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

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Why does deleting a big file take longer?

My understanding is that running rm on a file simply unlinks it, marking the space as free in the filesystem. It should then follow that deleting one file always takes roughly the same amount of time ...
26
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2answers
776 views

Why is it bad to have root writing files to a directory not owned by root?

This came up in a comment to another question and I'd love it if someone could explain the reasons for this to me. I suggested having Apache log the errors for a given VHost to a user's home ...
26
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4answers
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How useful is mounting /tmp noexec?

Many people (including the Securing Debian Manual) recommend mounting /tmp with the noexec,nodev,nosuid set of options. This is generally presented as one element of a 'defense-in-depth' strategy, by ...
26
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4answers
5k views

Does anyone really understand how HFSC scheduling in Linux/BSD works?

I read the original SIGCOMM '97 PostScript paper about HFSC, it is very technically, but I understand the basic concept. Instead of giving a linear service curve (as with pretty much every other ...
25
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14answers
12k views

How do I find out what version of Linux is running?

Sometimes your scripts need to behave differently on different Linux's. How can I determine which version of Linux a script is running on?
25
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3answers
2k views

Did a cat /dev/urandom - Broken terminal

Out of curiosity, when your shell character set breaks from doing something like cat /dev/urandom is there a way to fix that in place?
25
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5answers
3k views

Linux server out of space

I have been asked this question in two consecutive interviews, but after some research and checking with various systems administrators I haven't received a good answer. I am wondering if somebody can ...
25
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17answers
24k views

Caching/preloading files on Linux into RAM

I have a rather old server that has 4GB of RAM and it is pretty much serving the same files all day, but it is doing so from the hard drive while 3GBs of RAM are "free". Anyone who has ever tried ...
25
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7answers
45k 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 ...
25
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6answers
35k views

FreeBSD vs Linux performance? [closed]

I know that FreeBSD is famous for being highly stable, secure and of course open source. So I am going to try it on one of my servers. However, I am wondering what are the pros and cons of Using ...
25
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7answers
4k views

Reasons to disable / enable SELinux

In the line of this question on StackOverflow and the completely different crowd we have here, I wonder: what are your reasons to disable SELinux (assuming most people still do)? Would you like to ...
25
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4answers
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Where is the statement of deprecation of ifconfig (on linux)?

A lot of people is stating that the ifconfig command is deprecated in favor of the ipone (on linux at least). This is often used as an argumentation to switch from ifconfig to ip (see some comment ...
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6answers
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What happens if I delete lost+found

When you create some Linux filesystems like ext3 a 'lost+found' directory is created. According to this files will be placed there if files were damaged from some kind of system crash. What happens ...
25
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6answers
27k views

Linux: how to restore config file using apt-get/aptitude?

I've occasionally lost my config file "/etc/mysql/my.cnf", and want to restore it. The file belongs to package mysql-common which is needed for some vital functionality so I can't just purge ...
25
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15answers
2k views

How much is IP V6 addressing being used?

Just how much IP V6 addressing is really in typical use out there? I see that Linux/UNIX seems to be ready for this. But I don't see the readiness as much as the Windows side. Especially not for ...
25
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7answers
2k views

Ubuntu, how to setup a new machine like an existing one

I have one machine setup with the apps I need, and I'd like to install the same on a new Ubuntu box. Is there a script that will list all the installed apps, so that I can diff between the boxes and ...
25
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8answers
19k views

Why rsync is faster than NFS?

Few days ago I noticed something rather odd (at least for me). I ran rsync copying the same data and deleting it afterwards to NFS mount, called /nfs_mount/TEST. This /nfs_mount/TEST is ...
25
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3answers
9k views

The XFS filesystem is broken in RHEL/CentOS 6.x - What can I do about it?

Recent versions of RHEL/CentOS (EL6) brought some interesting changes to the XFS filesystem I've depended on heavily for over a decade. I spent part of last summer chasing down an XFS sparse file ...
25
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5answers
19k views

swap partition vs file for performance?

What is better for performance? A partition closer to the inside of the disk will have slower access times, and we must wait for the drive to switch between the OS and swap partitions. On the other ...
25
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5answers
28k views

How passively monitor for tcp packet loss? (Linux)

How can I passively monitor the packet loss on TCP connections to/from my machine? Basically, I'd like a tool that sits in the background and watches TCP ack/nak/re-transmits to generate a report on ...
25
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3answers
3k views

How to handle security updates within Docker containers?

When deploying applications onto servers, there is typically a separation between what the application bundles with itself and what it expects from the platform (operating system and installed ...
25
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1answer
1k views

Minimal rate and default class problem for HTB

I have some doubts about a HTB structure I'm using. My aim is to limit the download and upload speed of users in a local network. Each user of the network has a personal list of domains with a down ...
24
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6answers
27k views

shutdown: What is difference between “Power Off” and “Halt”?

With your typical Linux shutdown command, what is the difference between halting the system and powering off the system?
24
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13answers
6k views

Linux IO monitoring per file?

I am interested in a utility or process for monitoring disk IO per file on CentOS. On Win2008, the resmon utility allows this type of drilldown, but none of the Linux utilities I have found do this ...
24
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4answers
21k 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 ?
24
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6answers
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How can I prevent a DDOS attack on Amazon EC2?

