576
votes
2answers
124k views

Why was '~' chosen to represent the home directory?

I have often wondered why the ~ represents the home directory of a user. Is there a reason behind this, or is it just some infrequently used character?
538
votes
3answers
111k views

What is the exact difference between a 'terminal', a 'shell', a 'tty' and a 'console'?

I think these terms almost refer to the same thing, when used loosely: terminal shell tty console What exactly does each of these terms refer to?
308
votes
3answers
104k views

What is the purpose of the lost+found folder in Linux and Unix?

There is a folder at the root of Linux and Unix operating systems called /lost+found/ What is it for? Under what circumstances would I interact with it? How would I interact with it?
304
votes
7answers
27k views

How can I find the implementations of Linux kernel system calls?

I am trying to understand how a function, say mkdir, works by looking at the kernel source. This is an attempt to understand the kernel internals and navigate between various functions. I know mkdir ...
261
votes
9answers
109k views

When should I not kill -9 a process?

I am always very hesitant to run kill -9, but I see other admins do it almost routinely. I figure there is probably a sensible middle ground, so: When and why should kill -9 be used? When and why ...
238
votes
6answers
398k views

How to correctly add a path to PATH?

I'm wondering where a new path has to be added to PATH environment variable. I know this is accomplished editing .bash_rc (for example), but it's not clear how to do this. This way: export ...
221
votes
5answers
78k views

Why is printf better than echo?

I have heard that printf is better than echo. I can recall only one instance from my experience where I had to use printf because echo didn't work for feeding some text into some program on RHEL 5.8 ...
216
votes
14answers
111k views

What are the pros and cons of Vim and Emacs?

How would you compare these editors? What are the pros and cons of each? [note] This is not meant to be answered by those who "hate one and love another" or those who haven't used both.
208
votes
6answers
84k views

What does env x='() { :;}; command' bash do and why is it insecure?

There is apparently a vulnerability (CVE-2014-6271) in bash: Bash specially crafted environment variables code injection attack I am trying to figure out what is happening, but I'm not entirely sure ...
199
votes
23answers
128k views

Repeat a Unix command every x seconds forever

There's a built-in Unix command repeat whose first argument is the number of times to repeat a command, where the command (with any arguments) is specified by the remaining arguments to repeat. For ...
199
votes
4answers
66k views

Why is FreeBSD deprecating GCC in favor of Clang/LLVM?

So I was surfing the net and stumbled upon this article. It basically states that FreeBSD, starting from Version 10 and above will deprecate GCC in favor of Clang/LLVM. From what I have seen around ...
199
votes
10answers
23k views

In Bash, when to alias, when to script, and when to write a function?

It's taken me almost 10 years of Linux usage to ask this question. It was all trial and error and random late-night internet surfing. But people shouldn't need 10 years for this. If I were just ...
198
votes
12answers
164k views

What if 'kill -9' does not work?

I have a process I can't kill with kill -9 <pid>. What's the problem in such a case, especially since I am the owner of that process. I thought nothing could evade that kill option.
193
votes
13answers
56k views

Preserve bash history in multiple terminal windows

I consistently have more than one terminal open. Anywhere from two to ten, doing various bits and bobs. Now let's say I restart and open up another set of terminals. Some remember certain things, some ...
190
votes
5answers
12k views

What's the difference between $(stuff) and `stuff`?

There are two syntaxes for command substitution: with dollar-parentheses and with backticks. Running top -p $(pidof init) and top -p `pidof init` gives the same output. Are these two ways of doing the ...
188
votes
2answers
45k views

Difference between nohup, disown and &

What is the difference between $ nohup foo and $ foo & and $ foo & $ disown
171
votes
3answers
90k views

How to unfreeze after accidentally pressing Ctrl-S in a terminal?

It's a situation that has happened quite often to me: after I press (with a different intention) Ctrl-S in a terminal, the interaction (input or output) with it is frozen. It's probably a kind of ...
171
votes
6answers
23k views

What do the numbers in a man page mean?

