tt> is an absolute path to Apache's error log file. Assuming that the current directory is <tt>/var/log</tt> (another absolute path), the relative path to the same log file is given by <tt>apache2/error.log</tt>.

			<tt>.</tt> and <tt>..</tt> have a special meaning in paths. <tt>.</tt> refers to the current directory, whereas <tt>..</tt> refers to its parent.

	== What is the current directory? ==		== What is the current directory? ==

Admin at 18:18, 27 December 2020

2020-12-27T18:18:45Z

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	= What is Linux? =		= What is Linux? =

	'''[https://www.linuxfoundation.org/ Linux]''' is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on 1991.09.17 by [https://en.wikipedia.org/wiki/Linus_Torvalds ~~Linux~~ Torvalds].		'''[https://www.linuxfoundation.org/ Linux]''' is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on 1991.09.17 by [https://en.wikipedia.org/wiki/Linus_Torvalds Linus Torvalds].

	Linux is typically packaged in a Linux '''distribution'''. Distributions include the Linux kernel and supporting system software and libraries, many of which are provided by the [https://www.gnu.org/ GNU Project].		Linux is typically packaged in a Linux '''distribution'''. Distributions include the Linux kernel and supporting system software and libraries, many of which are provided by the [https://www.gnu.org/ GNU Project].

Admin: /* Creating or modifying an environment variable */

2020-12-27T18:13:15Z

Creating or modifying an environment variable

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 $ echo $MY_VAR
 this is a new value
 </pre>

Admin at 19:34, 26 December 2020

2020-12-26T19:34:27Z

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	Unlike the similar command <tt>su</tt>, users must, by default, supply their own password for authentication, rather than the password of the target user. After authentication, and if the configuration file, which is typically located at <tt>/etc/sudoers</tt>, permits the user access, the system invokes the requested command. The configuration file offers detailed access permissions, including enabling commands only from the invoking terminal; requiring a password per user or group; requiring re-entry of a password every time or never requiring a password at all for a particular command line. It can also be configured to permit passing arguments or multiple commands.		Unlike the similar command <tt>su</tt>, users must, by default, supply their own password for authentication, rather than the password of the target user. After authentication, and if the configuration file, which is typically located at <tt>/etc/sudoers</tt>, permits the user access, the system invokes the requested command. The configuration file offers detailed access permissions, including enabling commands only from the invoking terminal; requiring a password per user or group; requiring re-entry of a password every time or never requiring a password at all for a particular command line. It can also be configured to permit passing arguments or multiple commands.

			= Processes =

			== What is a process? ==

			A '''process''' is an instance of a computer program that is being executed by one or many threads. It contains the program code and its activity.

			Each process is uniquely identified by its '''process identifier''' (also known as '''process ID''' or '''PID''').

			== Listing processes ==

			A list of processes running on a Linux machine can be obtained by running
			<pre>
			$ top
			</pre>

			There is also a more advanced, interactive process viewer called
			<pre>
			$ htop
			</pre>

			== What is a thread? ==

			A '''thread''' of execution is the smallest sequence of programmed instructions that can be managed independently by a scheduler, which is typically part of the operating system. Multiple threads can exist within one process, executing concurrently and sharing resources such as memory, while different processes do not share these resources. In particular, the threads of a process share its executable code and the values of its dynamically allocated variables and non-thread-local global variables at any given time.

			= Environment variables =

			== What are environment variables? ==

			An '''environment variable''' is a dynamic-named value that can affect the way running processes will behave on a computer. They are part of the environment in which a process runs. For example, a running process can query the value of the <tt>TEMP</tt> environment variable to discover a suitable location to store temporary files or the <tt>HOME</tt> environment variable to find the directory structure owned by the user running the process.

			== Displaying the value of an environment variable ==

			To display the value of an environment variable, you can use
			<pre>
			$ echo $HOME
			/home/ubuntu
			</pre>

			== Listing all environment variables ==

			To obtain a list of all environment variables along with their values, you can use
			<pre>
			$ env
			SHELL=/bin/bash
			PWD=/home/ubuntu
			LOGNAME=ubuntu
			XDG_SESSION_TYPE=tty
			MOTD_SHOWN=pam
			HOME=/home/ubuntu
			LANG=C.UTF-8
			...
			</pre>

			== Creating or modifying an environment variable ==

			To create a new environment variable or change the value of an existing one, you can use the syntax
			<pre>
			$ MY_VAR="this is a new value"
			</pre>

			You can then check that the value has been set:
			<pre>
			$ echo $MY_VAR
			this is a new value
			</pre>

	= Packages =		= Packages =

Admin at 21:05, 24 December 2020

2020-12-24T21:05:09Z

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	Unlike the similar command <tt>su</tt>, users must, by default, supply their own password for authentication, rather than the password of the target user. After authentication, and if the configuration file, which is typically located at <tt>/etc/sudoers</tt>, permits the user access, the system invokes the requested command. The configuration file offers detailed access permissions, including enabling commands only from the invoking terminal; requiring a password per user or group; requiring re-entry of a password every time or never requiring a password at all for a particular command line. It can also be configured to permit passing arguments or multiple commands.		Unlike the similar command <tt>su</tt>, users must, by default, supply their own password for authentication, rather than the password of the target user. After authentication, and if the configuration file, which is typically located at <tt>/etc/sudoers</tt>, permits the user access, the system invokes the requested command. The configuration file offers detailed access permissions, including enabling commands only from the invoking terminal; requiring a password per user or group; requiring re-entry of a password every time or never requiring a password at all for a particular command line. It can also be configured to permit passing arguments or multiple commands.

