344
votes
Accepted
Why does Linux use LF as the newline character?
Windows uses CRLF because it inherited it from MS-DOS.
MS-DOS uses CRLF because it was inspired by CP/M which was already using CRLF.
CP/M and many operating systems from the eighties and earlier ...
- 60.4k
167
votes
If Linux is only a kernel, then how were its first versions used (without distribution)?
In the early stages of Linux, Linus Torvalds released the Linux kernel source in an alpha state to signal to others that work towards a new Unix-like kernel was in development. By that time, as @...
- 56k
139
votes
Accepted
Why can't Linux usernames begin with numbers?
Some commands (eg chown) can accept either a username or a numeric user ID, so allowing all-numeric usernames would break that.
A rule to allow names that start with a number and contain some alpha ...
- 1,481
104
votes
Accepted
Why is rm allowed to delete a file under ownership of a different user?
The reason why this is permitted is related to what removing a file actually does. Conceptually, rm's job is to remove a name entry from a directory. The fact that the file may then become unreachable ...
- 43.3k
102
votes
On what systems is //foo/bar different from /foo/bar?
This is a compilation and index of the answers given so far. This post is community wiki, it can be edited by anybody with 100+ reputation and nobody gets reputation from it. Feel free to post your ...
100
votes
Accepted
Why is Unix 'self-supporting'?
The question in your title is addressed immediately after your quote in the paper:
All Unix software is maintained on the system; likewise, this paper and all other documents in this issue were ...
- 415k
99
votes
Accepted
What does the letter 'u' mean in /dev/urandom?
Unlimited.
In Linux, comparing the kernel functions named random_read and random_read_unlimited
indicates that the etymology of the letter u in urandom isunlimited.
This is confirmed by line 114:
...
- 28.8k
94
votes
What did the sticky bit originally do when applied to files?
No, the sticky bit was not like the set-UID or set-GID flags. It didn't effect any changes to process credentials.
What the sticky bit did was make the program text "sticky". It wasn't a ...
- 67.1k
84
votes
Accepted
Is there any reason to have a shebang pointing at /bin/sh rather than /bin/bash?
There are systems not shipping bash by default (e.g. FreeBSD).
Even if bash is installed, it might not be located in /bin.
Most simple scripts don't require bash.
Using the POSIX shell is more ...
- 33.2k
83
votes
Accepted
Patching a binary with dd
Let's try it. Here's a trivial C program:
#include <stdio.h>
int main(int argc, char **argv) {
puts("/usr/tmp");
}
We'll build that into test:
$ cc -o test test.c
If we run it, it prints ...
- 75k
83
votes
Why can't Linux usernames begin with numbers?
here is a test on ubuntu 14.04 using numbers:
root@ubuntu:~# useradd 232
root@ubuntu:~# mkdir /home/232
root@ubuntu:~# chown 232.232 /home/232
root@ubuntu:~# passwd 232
Enter new UNIX password:
...
- 1,714
69
votes
Intended use of ctrl+T in bash?
It's very useful to quickly fix typos:
sl
becomes
ls
with a single CtrlT.
You can use AltT to swap words too (e.g. when switching between service and systemctl...).
Historically speaking, the ...
- 415k
66
votes
Accepted
Why are UNIX/POSIX system call namings so illegible?
It's due to the technical constraints of the time. The POSIX standard was created in the 1980s and referred to UNIX, which was born in the 1970. Several C compilers at that time were limited to ...
- 28.8k
62
votes
What's this I hear about First Edition Unix being restored?
In 2008–2009 The Unix Heritage Society managed to reconstruct the source for First Edition Unix kernel and parts of the shell from various sources, including magnetic tapes and paper documents. ...
- 67.1k
60
votes
Accepted
Why is the Intel HD Graphics driver called i915?
Well, that P4 chipset is the reason for the driver name. Starting with i810, Intel outsourced the driver to Tungsten Graphics, but commissioned it as an open source one for Linux.
The first 915 ...
- 79.6k
60
votes
Accepted
When did Unix stop storing passwords in clear text?
For the early history of Unix password storage, read Robert Morris and Ken Thompson's Password Security: A Case History. They explain why and how early Unix systems acquired most the features that are ...
57
votes
Accepted
Why is there a Linux kernel policy to never break user space?
The reason is not a historical one but a practical one. There are many many many programs that run on top of the Linux kernel; if a kernel interface breaks those programs then everybody would need to ...
56
votes
Accepted
Why was `cp` designed to silently overwrite existing files?
The default overwrite behavior of cp is specified in POSIX.
If source_file is of type regular file, the following steps shall be taken:
3.a. The behavior is unspecified if dest_file exists ...
- 88.3k
55
votes
Accepted
How do pipelines limit memory usage?
The data doesn’t need to be stored in RAM. Pipes block their writers if the readers aren’t there or can’t keep up; under Linux (and most other implementations, I imagine) there’s some buffering but ...
- 415k
53
votes
Does vi silently add a newline (LF) at the end of file?
POSIX requires this behavior, so it's not in any way unusual.
From the POSIX vi manual:
INPUT FILES
See the INPUT FILES section of the ex command for a description of the input files supported by the ...
- 1,906
52
votes
Accepted
Why don't man pages have examples?
That depends on the man pages... Traditionally, they have included a section with examples - but for some reason that is usually missing from the man pages under Linux (and I assume other using GNU ...
- 7,043
52
votes
How did Linux/xBSD boot before GRUB?
The first Linux distribution I used back in the 90s (Slackware 3.0 IIRC) used LILO as a bootloader. And many distros used LILO for years even when GRUB was becoming the "default" bootloader.
Moreover,...
- 4,127
49
votes
Accepted
Is there a reason why the first element of a Zsh array is indexed by 1 instead of 0?
Virtually all shell arrays (Bourne, csh, tcsh, fish, rc, es, yash) start at 1. ksh is the only exception that I know (bash just copied ksh).
Most interpreted languages at the time (early 90s): awk, ...
- 525k
47
votes
Accepted
What is the point of mv -f when default behavior already overwrites files?
The usage of -f is more clearly described in the man page from 4BSD, which was where the -f and -i options were added:
If file2 already exists, it is removed before file1 is moved. If file2 has a ...
- 25k
47
votes
Why is cp's option not to overwrite files called --no-clobber?
“Clobber” in the context of data manipulation means destroying data by overwriting it. In the context of files in a Unix environment, the word was used at least as far back as the early 1980s, ...
42
votes
Accepted
What do the large computers that real terminals were connected to look like?
That terminal would typically be connected to a PDP-11, or a VAX-11 (it can be used with many, many different types of computers though!). The PDP-11, like many mini-computers, was often housed in a ...
- 415k
40
votes
Accepted
What does "on-line" mean, as used in man(1)?
In contrast to a printed (hard-copy) manual, which you could read off-line (while not using a computer).
The term dates back (at least) to time-sharing systems. Users may have had a terminal which ...
- 75.2k
36
votes
Is the shell ksh93 dead?
It is not possible to give a real answer to this question, but the form of a comment is not sufficient. So I think it may be a good idea to collect points to a editable answer...
Two years ago, David ...
- 18.8k
36
votes
How do pipelines limit memory usage?
But I don't understand how this could limit memory usage considering the fact that the data has to be stored in RAM to transmit between programs.
This is your fundamental error. Early versions of ...
- 67.1k
Only top scored, non community-wiki answers of a minimum length are eligible