Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs.

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765
votes
8answers
540k views

How can I sort the output of 'ls' by last modified date?

How can I sort the output of ls by last modified date?
229
votes
27answers
126k views

How to execute a command whenever a file changes?

I want a quick and simple way to execute a command whenever a file changes. I want something very simple, something I will leave running on a terminal and close it whenever I'm finished working with ...
223
votes
8answers
235k views

What is the “You have new mail” message in Linux/UNIX?

Sometimes, when I log into a box and 'su' to root, I get a cute little message saying I have mail (thank GOD it's not AOL). Where is this mail? What does it contain? Who/What sent it? How important ...
196
votes
12answers
39k views

How can I change to the previous directory instead of going up?

I currently spend a lot of my working hours moving back and forth between two paths which are very far down the tree and divert from each other at root. It strikes me that my life would be a lot ...
191
votes
9answers
13k views

How far will you get with an 'rm -rf /' command?

I've often wondered how far the system will actually get if you run rm -rf /. I doubt the OS would be able to erase itself (?) Bonus Question: After the command has been executed, will rm have ...
189
votes
18answers
24k views

What is the advantage of using 'tar' today?

I know that tar was made for tape archives back in the day, but today we have archive file formats that both aggregate files and perform compression within the same logical file format. Questions: ...
182
votes
8answers
13k views

What do the parentheses and number after a Unix command or C function mean?

I keep seeing parentheses and a number after a command in Unix or Linux or C function. For example: man(8), ftok(2), mount(8), etc. What do these mean? I see them in man too.
172
votes
8answers
281k views

How to get the summarized sizes of folders and their subfolders?

Let's say I want to get the size of each folder of a linux file system. When I use ls -la I don't really get the summarized size of the folders. If I use df I get the size of each mounted file system ...
167
votes
10answers
81k views

How can I search the bash history and rerun a command?

Can I search history in bash and run the result?
161
votes
10answers
140k views

How to copy with cp to include hidden files and hidden directories and their contents?

How can I make cp -r copy absolutely all of the files and directories in a directory Requirements: Include hidden files and hidden directories. Be one single command with an flag to include the ...
138
votes
6answers
127k views

What's different between Ctrl+Z and Ctrl+C in Unix command line?

I'm using Mac OS X Terminal. And I use Ctrl+Z or Ctrl+C to stop some programs. But I realized that I don't know what they're exactly doing. What are they and what's the difference between them? ...
130
votes
3answers
36k views

less command with multiple files: How to navigate to next/previous

I just found out I can use less with multiple files. less status line tells me (END) - Next: file2.txt But how do I navigate previous/next from less?
128
votes
5answers
12k views

How does Linux know that the new password is similar to the previous one?

A few times I tried to change a user password on various Linux machines and when the new password was similar to the old one, the OS complained that they were too similar. I always wondered, how does ...
115
votes
5answers
273k views

How can I make chown work recursively?

I've got a directory called pdfs that contains a bunch of sub- and sub-sub-directories. I want to change ownership on all PDF files in all of the subfolders. I just tried this: chown -R ...
114
votes
10answers
80k views

How do I set up SSH so I don't have to type my password?

How do I set up SSH so I don't have to type my password when connecting to a host?
113
votes
18answers
11k views

What's the quickest way to count the number of each character in a file?

I want to count the A's T's C's G's N's and "-" characters in a file, or every letter if needed, is there a quick Unix command to do this?
109
votes
6answers
31k views

Choosing between .bashrc, .profile, .bash_profile, etc [duplicate]

This is embarrassing, but after many years of using POSIX systems full time, I still have a hard time figuring out if a shell customization should go in .bashrc, .profile, or somewhere else. Not to ...
104
votes
10answers
285k views

ls-command: how to display the file size in megabytes?

In Unix (Tru64), how do I make the ls command show the file size in megabytes? Currently I am able to show it in bytes using the following: ls -la
97
votes
7answers
177k views

If I know the PID number of a process, how can I get its name?

If I have the PID number for a process (on a UNIX machine), how can I find out the name of its associated process? What do I have to do?
93
votes
4answers
101k views

How can I find files that are bigger/smaller than x bytes?

In a terminal, how can I find files that are bigger or smaller than x bytes? I suppose I can do something like find . -exec ls -l {} \; and then pipe the result to awk to filter by file size. But ...
81
votes
4answers
111k views

What is the “wheel” user in OS X?

I recently reinstalled iTunes and I noticed something peculiar. The "original" version of iTunes has a user in the permissions known as "wheel" that is set to "Read Only". I dragged this version of ...
73
votes
2answers
38k views

Why does Windows use backslashes for paths and Unix forward slashes?

It annoys me having used Unix in college and now working on the Windows side. What's the history behind this decision? Anyone know why it worked out this way?
71
votes
3answers
100k views

What is the Windows equivalent of the Unix command cat?

I want to do exactly what unix "cat" does, but on my PC. Is there a simple equivalent command for the Windows command line? Specifically I want to create a file from all the files of a given type in ...
68
votes
8answers
17k views

Why is “Everything is a file” unique to the Unix operating systems?

I often hear people say "Unix's unique philosophy is that it treats everything as a file" or "In Unix, everything is a file". But I've never heard anyone explain why it is unique to Unix. So, why is ...
68
votes
13answers
96k views

Unix/Linux find and sort by date modified

How can I do a simple find which would order the results by most recently modified? Here is the current find I am using (I am doing a shell escape in PHP, so that is the reasoning for the variables): ...
67
votes
4answers
106k views

How to make new file permission inherit from the parent directory?