	= ~~Working with packages~~ =		= Packages =

	== What is a package? ==		== What is a package? ==
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	</pre>		</pre>

	= ~~Working with files~~ and directories =		= Files and directories =

	== Basics ==		== Basics ==

Admin at 23:26, 23 December 2020

2020-12-23T23:26:18Z

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	The learning curve for both these editors is quite steep.		The learning curve for both these editors is quite steep.

	= Deleting a file =		== Deleting a file or directory ==

	Once a file has been created, e.g. with		Once a file has been created, e.g. with

Admin at 23:25, 23 December 2020

2020-12-23T23:25:51Z

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	The learning curve for both these editors is quite steep.		The learning curve for both these editors is quite steep.

			= Deleting a file =

			Once a file has been created, e.g. with
			<pre>
			$ touch test.txt
			</pre>
			it can be deleted using <tt>rm</tt>:
			<pre>
			$ rm test.txt
			</pre>

			Useful options for <tt>rm</tt> are <tt>-f</tt> (ignore nonexistent files and arguments, never prompt) and <tt>-R</tt> (remove directories and their contents recursively).

			For example, we can create a directory,
			<pre>
			$ mkdir test
			</pre>
			possibly some files and/or directories underneath it,
			<pre>
			$ mkdir test/subdir
			$ touch test/foo.txt
			</pre>
			then delete the entire directory subtree starting with and including <tt>test</tt> using
			<pre>
			$ rm -R test
			</pre>

	= DevOps =		= DevOps =

Admin at 19:18, 23 December 2020

2020-12-23T19:18:23Z

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	= What is Linux? =		= What is Linux? =

	'''[https://www.linuxfoundation.org/ Linux]''' is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on 1991.09.17 by [https://en.wikipedia.org/wiki/Linus_Torvalds Linux Torvalds]~~. Linux is typically packaged in a Linux distribution~~.		'''[https://www.linuxfoundation.org/ Linux]''' is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on 1991.09.17 by [https://en.wikipedia.org/wiki/Linus_Torvalds Linux Torvalds].

	Distributions include the Linux kernel and supporting system software and libraries, many of which are provided by the [https://www.gnu.org/ GNU Project].		Linux is typically packaged in a Linux '''distribution'''. Distributions include the Linux kernel and supporting system software and libraries, many of which are provided by the [https://www.gnu.org/ GNU Project].

	Popular Linux distributions include [https://www.debian.org/ Debian], [https://getfedora.org/ Fedora], and [https://ubuntu.com/ Ubuntu]. Commercial distributions include [https://www.redhat.com/ Red Hat Enterprise Linux] and [https://www.suse.com/ SUSE Linux Enterprise Server].		Popular Linux distributions include [https://www.debian.org/ Debian], [https://getfedora.org/ Fedora], and [https://ubuntu.com/ Ubuntu]. Commercial distributions include [https://www.redhat.com/ Red Hat Enterprise Linux] and [https://www.suse.com/ SUSE Linux Enterprise Server].

Admin at 19:15, 23 December 2020

2020-12-23T19:15:39Z

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	Popular Linux distributions include [https://www.debian.org/ Debian], [https://getfedora.org/ Fedora], and [https://ubuntu.com/ Ubuntu]. Commercial distributions include [https://www.redhat.com/ Red Hat Enterprise Linux] and [https://www.suse.com/ SUSE Linux Enterprise Server].		Popular Linux distributions include [https://www.debian.org/ Debian], [https://getfedora.org/ Fedora], and [https://ubuntu.com/ Ubuntu]. Commercial distributions include [https://www.redhat.com/ Red Hat Enterprise Linux] and [https://www.suse.com/ SUSE Linux Enterprise Server].

			As a Unix-like system, Linux subscribes to what has become known as the [https://en.wikipedia.org/wiki/Unix_philosophy Unix philosophy]. In their preface to the 1984 book, [https://amzn.to/3aQeAq7 The UNIX Programming Environment], [https://en.wikipedia.org/wiki/Brian_Kernighan Brian Kernighan] and [https://en.wikipedia.org/wiki/Rob_Pike Rob Pike], both from [https://en.wikipedia.org/wiki/Bell_Labs Bell Labs], give a brief description of the Unix design and the Unix philosophy:

			<blockquote>
			Even though the UNIX system introduces a number of innovative programs and techniques, no single program or idea makes it work well. Instead, what makes it effective is the approach to programming, a philosophy of using the computer. Although that philosophy can't be written down in a single sentence, at its heart is the idea that the power of a system comes more from the relationships among programs than from the programs themselves. Many UNIX programs do quite trivial things in isolation, but, combined with other programs, become general and useful tools.
			</blockquote>

	= Learning about the system =		= Learning about the system =