So, for example, when I type man ls I see LS(1). But if I type man apachectl I see APACHECTL(8) and if I type man cd I end up with cd(n). I'm wondering what the significance of the numbers in the ...
170
votes
10answers
57k views

What is the real difference between “apt-get” and “aptitude”? (How about “wajig”?)

I know that both apt-get and aptitude are command line package management interfaces on Debian derived Linux, with different options, but I'm still somewhat confused. Under the hood, aren't they ...
159
votes
10answers
59k views

Turn off buffering in pipe

I have a script which calls two commands: long_running_command | print_progress The long_running_command prints a progress but I'm unhappy with it. I'm using print_progress to make it more nice ...
159
votes
4answers
28k views

/usr/bin vs /usr/local/bin on Linux

Why are there so many places to put a binary in Linux? There are atleast these five: /bin/ /sbin/ /usr/bin/ /usr/local/bin/ /usr/local/sbin/ And on my office box, I do not have write permissions ...
159
votes
3answers
190k views

Check package version using apt-get/aptitude?

Before I install a package I'd like to know what version i would get. How do I check the version before installing using apt-get or aptitude on debian or ubuntu?
158
votes
31answers
92k views

Tracking down where disk space has gone on Linux?

When administering Linux systems I often find myself struggling to track down the culprit after a partition goes full. I normally use du / | sort -nr but on a large filesystem this takes a long time ...
153
votes
3answers
66k views

How do directory permissions in Linux work?

In my CMS, I noticed that directories need the executable bit (+x) set for the user to open them. Why is the execute permission required to read a directory, and how do directory permissions in Linux ...
150
votes
16answers
55k views

Any reason NOT to run Linux in a VM all the time?

I've switched to using Arch Linux for most of my day to day work and don't need Windows for anything but gaming and the couple of apps that aren't ported to Linux like OneNote. My Linux distro is ...
149
votes
5answers
20k views

What's the story behind Super Cow Powers?

As we know, apt-get has Super Cow Powers and aptitude does not: $ apt-get --help | grep -i cow This APT has Super Cow Powers. $ aptitude --help | grep -i cow ...
141
votes
16answers
85k views

How to do integer & float calculations, in bash or other languages/frameworks?

Using echo "20+5" literally produces the text "20+5" What command can I use to get the numeric sum, e.g. 25 in this case. Also, what's the easiest way to do it just using bash for floating point, ...
136
votes
5answers
25k views

I deleted /bin/rm. How do I recover it?

Just for fun, I thought I would use this command on my Raspberry Pi running Raspbian: sudo rm -f /bin/rm I thought I could just reinstall coreutils: I was wrong! apt-get install --reinstall ...
136
votes
10answers
31k views

tmux vs. GNU Screen

Browsing through questions I found about tmux (I normally used GNU Screen). My question is what are pros and cons of each of them. Especially I couldn't find much about tmux.
121
votes
15answers
55k views

How to have tail -f show colored output

I'd like to be able to tail the output of a server log file that has messages like: INFO SEVERE etc, and if it's SEVERE, show the line in red; if it's INFO, in green. What kind of alias can I ...
119
votes
12answers
50k views

What are the pros/cons of deb vs. rpm?

For whatever reasons, I've always used RPM based distros (Fedora, Centos and currently openSUSE). I have often heard it stated that deb is better than rpm, but when asked why, have never been able to ...
119
votes
7answers
29k views

Why is it better to use “#!/usr/bin/env NAME” instead of “#!/path/to/NAME” as my shebang?

I notice that some scripts which I have acquired from others have the shebang #!/path/to/NAME while others (using the same tool, NAME) have the shebang #!/usr/bin/env NAME. Both seem to work ...
119
votes
6answers
448k views

How to switch between users on one terminal?

