Linux is the generic term for a UNIX-like open source operating system based on the Linux kernel.

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445
votes
34answers
215k views

How can I sort du -h output by size

I need to get a list of human readable du output. However, du does not have a "sort by size" option, and piping to sort doesn't work with the human readable flag. For example, running: du | sort -...
367
votes
5answers
140k views

Anyone else experiencing high rates of Linux server crashes during a leap second day?

*NOTE: if your server still has issues due to confused kernels, and you can't reboot - the simplest solution proposed with gnu date installed on your system is: date -s now. This will reset the ...
257
votes
8answers
295k views

Can scp copy directories?

Now I can only copy a single tar file,how can I copy directories recursively with scp?
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 ...
209
votes
2answers
76k views

What exactly do the colors in htop status bars mean?

By default, htop shows colored status bars for processors, memory, and swap. From left to right, the bars are colored green, blue, yellow, and red depending on some thresholds. What does it mean when ...
200
votes
2answers
46k views

Why is “chmod -R 777 /” destructive?

This is a Canonical Question about File Permission and Why 777 is "destructive". I'm not asking how to fix this problem, as there are a ton of references of that already on Server Fault (reinstall ...
196
votes
22answers
56k views

Is it normal to get hundreds of break-in attempts per day?

I just checked my server's /var/log/auth.log and found that I'm getting over 500 failed password/break-in attempt notifications per day! My site is small, and its URL is obscure. Is this normal? ...
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 ...
186
votes
5answers
176k views

What permissions should my website files/folders have on a Linux webserver?

This is a Canonical Question about File Permissions on a Linux web server. I have a Linux web server running Apache2 that hosts several websites. Each website has its own folder in /var/www/. /var/...
185
votes
3answers
133k views

What's the best way of handling permissions for apache2's user www-data in /var/www?

Has anyone got a nice solution for handling files in /var/www? We're running Name Based Virtual Hosts and the apache2 user is www-data. We've got two regular users & root. So when messing with ...
184
votes
20answers
146k views

How can I run a server on Linux on port 80 as a normal user?

I have googled about it for quite some time, but couldn't find it. I am on Ubuntu Linux and want to run a server on port 80, but due to security mechanism of Ubuntu, I get the following error: ...
167
votes
15answers
451k views

Why do I get “Permission denied (publickey)” when trying to SSH from local Ubuntu to a Amazon EC2 server?

I have an instance of an application running in the cloud on Amazon EC2 instance, and I need to connect it from my local Ubuntu. It works fine on one of local ubuntu and also laptop. I got message "...
164
votes
5answers
259k views

MySQL: Bind to more than one IP address?

Is there a secret way to bind MySQL to more than one IP address? As far as I can see the bind-address parameter in the my.cnf does not support more than one IP and you can't have it more than once.
151
votes
4answers
49k views

LVM dangers and caveats

I have recently started using LVM on some servers for hard drives larger than 1 TB. They're useful, expandable and quite easy to install. However, I could not find any data about the dangers and ...
142
votes
39answers
78k views

What useful things can one add to one's .bashrc? [closed]

Is there anything that you can't live without and will make my life SO much easier? Here are some that I use ('diskspace' & 'folders' are particularly handy). # some more ls aliases alias ll='ls -...
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 ...
134
votes
2answers
65k views

Why does my hostname appear with the address 127.0.1.1 rather than 127.0.0.1 in /etc/hosts?

This may be a bit of a noobish question, but I was taking a look at /etc/hosts on my new Xubuntu install and saw this: 127.0.0.1 localhost 127.0.1.1 myhostname On most 'nixes I've used, the second ...
124
votes
9answers
71k views

Keeping a linux process running after I logout

I'm connecting to a Linux machine through SSH, and I'm trying to run a heavy bash script that makes filesystem operations. It's expected to keep running for hours, but I cannot leave the SSH session ...
124
votes
3answers
74k views

Moving an already-running process to Screen

My office job routinely sees me connected to a Linux box via VNC. Sometimes I start a remote job on the console, and realize later that it runs much longer than expected. (Should have started that ...
123
votes
8answers
103k views

How can I run Debian stable but install some packages from testing?

Say you're running a server and you don't want to upgrade to Testing (Squeeze) from Stable (Lenny) to just install a required package or two. What's the best way of installing only certain packages ...
121
votes
27answers
45k views

How do I prevent accidental rm -rf /*?

I just ran rm -rf /* accidentally, but I meant rm -rf ./* (notice the star after the slash). alias rm='rm -i' and --preserve-root by default didn't save me, so are there any automatic safeguards for ...
116
votes
23answers
12k views

How do you make it obvious you are on a production system?

A few of us at my company have root access on production servers. We are looking for a good way to make it exceedingly clear when we have ssh'd in. A few ideas we have had are: Bright red prompt ...
116
votes
7answers
131k views

Setting the hostname: FQDN or short name?

I've noticed that the "preferred" method of setting the system hostname is fundamentally different between Red Hat/CentOS and Debian/Ubuntu systems. CentOS documentation and the RHEL deployment guide ...
114
votes
5answers
9k views

How to cd into a directory with this name “-2” (starting with the hyphen)?

