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

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12answers
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How much SWAP space on a 2-4GB system?

How should I decide what size to make my swap on a new Linux machine (Debian) with 2-4 GB of RAM? Do I really need swap space?
49
votes
3answers
36k views

Linux command to inspect TXT records of a domain

Is there a linux shell command that I can use to inspect the TXT records of a domain?
49
votes
6answers
54k views

Find out symbolic link target via command line

Say that I setup a symbolic link: ln -s /root/Public/mytextfile.txt /root/Public/myothertextfile.txt is there a way to see what the target of myothertextfile.txt is using the command line?
49
votes
6answers
33k views

How to add a security group to a running EC2 Instance?

I have an Amazon EC2 instance running and I will like to add another security group to that instance and then remove the current security group from that instance. Is this possible?
49
votes
10answers
50k views

Reread partition table without rebooting?

Sometimes, when resizing or otherwise mucking about with partitions on a disk, cfdisk will say: Wrote partition table, but re-read table failed. Reboot to update table. (This also happens with ...
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 ...
48
votes
8answers
9k views

How do I know I'm running inside a linux “screen” or not?

The "screen" refers to a program mentioned in How to reconnect to a disconnected ssh session . That is a good facility. But there is a question I'd really like to know. How do I know whether I'm ...
48
votes
3answers
65k views

How to get the url of the current svn repo?

I have 2 svn checkouts that someone setup for me. Now I need to check these same files on another computer, but since I didn't check them out initially I don't know the urls to use when running the ...
48
votes
3answers
12k views

Isn't Ctrl-Alt-Delete on Linux *really* dangerous?

Is the default Ctrl-Alt-Delete shutdown -r functionality on Linux systems a dangerous feature? Years ago, when I deployed physical systems with attached keyboards and monitors, I'd sometimes modify ...
47
votes
6answers
91k views

How do I verify the speed of my NIC?

I just installed a new gigabit network interface card (NIC) in Linux. How do I tell if it is really set to gigabit speeds? I see ethtool has an option to set the speed, but I can't seem to figure ...
47
votes
4answers
121k views

Postfix - how to retry delivery of mail in queue?

I have a backup mail server in case of a failure on the main one. In that case of failure, mails come on the backup server and stay there until the main one is back. If I wait some times, the ...
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 ...
47
votes
5answers
93k views

Practical maximum open file descriptors (ulimit -n) for a high volume system

We recently began load testing our application and noticed that it ran out of file descriptors after about 24 hours. We are running RHEL 5 on a Dell 1955: CPU: 2 x Dual Core 2.66GHz 4MB 5150 / ...
46
votes
6answers
109k views

How do I sleep for a millisecond in bash or ksh

sleep is a very popular command and we can start sleep from 1 second: # wait one second please sleep 1 but what the alternative if I need to wait only 0.1 second or between 0.1 to 1 second ? ...
46
votes
8answers
94k views

df in linux not showing correct free space after file removal

I have file servers which are used to store files. Files might reside there for a week, or for a year. Unfortunately, when I remove files from the server, df command doesn't reflect the freed up space....
46
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5answers
5k views

Why is tampering with the TTL of IP dangerous?

I've been reading the iptables man-page (light bedtime reading) and i came across the 'TTL' target, but it warns: Setting or incrementing the TTL field can potentially be very dangerous and ...
46
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9answers
15k views

Linux: how to explicitly unswap everything possible?

I've launched something that took lots of memory and now everything lags a lot. I guess all applications' memory has gone to swap in order to free some space for the memory-intensive process, and now ...
45
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5answers
38k views

REJECT vs DROP when using iptables

Is there any reason why I would want to have iptables -A INPUT -j REJECT instead of iptables -A INPUT -j DROP
45
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5answers
23k views

How to understand the memory usage and load average in linux server

I am using a linux server which has 128GB of memory and 24 cores. I use top to see how much it is used. Its output is pasted at the end of the post. Here are two questions: (1) I see that each of the ...
44
votes
9answers
65k 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 ...
44
votes
3answers
4k views

nice sudo or sudo nice?

is there any difference between running an intensive task over sudo with the following commands?: nice sudo [intensive command here] sudo nice [intensive command here] BTW this is for Linux 3.x.
44
votes
7answers
45k 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
3answers
15k views

Does getting disconnected from an SSH session kill your programs?

So, say I get disconnected from an SSH-session after I've started rsync or cp or any other command that can be long running. Does that command keep running until it's finished after I get disconnected ...
44
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15answers
20k views

iptables Tips & Tricks

I'm sure Linux sysadmins are quite familiar with iptables, the userland interface to the netfilter packet-filtering framework. Now, this "Question" is meant to be a Community Wiki for collecting ...
44
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10answers
7k views

How Often Should I Update our Linux Server?

