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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210
votes
11answers
66k views

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 ...
192
votes
5answers
74k views

What is the difference between double and single square brackets in bash?

I just wondered what exactly the difference between [[ $STRING != foo ]] and [ $STRING != foo ] is, apart from that the latter is posix-compliant, found in sh and the former is an extension ...
138
votes
13answers
152k views

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 ...
111
votes
10answers
158k views

How do I get current Unix time in milliseconds using bash?

How do I get the current Unix time in milliseconds (i.e number of milliseconds since Unix epoch January 1 1970)?
110
votes
14answers
99k views

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 ...
106
votes
10answers
57k views

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 ...
85
votes
7answers
126k views

How to forcibly close a socket in TIME_WAIT?

I run a particular program on linux which sometimes crashes. If you open it quickly after that, it listens on socket 49201 instead of 49200 as it did the first time. netstat reveals that 49200 is in ...
83
votes
7answers
51k views

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 ...
79
votes
1answer
48k views

How do I change my private key passphrase?

I have an existing public/private key pair, the private key is password protected, the encryption may be either RSA or DSA. These keys are the kind you generate with ssh-keygen and generally store ...
77
votes
9answers
60k views

Is it possible to detach a process from its terminal? (Or, “I should have used screen!”) [duplicate]

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 ...
63
votes
6answers
72k views

When does /tmp get cleared?

I'm taking to putting various files in /tmp, and I wondered what the rules on deleting them are? I'm imagining it's different for different distributions, and I'm particularly interested in Ubuntu ...
56
votes
5answers
56k views

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's scroll up only with up key line by line. What's more powerful less usages which provide scrolling by page, pattern search backward and so on?
50
votes
3answers
6k views

What is the difference between a soft (symbolic) link and a hard link?

I hear that you can now create soft links in Vista too. So, what is the difference between a soft (symbolic) link and a hard link on UNIX/Linux/Vista? Are there advantages of using one over the other?...
49
votes
4answers
23k views

Drawbacks of mounting a filesystem with noatime?

Having every file be updated just when accessing them sounds like a waste. What's the catch with mounting a file system with the noatime option. What kind of applications/servers relies on the access ...
47
votes
16answers
14k views

Favorite rsync tips and tricks

The more I use rsync the more I realise that it's a swiss army knife of file transfer. There are so many options. I recently found out that you can go --remove-source-files and it'll delete a file ...
44
votes
7answers
46k views

What is a good solution to encrypt some files in unix? [closed]

I am looking for a utility to encrypt certain directories in Linux. I am not looking for any full disk encryption services, but simply to encrypt a few directories for the purposes of storing files in ...
43
votes
6answers
15k views

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 ...
42
votes
2answers
5k views

Why do I have to edit /etc/sudoers with visudo?

I've noticed that the sudoers file and cron config files act in a special way compared to other config files on Linux. They need to be edited with a special wrapper rather than any text editor. Why is ...
41
votes
10answers
10k views

Learning to compile things from source (on Unix/Linux/OSX)

While I install software from packages (MacPorts / apt-get) where-ever possible, I often find myself needing to compile packages from source. ./configure && make && sudo make install ...
40
votes
3answers
37k views

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

I'm trying to figure out how to add "mount --bind /proc/ /chroot/mysql/proc" to /etc/fstab, but how can I to do this?
37
votes
52answers
7k views

Favorite Unix command line aliases? [closed]

What are your favorite command line aliases (bash/sh/tcsh) aliases? Here are a few of mine. alias lsr='ls -lrt' alias gon='cd $HOME/Notes' alias devdb='mysql -h dev --user=x --password=secret ...
36
votes
3answers
4k views

Why is Linux reporting “free” memory strangely?

This is a canonical question about how Unix operating systems report memory usage. Similar Questions: Server refuses to use swap partition Memory Usage in LINUX I have production ...
35
votes
4answers
18k views

How to make a global ~/.vimrc?

Right now, I make everyone do ~/.vimrc and put their settings there. How can I make a global, default .vimrc for new users?
35
votes
3answers
36k views

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? Thanks.
35
votes
9answers
36k views

How to reduce memory usage on a Unix webserver

I'm currently using a Joyent Accelerator to host my webapps, and it's working fine, however I need to reduce costs so I'm downgrading my current plan and that imposes some new memory limits (256M rss, ...
34
votes
30answers
9k views

Command line safety tricks [closed]

Command line and scripting is dangerous. Make a little typo with rm -rf and you are in a world of hurt. Confuse prod with stage in the name of the database while running an import script and you are ...
34
votes
5answers
11k views

Can you have more than one ~/.ssh/config file?

We have a bastion server that we use to connect to multiple hosts, and our .ssh/config has grown to over a thousand lines (we have hundreds of hosts that we connect to). This is beginning to get a ...
33
votes
42answers
8k views

Tools a Unix administrator cannot live without [closed]

After administering Unix or Unix-like servers, what tools (command-line preferably) do you feel you cannot live without?
33
votes
23answers
54k views

Finding the Public IP address in a shell script

I can find my IP address using ifconfig or hostname -i command. But how do I find my Public IP? (I have a static public IP but I want to find it out using unix command)
33
votes
7answers
66k views

How to get e-mail from (failed) cron-jobs in Ubuntu?