I have a directory called data. Then I am running a script under the user id 'robot'. robot writes to the data directory and update files inside. The idea is data is open for both me and robot to ...
67
votes
4answers
235k views

What is the .bashrc file?

Unix shells when starting read the .bashrc file and execute commands written in it. What is this file and what does it execute?
66
votes
12answers
6k views

What command do I need to use to remove a file called `-rf`?

I did something crazy at some point that created a file called -rf on my filesystem. Now I can't figure out how to delete it... I've tried: rm "-rf" rm \-rf These just exit immediately, arrgh! ...
65
votes
6answers
45k views

Is there a way to see any tar progress per file?

I have a couple of big files that I would like to compress. I can do this with for example tar cvfj big-files.tar.bz2 folder-with-big-files The problem is that I can't see any progress, so I don't ...
60
votes
13answers
8k views

Is Unix not a PC operating system? [closed]

I am doing my bachelor's degree at a university. In a written assignment, the professor posted the task: "Name three PC operating systems". Well, I went on an included a variety of OSes (Linux, ...
57
votes
2answers
19k views

How to start vim without executing /etc/vimrc?

On my Linux server at work, the admins did not install cscope, and I installed it from source in my home directory and added it to the $PATH. The trouble is, the /etc/vimrc has a reference to ...
55
votes
10answers
42k views

How to sort first directories then files etc… when using “ls” in Unix

I would like to use ls command to first show direcotries and then files. I tried ls -la | sort -k 1 But I got a wrong order.
55
votes
3answers
11k views

What does the 'rc' in `.bashrc`, etc. mean?

Well, this is embarrassing. I feel like I just realised I don't know a loyal colleague's name after working with them for 10 years. "Hey, er... *cough!*, thanks for that great job you did on setting ...
55
votes
4answers
44k views

What does load average mean in unix/linux?

If I run uptime, I get something like this: 10:50:30 up 366 days, 23:27, 1 user, load average: 1.27, 2.06, 1.54 What do those numbers on the end mean? The man page tells me it's "the load ...
54
votes
3answers
58k views

How to find files with certain text in the Terminal

I'd like to find all files that contain a certain string of text. How would you do that in the Terminal?
54
votes
4answers
23k views

Bash: Spaces in alias name

I am trying to create an aliases in bash. What I want to do is map ls -la to ls -la | more In my .bashrc file this is what I attempted: alias 'ls -la'='ls -la | more' However it does not work ...
53
votes
11answers
29k views

How can I trigger a notification when a job/process ends?

The place I work at has commands that take a long time to execute. Is there a command/utility that I can use to notify me when the command execution is over? It could be a popup window or maybe a ...
51
votes
4answers
19k views

less is more? Is more less? I'm so confused

When I first heard their names, I got really confused when I tried to guess what these tools might do, I thought I'd misheard something. What's the difference between more and less, i.e. why ...
50
votes
5answers
72k views

List the current folder folder's sizes with the terminal?

I want a list of the folders from the current directory or one that I specify with their size. I have tried with du but I only get the size of the directories I specify (du . ./f1), and ls doesn't ...
49
votes
2answers
9k views

'less' command clearing screen upon exit - how to switch it off?

How to force the less program to not clear the screen upon exit? I'd like it to behave like git log command: it leaves the recently seen page on screen upon exiting it does not exit the less even ...
48
votes
2answers
45k views

How do file permissions apply to symlinks?

Let's say you have this structure: + directory -- file1 -- file2 -- file3 -> /tmp/file3 file3 is a link to another file3 somewhere else on the system. Now let's say I chmod 777 the directory ...
47
votes
11answers
20k views

What is the relationship between Unix, Linux, Ubuntu, Debian and Android?

What is the relationship between Unix, Linux, Ubuntu, and Debian? I suspect they are all Operating Systems that are based upon one another - similar to how Windows is based on DOS. Is my suspicion ...
47
votes
4answers
68k views

What is effect of CTRL + Z on a unix\Linux application

I was curious and confused that what exactly is the behaviour of CTRl+Z. I know, If a process in running in foreground, and we press ctrl+z, it goes to background. But what exactly happens. Does it ...
46
votes
4answers
14k views

which/whereis differences

What's the difference between which and whereis ?
44
votes
3answers
108k views

How do I list the groups that a UNIX user is a member of?

With pts m groupname I can list the users in a group. How can I do the opposite - that is, list all the groups that a user is a member of? (I need this to find the name of a group that I know a user ...
44
votes
4answers
16k views

What is the origin of the UNIX $ (dollar) prompt?

The UNIX prompt uses a $ symbol to indicate that your input is expected. I was wondering why this symbol was chosen—if there is a reason. Dollar just seems a little unexpected. A > symbol would ...
42
votes
14answers
43k views

Is there a way to display a countdown or stopwatch timer in a terminal?

How can I display a real-time countdown timer on the Linux terminal? Is there an existing app or, even better, a one liner to do this?
41
votes
5answers
24k views

How do I configure SSH so it dosen't try all the identity files automatically?

I have been putting my ssh identity files inside my ~/.ssh/ folder. I have probably about 30 files in there. When I connect to servers, i will specify the identity file to use, with something like ...
41
votes
3answers
30k views

Ubuntu shutdown fail “* Killing all remaining processes…”

I have a fresh installation of Ubuntu Server. "reboot" works, but shutdown fails on the * Killing all remaining processes... step. I am using: sudo shutdown now After the failure, indicated by ...
41
votes
3answers
13k views

How do I reload .inputrc?

Background I have heard that the readline module is reading ~/.inputrc and that is how it changes the behaviour of keystrokes under programs such as bash. Question How can I reload this after ...