I am responsible for managing both our production server (mail, web, database are all on one server) and our test server. Both are built on Debian. However as I am very new to system administration, I ...
43
votes
14answers
85k views

How to create an SHA-512 hashed password for shadow?

The previous SF questions I've seen have lead to answers that produce MD5 hashed password. Does anyone have a suggestion on to produce an SHA-512 hashed password? I'd prefer a one liner instead of a ...
43
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17answers
35k 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 ...
43
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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 ...
43
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7answers
38k views

What is the best Linux filesystem for MySQL (InnoDB)?

I tried to look for benchmark on the performances of various filesystems with MySQL InnoDB but couldn't find any. My database workload is the typical web-based OLTP, about 90% read, 10% write. Random ...
42
votes
5answers
3k views

Is there a linux terminal command that lets you see the amount of data coming in to your tcp connections in real time?

It would be something similar to top, where you see your cpu processes in real time. I'm not looking for a GUI like Wireshark to do it.
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 ...
42
votes
3answers
28k views

Force dig to resolve without using cache

I'm wondering if there is a way to query a DNS server and bypass caching (with dig). Often I change a zone on the DNS server and I want to check if it resolves correctly from my workstation. But since ...
42
votes
2answers
28k 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?
42
votes
1answer
11k views

Why are my XFS filesystems suddenly consuming more space and full of sparse files?

I've run XFS filesystems as data/growth partitions for nearly 10 years across various Linux servers. I've noticed a strange phenomenon with recent CentOS/RHEL servers running version 6.2+. Stable ...
42
votes
10answers
6k views

PostgreSQL Replication

We constantly bat this around the office, and the question continues to come up. How do you deal with PostgreSQL replication? I'm not even necessarily talking about advanced clusters, just keeping it ...
41
votes
6answers
4k views

How can you tell what a server actually does? [closed]

I've been handed 3 Linux boxes, 1 front facing with apache on it and another 2 which, as far as I can tell, don't do an awful lot. All running on Redhat. The question is simple: How can I tell what ...
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 ...
41
votes
9answers
107k views

How to display certain lines from a text file in Linux?

I guess everyone knows the useful Linux cmd line utilities head and tail. head allows you to print the first X lines of a file, tail does the same but prints the end of the file. What is a good ...
41
votes
10answers
7k views

Linux on VMware - why use partitioning?

When installing Linux VMs in a virtualized environment (ESXi in my case), are there any compelling reasons to partition the disks (when using ext4) rather than just adding separate disks for each ...
40
votes
22answers
15k views

Why change default ssh port? [closed]

I've noticed that a lot of admins change the default ssh port. Is there any rational reason to do so?
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?
40
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3answers
22k views

What is the difference between unlink and rm?

Is unlink any faster than rm?
40
votes
1answer
17k views

Linux FHS: /srv vs /var … where do I put stuff?

My web development experience has started with Fedora and RHEL but I'm transitioning to Ubuntu. In Fedora/RHEL, the default seems to be using the /var folder while Ubuntu uses /srv. Is there any ...
39
votes
7answers
48k views

Poodle: Is disabling SSL V3 on server really a solution?

I've been reading all day about the Poodle vulnerability and it I am bit confused now vs Security and Revenue. If I disable SSL V3 on Server (SSL V2 and V3 both will be disabled for Apache) clients ...
39
votes
11answers
99k views

create home directories after create users

I created some users with: $ useradd john and I forgot to specify the parameter -m to create the home directory and to have the skeleton files copied to each user. now I want to do that, and I ...
39
votes
6answers
25k 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 ...
39
votes
3answers
79k views

How do I extract login history?

I need to know the login history for specific user (i.e. login and logout time), How do I extract this history for a specific date range in Linux ?
39
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6answers
32k views

How do I permanently disable Linux's console screen saver, system-wide?

I've got an Ubuntu server that boots up in text mode. It rarely has a screen or keyboard attached to it, but when I do attach a screen, I usually have to attach a keyboard too, because the darn ...
39
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1answer
11k views

What is the difference between a 'Login' and an 'Interactive' bash shell

What is the difference between a ‘Login’ and an ‘Interactive’ bash shell? I have quoted Wikipedia below but can anybody give a better answer? EDIT: This is a community wiki so maybe rather than ...
38
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4answers
65k views

EPEL Repo for CentOS 6 causing error

I've got a VM running CentOS 6 (64bit) and I'm attempting to add the EPEL repo like usual to install various packages as I do quite regularly. Today, I'm experiencing some strange errors yet I'm ...