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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.

370
votes
5answers
162k 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 found ...
253
votes
12answers
80k 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 ...
245
votes
2answers
161k views

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 ...
224
votes
14answers
265k 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 ...
213
votes
13answers
122k 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 ...
208
votes
15answers
297k views

How to get 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)?
157
votes
14answers
160k 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 ...
154
votes
10answers
213k 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?
143
votes
8answers
102k 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 ...
132
votes
9answers
114k 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 realize ...
129
votes
7answers
157k views

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 ...
126
votes
3answers
116k views

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?
112
votes
7answers
209k 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 ...
100
votes
4answers
173k 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.
99
votes
7answers
35k 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 ...
76
votes
7answers
31k 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 ...
74
votes
3answers
71k views

How can I rename an unix user?

I have a user named hedgehog and I want him to be named squirrel, but I don't want to change his numeric user ID. How can I accomplish this?
61
votes
4answers
30k 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 ...
57
votes
9answers
128k views

How to disable everything in crontab -l?

I just want to pause everything. Don't execute anything listed on crontab -l.
57
votes
8answers
129k 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&...
57
votes
3answers
8k 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?...
56
votes
16answers
21k 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 ...
56
votes
4answers
43k views

What is the “slash” after the IP? [duplicate]

In Amazon EC2, where I set "security groups", It says: Source: 0.0.0.0/0 And then it gives an example of: 192.168.2.0/24 What is "/24"? I know what port and IP is.
55
votes
6answers
70k 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 ?
50
votes
2answers
7k 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 ...
48
votes
8answers
51k 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 ...
47
votes
9answers
180k views

How to check what port mysql is running on

On my windows dev box mysql is running on port 3306 How can I check what port it is running on the unix server that I have to upload the app to.
47
votes
10answers
13k 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 ...
47
votes
5answers
60k views

How can I run mongod in the background on unix (mac osx)?

I would like to run mongod in the background as an always present sort of thing. What would be the best way to do this? Kind of like the way I can run MySQL on startup and it's just always there ...
46
votes
7answers
155k views

How to remove empty/blank lines in a file in Unix?

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:<blank> 2:AAA 3:<blank> 4:BBB 5:<blank> 6:...
45
votes
5answers
32k 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?
45
votes
3answers
81k views

Getting the last match in a file using grep

What's the best way of getting only the final match of a regular expression in a file using grep? Also, is it possible to begin grepping from the end of the file instead of the beginning and stop ...
44
votes
7answers
73k 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 ...
44
votes
7answers
96k 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 ...
44
votes
3answers
7k 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 ...
41
votes
4answers
19k views

A better unix find with parallel processing?

The unix find(1) utility is very useful allowing me to perform an action on many files that match certain specifications, e.g. find /dump -type f -name '*.xml' -exec java -jar ProcessFile.jar {} \; ...
41
votes
1answer
27k 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 ...
40
votes
7answers
43k 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 ...
40
votes
6answers
57k 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 ...
40
votes
3answers
32k views

Better logging for cronjobs? Send cron output to syslog?

I am looking for a better way to log cronjobs. Most cronjobs tend to spam email or the console, get ignored, or create yet another logfile. In this case, I have a Nagios NSCA script which sends data ...
40
votes
3answers
20k 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 ...
39
votes
23answers
62k 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)
39
votes
8answers
77k 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?
39
votes
6answers
35k 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.
38
votes
52answers
8k 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 dbname'...
38
votes
6answers
124k views

How to change owner of mount point

We have moved mysql data directory to another disk, so now /var/lib/mysql is just a mount point to another partition. We set the owner of the /var/lib/mysql directory to mysql.mysql. But everytime we ...
37
votes
4answers
26k 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"
37
votes
5answers
23k 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 ...
36
votes
4answers
16k 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?
36
votes
7answers
38k views

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

Virtual Shotguns?