One of the servers I use is hosted on the Amazon EC2 cloud. Every few months we appear to have a DDOS attack on this sever. This slows the server down incredibly. After around 30 minutes, and ...
24
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11answers
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How to make `rm` faster on ext3/linux?

I have ext3 filesystem mounted with default options. On it I have some ~ 100GB files. Removal of any of such files takes long time (8 minutes) and causes a lot of io traffic, which increases load on ...
24
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7answers
19k views

Unzipping files that are flying in through a pipe

Can I make unzip or any similar programs work on the standard output? The situation is I'm downloading a zip file, which is supposed to be unzipped on the fly. Related issue: ...
24
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2answers
14k views

I have a keypair. How do I determine the key length?

Using OpenSSL from the command line in Linux, is there some way to examine a key (either public or private) to determine the key size?
24
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12answers
4k views

Storing and backing up 10 million files on Linux

I run a website where about 10 million files (book covers) are stored in 3 levels of subdirectories, ranging [0-f]: 0/0/0/ 0/0/1/ ... f/f/f/ This leads to around 2400 files per directory, which is ...
24
votes
3answers
19k views

iptables port redirect not working for localhost

I want to redirect all traffic from port 443 to the internal port 8080. I'm using this config for iptables: iptables -t nat -I PREROUTING --source 0/0 --destination 0/0 -p tcp ...
24
votes
1answer
3k views

When using software RAID and LVM on Linux, which IO scheduler and readahead settings are honored?

In the case of multiple layers (physical drives -> md -> dm -> lvm), how do the schedulers, readahead settings, and other disk settings interact? Imagine you have several disks (/dev/sda - /dev/sdd) ...
23
votes
4answers
17k views

How to add message that will be read with dmesg?

I am trying to write some custom messages in my dmesg output. I tried: logger "Hello" but this does not work. It exits without error, but no "Hello" appears int the output of: dmesg I am using a ...
23
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5answers
1k views

What are the actual benefits of assigning sudo privileges to a user instead of using root?

I am fairly new to server administration, and I have seen a lot of sites recommending to assign sudo privileges to a user created by the root user and giving the root user an insanely long password ...
23
votes
4answers
5k 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"
23
votes
9answers
23k views

Disk full, du tells different. How to further investigate?

I have a SCSI disk in a server (hardware Raid 1), 32G, ext3 filesytem. df tells me that the disk is 100% full. If I delete 1G this is correctly shown. However, if I run a du -h -x / then du tells me ...
23
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6answers
14k views

How do I join two named pipes into single input stream in linux

Using the pipes (|) feature in Linux I can forward chain the standard input to one or several output streams. I can use tee to split the output to separate sub processes. Is there a command to ...
23
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5answers
3k views

Can a program tell it is being run under sudo?

I have a program that should behave differently if it is being run under "sudo". Is there a way it can find out if it was run under sudo? Update: Someone asked why would I want to do this. In this ...
23
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6answers
84k views

add new user with root access in Linux

I want to add new user and have/grant that new user to have all the root access, how can I do that ? I did sudo adduser --system testuser but this is not working as I expected. Thanks for help.
23
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3answers
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How can I allow one user to su to another without allowing root access?

I'd like to allow certain users to su to another user account without having to know that account's password, but not allow access to any other user account (i.e. root). For instance, I'd like to ...
23
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3answers
13k views

What is the difference between unlink and rm?

Is unlink any faster than rm?
23
votes
3answers
27k views

Why is my rsync so slow?

My Laptop and my workstation are both connected to a Gigabit Switch. Both are running Linux. But when I copy files with rsync, it performs badly. I get about 22 MB/s. Shouldn't I theoretically get ...
23
votes
3answers
14k views

How to disconnect from lxc-console

I'm working with lxc in Ubuntu 12.04, and it's really great. However, I am unable to disconnect from a lxc-console session after I've connected. I read somewhere that Ctrl-aq will disconnect me from ...
22
votes
9answers
6k views

How to hide a password passed as command line argument?

I'm running a software daemon that requires for certain actions to enter a passphrase to unlock some features which looks for example like that: $ darkcoind masternode start <mypassphrase> ...
22
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10answers
4k views

I just did a chmod -x chmod

So I did a chmod -x chmod. How I can fix this problem? How do I give execute rights back to chmod?
22
votes
10answers
19k views

How do I know if my Linux server has been hacked?

What are the tell-tale signs that a Linux server has been hacked? Are there any tools that can generate and email an audit report on a scheduled basis?
22
votes
5answers
6k 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 ...
22
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8answers
4k views

accidently ran “chown www-data:www-data / -R” as root

i just ran this a few seconds ago, i managed to do ctrl-c as soon as i realized what i started doing. so far in the only dir its started going through is /bin im afraid to do anything else, so far ...
22
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3answers
17k views

Linux - How can I see what's waiting for disk IO

I have a server that has a really high load. Nothing is jumping out at me in terms of CPU usage, and it's not swapping. I think it's cause some processes are waiting for disk IO, and I want to see ...
22
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7answers
22k views

how can I limit per user bandwidth?

To summarise: I have a dedicated server with a few friends running a torrent client with web gui. Each user is running a client under their username on the server so downloads go in their user dir, ...