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UNIX is the name of the original system designed by Ken Thompson and Dennis Ritchie during their time at AT&T in the 1970s. It featured a great deal of novelties at the time, including multi-tasking, multi-user support, time sharing, etc. This manual provides a brief introduction how to get started using powerful Linux command-line utilities. How to Get Access? Install your preferred GNU/Linux distribution on your local machine (not required!!!) Users at UC Riverside can apply for an account on our Linux clusters by sending an account request to Rakesh Kaundal ([email protected]). This really deserves a book-length answer, but here's the TL;DR version. Key concepts. Unix is not one thing; there are dozens of Unix variants. Unix is a trademark, but people are not rigorous about using that word to refer to the trademarked.
Version 7 Unix: /etc listing, showing init and rc
Version 7 Unix: contents of an /etc/rcBourne shell script
In Unix-based computer operating systems, init (short for initialization) is the first process started during booting of the computer system. Init is a daemon process that continues running until the system is shut down. It is the direct or indirect ancestor of all other processes and automatically adopts all orphaned processes. Init is started by the kernel during the booting process; a kernel panic will occur if the kernel is unable to start it. Init is typically assigned process identifier 1.
In Unix systems such as System III and System V, the design of init has diverged from the functionality provided by the init in Research Unix and its BSD derivatives. Up until recently, most Linux distributions employed a traditional init that is somewhat compatible with System V, while some distributions such as Slackware use BSD-style startup scripts, and others such as Gentoo have their own customized versions.
Since then, several additional init implementations have been created, attempting to address design limitations in the traditional versions. These include launchd, the Service Management Facility, systemd and OpenRC.
Research Unix-style/BSD-style[edit]
Research Unix init ran the initialization shell script located in
/etc/rc ,[1] then launched getty on terminals under the control of /etc/ttys .[2] There are no runlevels; the /etc/rc file determines what programs are run by init. The advantage of this system is that it is simple and easy to edit manually. However, new software added to the system may require changes to existing files that risk producing an unbootable system.
BSD init was, prior to 4.3BSD, the same as Research UNIX's init;[3][4] in 4.3BSD, it added support for running a windowing system such as X on graphical terminals under the control of
/etc/ttys .[5][6] To remove the requirement to edit /etc/rc , BSD variants have long supported a site-specific /etc/rc.local file that is run in a sub-shell near the end of the boot sequence.
A fully modular system was introduced with NetBSD 1.5 and ported to FreeBSD 5.0 and successors. This system executes scripts in the
/etc/rc.d directory. Power mac g4 cube manual. Unlike System V's script ordering, which is derived from the filename of each script, this system uses explicit dependency tags placed within each script.[7] The order in which scripts are executed is determined by the rcorder script based on the requirements stated in these tags.
SysV-style[edit]
Sysv-rc-conf, a TUI utility that selects which SysV-style init scripts will be run in each runlevel
When compared to its predecessors, AT&T's UNIX System III introduced a new style of system startup configuration,[8] which survived (with modifications) into UNIX System V and is therefore called the 'SysV-style init'.
At any moment, a running System V is in one of the predetermined number of states, called runlevels. At least one runlevel is the normal operating state of the system; typically, other runlevels represent single-user mode (used for repairing a faulty system), system shutdown, and various other states. Switching from one runlevel to another causes a per-runlevel set of scripts to be run, which typically mount filesystems, start or stop daemons, start or stop the X Window System, shutdown the machine, etc.
Runlevels[edit]
The runlevels in System V describe certain states of a machine, characterized by the processes and daemons running in each of them. In general, there are seven runlevels, out of which three runlevels are considered 'standard' as they are essential to the operation of a system:
Aside from these standard ones, Unix and Unix-like systems treat runlevels somewhat differently. The common denominator, the
/etc/inittab file, defines what each configured runlevel does in a given system.
Default runlevels[edit]
On Linux distributions defaulting to runlevel 5 in the table on the right, runlevel 5 invokes a multiuser graphical environment running the X Window System, usually with a display manager like GDM or KDM. However, the Solaris and illumos operating systems typically reserve runlevel 5 to shut down and automatically power off the machine.
On most systems, all users can check the current runlevel with either the
runlevel or who -r command.[11] The root user typically changes the current runlevel by running the telinit or init commands. The /etc/inittab file sets the default runlevel with the :initdefault: entry.
On Unix systems, changing the runlevel is achieved by starting only the missing services (as each level defines only those that are started / stopped).[citation needed] For example, changing a system from runlevel 3 to 4 might only start the local X server. Going back to runlevel 3, it would be stopped again.
Other implementations[edit]
Traditionally, one of the major drawbacks of init is that it starts tasks serially, waiting for each to finish loading before moving on to the next. When startup processes end up Input/output (I/O) blocked, this can result in long delays during boot. Speeding up I/O, e.g. by using SSDs, may shorten the delays but it does not address the root cause.
Various efforts have been made to replace the traditional init daemons to address this and other design problems, including:
As of February 2019, systemd has been adopted by most major Linux distributions.[24]
See also[edit]
References[edit]![]()
External links[edit]
Retrieved from 'https://en.wikipedia.org/w/index.php?title=Init&oldid=908189997'
Active1 year, 7 months ago
I've been looking for an all around Linux Programmers manual but there isn't one.. So that leads me to ask if the Unix Programmers Manual is relevant for Linux?
The manual is here: http://cm.bell-labs.com/7thEdMan/v7vol1.pdf
Linux Programmer's Manual Versus Bsd System Call Manual On Mac Free
user1529891
user1529891user1529891
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2 Answers
The Unix Programmers Manual you linked to is probably mostly relevant for Linux also. However, that manual was published in 1979. Things have changed since then in all descendants of the original Unix.
Greg HewgillGreg Hewgill
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This is the collection of man pages for an antique version of Unix. There is little point in reading that: it is of historical interest only. Man page collection exist for your current version of Unix. You can read a man page with the
man command , or in various online databases (Ubuntu, FreeBSD, etc.).
Don't try to read all the man pages. I have about 23000 man pages installed on my home computer… Man pages are reference documents, to be consulted when you need to know about one specific command or function. They are often incomprehensible if you don't know anything about that part of the system.
If you want to learn programming from a book, what you need is a book written to teach Unix programming. Advanced Programming in the UNIX Environment (APUE) is a classic; there are many others.
GillesGilles
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Not the answer you're looking for? Browse other questions tagged linuxdocumentationprogramming or ask your own question.Comments are closed.
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