I create cron-jobs in Ubuntu by placing the executable in one of /etc/cron.{daily,hourly,monthly,weekly}. There are lots of directories starting with cron: kent@rat:~$ ls -ld /etc/cron* drwxr-xr-x 2 ...
33
votes
7answers
29k views

Why can't you fsck a mounted partition?

It's well-known that you should never fsck a mounted partition. I can understand how this could easily lead to corruption if the filesystem is written to by fsck (e.g., the -a option is used), but why ...
33
votes
8answers
28k views

What is a zombie process and how do I kill it?

Virtual Shotguns?
32
votes
4answers
35k views

How to convert line breaks in a text file between the Windows and Unix/Linux formats?

How do I convert line breaks in a text file between the Windows and Unix/Linux formats? I have a *nix environment, but that I need to import and export data with the Windows-style line breaks. I ...
32
votes
4answers
35k views

How to sort ps output by process start time?

Is there a way to sort ps output by process start time, so newest are either at the top or bottom ? On Linux ? On SysV5 ? On Mac ?
32
votes
8answers
28k views

Is there a way to redirect output to a file without buffering on unix/linux?

I have a long running batch process that outputs some debug and process information to stdout. If I just run from a terminal I can keep track of 'where it is' but then the data gets too much and ...
31
votes
9answers
39k views

Why is Linux more popular than BSD? [closed]

Some would argue that BSD/Unix has always been more reliable and stable than Linux (not me, of course, don't hurt me!). Why does Linux always seem to beat BSD? Is it the romance of the Linux story? I ...
30
votes
6answers
19k views

How to determine the “age” of a linux system since installation?

I thought that I could easily check the timestamp of particular files. Then I realized that it wouldn't be so easy when I saw timestamps like 1991.
29
votes
7answers
55k views

How do I determine the block size of an ext3 partition on Linux?

How do I determine the block size of an ext3 partition on Linux?
29
votes
11answers
1k views

Best practices for keeping UNIX packages up to date?

How do you keep your servers up to date? When using a package manager like Aptitude, do you keep an upgrade / install history, and if so, how do you do it? When installing or upgrading packages on ...
28
votes
5answers
51k views

How to get pid of just started process

I want to start process (eg. myCommand) and get its pid (to allow to kill it later). I tried ps and filter by name, but I can not distinguish process by names myCommand ps ux | awk '/<myCommand&...
27
votes
4answers
11k views

In tail -f, how do I filter out stuff that has certain keywords?

I want to tail -f my logs. However, I want to filter out everything that has the words: "ELB", "Pingdom", "Health"
27
votes
1answer
15k views

rsync - Exclude files that are over a certain size?

I am doing a backup of my desktop to a remote machine. I'm basically doing rsync -a ~ example.com:backup/ However there are loads of large files, e.g. wikipedia dumps etc. Most of the files I care a ...
26
votes
5answers
11k views

What do these abbreviations stand for?

Some directories are easy to understand the meaning /usr /bin ... But for the next ones, I have no idea. /etc /opt opt for optional? etc for electronic t...... configuration (no idea for t) I ...
26
votes
4answers
8k views

What is the first digit for in 4-digit octal Unix file permission notation?

3-digit: 644 ugo (user group other) 4-digit: 0644 ?ugo (??? user group other) What is the first octal digit for in 4-digit octal Unix file permission notation?
25
votes
6answers
12k views

Bash Loop - How to stop the loop when I press Control-C inside a command?

I am rsyncing a few directories. I have a bash terminal open and am executing something like this: for DIR in * ; do rsync -a $DIR example.com:somewhere/ ; done However if I want to stop the whole ...
25
votes
6answers
30k views

How to find other end of unix socket connection?

I have a process (dbus-daemon) which has many open connection over UNIX sockets. One of these connections is fd #36: =$ ps uw -p 23284 USER PID %CPU %MEM VSZ RSS TTY STAT START TIME ...
25
votes
2answers
12k views

Unix socket vs TCP/IP host:port

Could someone please describe to me the pros and cons of using a Unix socket file vs a tcp/ip localhost:port when setting up services on a server (Ubuntu, FWIW)? In this particular instance it's for ...
24
votes
10answers
15k views

Alternative to Daemontools (djbtools) to supervise unix processes?

I've used Daemontools to provide a simple and reliable way to supervise Unix services on my servers. It works well, but it requires a different way of thinking (The DJB Way) and some common complaints ...
24
votes
3answers
13k views

How i configure monit to start a process with a specific user?

Monit runs with root, but i don't want to start my processes as root.. like mysql, mongrel, apache..
24
votes
5answers
3k views

Turning off atime on a filesystem

I am setting up a mongoDB replica set and one of the first things I am suppose to do is turn off atime on the file system. After researching this a bit, I am not opposed to doing this, but I have to ...