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Questions tagged [unix]

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

Excerpt from the Wikipedia page

Unix was originally written in assembly but has since been rewritten in C. It has been branched many times both commercially and open source. One popular variant is the BSD variant which originated from the University of California, Berkeley. It also gave rise to Linux.

Notable variants

  • Silicon Graphics - Irix
  • IBM - AIX
  • Sun Microsystems - Solaris
  • Hewlett Packard - HP-UX

Unix is officially trademarked as UNIX.

Unix on Wikipedia

1962 questions
434
votes
3 answers

How do I change my private key passphrase?

I have an existing public/private key pair. The private key is password protected, and the encryption may be either RSA or DSA. These keys are the kind you generate with ssh-keygen and generally store under ~/.ssh. I'd like to change the private…
kch
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389
votes
20 answers

How do I get the current Unix time in milliseconds in Bash?

How do I get the current Unix time in milliseconds (i.e number of milliseconds since Unix epoch January 1 1970)?
Richard
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345
votes
18 answers

Copying a large directory tree locally? cp or rsync?

I have to copy a large directory tree, about 1.8 TB. It's all local. Out of habit I'd use rsync, however I wonder if there's much point, and if I should rather use cp. I'm worried about permissions and uid/gid, since they have to be preserved in the…
Amandasaurus
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285
votes
13 answers

Can I nohup/screen an already-started process?

I'm doing some test-runs of long-running data migration scripts, over SSH. Let's say I start running a script around 4 PM; now, 6 PM rolls around, and I'm cursing myself for not doing this all in screen. Is there any way to "retroactively" nohup a…
ojrac
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246
votes
13 answers

Environment variables of a running process on Unix?

I need to troubleshoot some problems related to environment variables on a Unix system. On Windows, I can use a tool such as ProcessExplorer to select particular a process and view values of each environment variable. How can I accomplish the same…
Gant
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246
votes
4 answers

How do I do 'mount --bind' in /etc/fstab?

I'm trying to add mount --bind /proc/ /chroot/mysql/proc to /etc/fstab. How can I do this?
Some Linux Nerd
  • 3,597
241
votes
19 answers

In my /etc/hosts/ file on Linux/OSX, how do I do a wildcard subdomain?

I need to test sub-domains on my localhost. How can I effectively have this result of adding *.localhost.com to my /etc/hosts/ file? If it's not possible, how do I work around this problem? I need to test wildcard sub-domains on my localserver. …
MikeN
  • 8,602
240
votes
9 answers

When does /tmp get cleared?

I'm taking to putting various files in /tmp, and I wondered about the rules on deleting them? I'm imagining it's different for different distributions, and I'm particularly interested in Ubuntu and Fedora desktop versions. But a nice general way of…
John Lawrence Aspden
  • 2,676
221
votes
11 answers

How to read backward from the end of file in less or more?

I've found one way so far: less +G filename, but it scrolls up line-by-line only with ↑. What's a more powerful less usage which provides scrolling by page, backward pattern search, and so on?
yetanothercoder
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203
votes
6 answers

SSL Certificate Location on UNIX/Linux

Is there any standard or convention for where SSL certificates and associated private keys should go on the UNIX/Linux filesystem?
John Topley
  • 2,335
178
votes
9 answers

Is it possible to detach a process from its terminal? (Or, "I should have used screen!")

Possible Duplicate: Can I nohup/screen an already-started process? On Unix (specifically, Linux), I've started a job in a regular ssh->bash session. I'd like to leave work soon, but I now realize that the job is going to take several hours. If I…
mike
  • 4,123
176
votes
10 answers

How to remove empty/blank lines from a file in Unix (including spaces)?

How do I remove empty/blank (including spaces only) lines in a file in Unix/Linux using the command line? contents of file.txt Line:Text 1: 2:AAA 3: 4:BBB 5: 6:CCC 7: 8:DDD output desired…
Michael Ellick Ang
  • 2,279
165
votes
8 answers

What does 'set -e' do, and why might it be considered dangerous?

This question has appeared on a pre-interview quiz and it's making me crazy. Can anyone answer this and put me at ease? The quiz has no reference to a particular shell but the job description is for a unix sa. again the question is simply... What…
egorgry
  • 2,911
150
votes
7 answers

Meaning of directories on Unix and Unix like systems

I've been using Linux for a couple of years now but I still haven't figured out what the origin or meaning of some the directory names are on Unix and Unix like systems. E.g. what does etc stand for or var? Where does the opt name come from? And…
Luke
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148
votes
3 answers

How can I rename a Unix user?

I have a user named old_username and I want him to be named new_username, but I don't want to change his numeric user ID. How can I accomplish this?
Szymon Jeż
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