I have a directory named: -2 I want to cd into it but the cd complains: bash: cd: -2: invalid option With no success, I've tried: cd "-2" cd '-2' cd \-2 Any solution? Edit: no file browsers ...
113
votes
4answers
29k views

Should I quit using Ifconfig?

With the servers that mount Infiniband cards, when I use the ifconfig command, I get this warning: Ifconfig uses the ioctl access method to get the full address information, which limits hardware ...
112
votes
9answers
120k views

When does `cron.daily` run?

When do entries in cron.daily (and .weekly and .hourly) run, and is it configurable? I haven't found a definitive answer to this, and am hoping there is one. I'm running RHEL5 and CentOS 4, but for ...
111
votes
85answers
16k views

Linux command line best practices and tips?

I would like to open a discussion that would accumulate your Linux command line (CLI) best practices and tips. I've searched for such a discussion to share the below comment but haven't found one, ...
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 ...
107
votes
3answers
54k views

ssh hostname returns “Bad owner or permissions on ~/.ssh/config”

When I try to ssh to another box, I get this strange error $ ssh hostname Bad owner or permissions on ~/.ssh/config But I made sure that I own and have rw permissions on the file: ls -la ~/.ssh/ ...
106
votes
7answers
126k views

Can I automatically add a new host to known_hosts?

Here's my situation: I'm setting up a test harness that will, from a central client, launch a number of virtual machine instances and then execute commands on them via ssh. The virtual machines will ...
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 ...
102
votes
4answers
136k views

Why is my crontab not working, and how can I troubleshoot it?

This is a Canonical Question about using cron & crontab. The answer for 'Why is my crontab not working, and how can I troubleshoot it?' can be seen below. This addresses the cron system with ...
99
votes
3answers
91k views

Meaning of the buffers/cache line in the output of free

Why does my server show total used free shared buffers cached Mem: 12286456 11715372 571084 0 81912 6545228 -/+ buffers/cache: 5088232 7198224 Swap:...
98
votes
16answers
89k views

Showing total progress in rsync: is it possible?

I have searched for this option already, but have only found solutions that involve custom patching. The fact that it does not show in --help and no more info can be found probably indicates the ...
94
votes
10answers
107k views

Shell command to monitor changes in a file - What's it called again?

I know there was a command on Unix that I could use to monitor a file and see changes that are getting written to it. This was quite useful especially for checking log files. Do you know what it is ...
94
votes
14answers
114k views

How to reconnect to a disconnected ssh session

Is there a way to connect to an ssh session that was disconnected? We are having problems with our network connection to a remote site that we are working on separately; however, in the mean time we ...
94
votes
6answers
198k views

What version of RHEL am I using?

From the shell and without root privileges, how can I determine what Red Hat Enterprise Linux version I'm running? Ideally, I'd like to get both the major and minor release version, for example RHEL ...
93
votes
4answers
83k views

how to disable SSH login with password for some users?

On Linux (Debian Squeeze) I would like to disable SSH login using password to some users (selected group or all users except root). But I do not want to disable login using certificate for them. edit:...
88
votes
11answers
101k views

How to know from which yum repository a package has been installed?

After I have installed a package by yum (with multiple repositories configured), how can I find from which repository it has been installed? If I run yum info package-name (or yum list package-name), ...
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 ...
85
votes
12answers
15k views

What solutions exist to allow the use of revision control for server configuration files? [closed]

In an environment with multiple system administrators, I see a few advantages to adding the server config files into a revision control system. Most notable are the ability to track changes, who made ...
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 ...
82
votes
11answers
18k views

How bad is it really to install Linux on one big partition?

We will be running CentOS 7 on our new server. We have 6 x 300GB drives in raid6 internal to the server. (Storage is largely external in the form of a 40TB raid box.) The internal volume comes to ...
80
votes
8answers
40k views

Heartbleed: how to reliably and portably check the OpenSSL version?

I was looking at a reliable and portable way to check the OpenSSL version on GNU/Linux and other systems, so users can easily discover if they should upgrade their SSL because of the Heartbleed bug. ...
80
votes
4answers
15k views

Why is TCP accept() performance so bad under Xen?

The rate at which my server can accept() new incoming TCP connections is really bad under Xen. The same test on bare metal hardware shows 3-5x speed ups. How come this is so bad under Xen? Can you ...
78
votes
5answers
13k views

Tips for Securing a LAMP Server

This is a Canonical Question about Securing a LAMP stack What are the absolute guidelines for securing a LAMP server?
77
votes
5answers
95k views

How to setup passwordless `sudo` on Linux?

How can passwordless sudo access be setup on either RHEL (Fedora, CentOS, etc) or Ubuntu distributions? (If it's the same across distros, that's even better!) Setting: personal and/or lab/training ...
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 ...
76
votes
4answers
173k views

How to make Windows 7 USB flash install media from Linux?

I have: ISO image of Windows 7 install media 4 GB USB flash drive no DVD drive Linux installed
73
votes
14answers
5k views

What's wrong with always being root?

I have a feeling this is a stupid question, but this is something I've wondered for awhile. I have a VPS and this is my first big linux venture. I am the only person who has access to it. My question ...