I'd like to log in as a different user without logging out of the current one (on the same terminal). How do I do that?
117
votes
8answers
12k views

How to chmod without /usr/bin/chmod?

Today I was told a tale by a Unix trainer, a tale where the root password got leaked to the students, and one of the fellas removed the execute permission from /usr/bin/chmod itself. How do you ...
115
votes
3answers
20k views

What does “rc” in .bashrc stand for?

Is it "resource configuration", by any chance?
114
votes
5answers
33k views

Is Linux a Unix?

So, there are lots of different versions of Unix out there: HP-UX, AIX, BSD, etc. Linux is considered a Unix clone rather than an implementation of Unix. Are all the "real" Unices actual descendants ...
114
votes
5answers
34k views

Moving tmux window to pane

How do I move an existing window into a pane in another window in tmux when I have multiple windows, and vice versa? I'm coming from screen, where I can switch to the pane and then switch windows ...
114
votes
3answers
29k views

What does “--” (double-dash) mean? (also known as “bare double dash”)

I have seen -- used in the compgen command. For example: compgen -W "foo bar baz" -- b What is the meaning of the --.
114
votes
9answers
99k views

SSH easily copy file to local system

If I'm logged in to a system via SSH, is there a way to copy a file back to my local system without firing up another terminal or screen session and doing scp or something similar or without doing SSH ...
113
votes
3answers
8k views

What does the Windows flag in the Linux logo of kernel 3.11 mean?

I've just installed kernel-3.11.0-1.fc20 for my Fedora 19 installation. During the rebooting progress, I saw the Linux logo with a Windows flag in it, what does it mean? The Fedora 19 is installed ...
111
votes
7answers
80k views

How can I close a terminal without killing the command running in it?

Sometimes I want to start a process and forget about it. If I start it from the command line, like this: redshift I can't close the terminal, or it will kill the process. Can I run a command in ...
110
votes
19answers
144k views

Why am I still getting a password prompt with ssh with public key authentication?

I'm working from the URL I found here: http://jaybyjayfresh.com/2009/02/04/logging-in-without-a-password-certificates-ssh/ My ssh client is Ubuntu 64 bit 11.10 desktop and my server is Centos 6.2 64 ...
110
votes
9answers
10k views

The myths about malware in Unix / Linux

Is it possible for my Linux box to become infected with a malware? I haven't heard of it happening to anyone I know, and I've heard quite a few times that it isn't possible. Is that true? If so, ...
109
votes
1answer
8k views

Bash: What does “>|” do?

I have just seen this written down; $ some-command >| /tmp/output.txt Vertical pipes are used in standard redirects "piping" the output of one command to another, is >| in fact completely ...
108
votes
3answers
7k views

Why not use “which”? What to use then?

When looking for the path to an executable or checking what would happen would you enter a command name in a Unix shell, there's a plethora of different utilities (which, type, command, whence, where, ...
103
votes
8answers
151k views

How do I remove a user from a group?

Which command should I use to remove a user from a group in Debian? When adding a user to a group, it can be done with: usermod -a -G group user However, I could not find a similar command ...
103
votes
15answers
144k views

Colorizing your terminal and shell environment?

I spend most of my time working in Unix environments and using terminal emulators. I try to use color on the command line, because color makes the output more useful and intuitive. What options exist ...
102
votes
14answers
141k views

How can I resolve a hostname to an IP address in a Bash script?

What's the most concise way to resolve a hostname to an IP address in a Bash script? I'm using Arch Linux.
102
votes
6answers
121k views

How to split the terminal into more than one “view”?

From vi, if you issue the command :sp, the screen splits into two "views", allowing you to edit more than one file from the same terminal. Along those same lines, is there a way to have multiple ...
101
votes
7answers
12k views

How to kill a runaway cat?

Many times I accidentally run the cat command on files that have contents up to few thousand lines. I try to kill the cat command with Ctrl+ C or Ctrl+Z, but both only take effect after the total ...

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