Following in the spirit of Hidden Features of PowerShell and various others on Stack Overflow, what Linux commands or combinations of commands do you find essential in your work?
Also See:
Hidden Features of MySQL
Hidden Features of PowerShell
Hidden features of Oracle Database
Hidden Features of Windows 2008
Hidden Features of Solaris/OpenSolaris
Hidden Features of SQL Server
Hidden Features of IIS (6.0 / 7.0)
To get the ball going, I find screen to be essential:
When screen is called, it creates a single window with a shell in it (or the specified command) and then gets out of your way so that you can use the program as you normally would. Then, at any time, you can create new (full-screen) windows with other programs in them (including more shells), kill the current window, view a list of the active windows, turn output logging on and off, copy text between windows, view the scrollback history, switch between windows, etc. All windows run their programs completely independent of each other. Programs continue to run when their window is currently not visible and even when the whole screen session is detached from the users terminal.
Maybe I don't use these every day, but I use them frequently:
I was surprised to find that you can run remote GUI applications over SSH, using the "-X" parameter. For example:
# on my machine
$ ssh -X linuxserver
# on remote machine
$ gedit /etc/my.cnf &
The gedit window appears on my local machine, editing the "my.cnf" file on the server.
I'm assuming this only works if your client machine has an X environment -- in other words, not on Windows. But it works great on my Mac!
lsof often gets ignored, its a very useful tool. lsof lets you view a list of every open file on the system, who / what is using it, etc.
For instance:
root@tower:~ # umount /mnt/hardy
umount: /mnt/hardy: device is busy
umount: /mnt/hardy: device is busy
root@tower:~ # lsof | grep /mnt/hardy
bash 5966 root cwd DIR 253,2 1024 2 /mnt/hardy
root@tower:~ #
Now I see that I'm logged into a shell in another terminal, and /mnt/hardy is my current working directory. So I can either kill that shell, or go to the other terminal and get out of that directory so it can be unmounted.
That's really a trivial example, its very handy for cleaning up the occasional 'bot' infestation too. The options are extensive, see man lsof for more.
Network stack can be left running after a system halt. I don't know if this is current with the 2.6.x series of kernels, but on older versions, you could configure the firewall/routing, then halt the system without a shutdown, leaving just the network stack running. This would allow you to make a cheap (although static) firewall that "can't be hacked" - because there's nothing to hack, as there are no programs or services, just the network stack portion of the kernel passing packets back and forth...
I like "locate" - a much easier way to find files than the gnarly "find . -name xxxx -print". Note that you have to use the updatedb command with it to a your file index up to date; see the man pages for details.
I find "ngrep" really useful for debugging network code on remote servers without having to punt tcpdump files around:
ngrep -d any -W byline port 80
for example, will show you live HTTP requests and responses.
One other thing I've found useful frequently is the "-e" switch to strace:
strace -p <pid> -e trace=open
will show all open() syscalls for a given pid, and
strace -p <pid> -e trace=\!rt_sigprocmask
will exclude all calls to rt_sigprocmask() from the output (useful for debugging RoR code, which seems to make an awful lot of those calls when built to use pthreads).
apropos
DESCRIPTION Each manual page has a short description available within it. apropos searches the descriptions for instances of keyword.
gyaresu@debian:~/bin$ apropos ettercap
etter.conf (5) - Ettercap configuration file
ettercap (8) - (unknown subject)
ettercap_curses (8) - (unknown subject)
ettercap_plugins (8) - (unknown subject)
Thankfully I've only needed a couple of times, but the Magic SysRq key still remains one of my all time favorite hidden features.
Alt+SysRq+RSEIUB
NAME
units -- conversion program
DESCRIPTION
The units program converts quantities expressed in various scales to their
equivalents in other scales. The units program can only handle multiplicative
scale changes. It cannot convert Celsius to Fahrenheit, for example.
It works interactively by prompting the user for input:
You have: meters
You want: feet
* 3.2808399
/ 0.3048
You have: cm^3
You want: gallons
* 0.00026417205
/ 3785.4118
You have: meters/s
You want: furlongs/fortnight
* 6012.8848
/ 0.00016630952
You have: 1|2 inch
You want: cm
* 1.27
/ 0.78740157
ssh-copy-id for transferring ssh keys. The old way was to cat the key over ssh and even older was to scp the key and then cat the key. If you are using a non-standard ssh port then this will do the trick for you ..
ssh-copy-id -i /path/to/key '-p nonstandardport hostname'
Other wise..
ssh-copy-id -i /path/to/key hostname
It's not really that hidden, but may be so for those with no experience, but I really like being able to give lists and let bash expand them like in:
cp arq{,.bak}
which is the same as typing
cp arq arq.bak
I also use the history shortcuts (I don't think that's the right term, but...) like
!!
to repeat the last command, or
^foo^bar
to replace foo by bar in the last command
I like the debian-goodies package:
Description: Small toolbox-style utilities for Debian systems
These programs are designed to integrate with standard shell tools,
extending them to operate on the Debian packaging system.
.
dgrep - Search all files in specified packages for a regex
dglob - Generate a list of package names which match a pattern
.
These are also included, because they are useful and don't justify
their own packages:
.
debget - Fetch a .deb for a package in APT's database
dpigs - Show which installed packages occupy the most space
debman - Easily view man pages from a binary .deb without extracting
debmany - Select manpages of installed or uninstalled packages
checkrestart - Help to find and restart processes which are using old
versions of upgraded files (such as libraries)
popbugs - Display a customized release-critical bug list based on
packages you use (using popularity-contest data)
and also moreutils, which is basically awesome pipe tools on wheels:
Description: additional unix utilities This is a growing collection of the unix tools that nobody thought to write thirty years ago. . So far, it includes the following utilities: - sponge: soak up standard input and write to a file - ifdata: get network interface info without parsing ifconfig output - ifne: run a program if the standard input is not empty - vidir: edit a directory in your text editor - vipe: insert a text editor into a pipe - ts: timestamp standard input - combine: combine the lines in two files using boolean operations - pee: tee standard input to pipes - zrun: automatically uncompress arguments to command - mispipe: pipe two commands, returning the exit status of the first - isutf8: check if a file or standard input is utf-8 - lckdo: execute a program with a lock held Homepage: http://kitenet.net/~joey/code/moreutils/
Virtual consoles. Most people know how to use (Ctrl-)Alt-F1 to get to the first console, etc., but what if you have more than 12? You can use Alt-Left and Alt-Right to cycle through the rest of them. :-D
Perhaps one of the great features of Linux, as opposed to Windows, is that there are next to no hidden features by design. Hopefully if any undocumented features are important enough to know about the community does its job.
A great "feature" I use every day at work: The ability to have SSH listen on port 443 so I can create a tunnel which bypasses my work firewall, allowing me to run a local SOCKS proxy tunneled through SSH to my internet facing Linux server.
I can completely ignore my corporate firewall.
Here are a few I've used on a regular basis:
sar - shows system activityvmstat - virtual memory statsiostat - io statspkill - like pgrep but allows you to kill the returned process IDxargs -I<string> - allows for replacing strings with piped dataat - schedule a tasktkdiff - graphical diff utilityThese are probably not "hidden", but I find them extremely useful:
df -hk - show disk usage in human readable formatls -ltr - list files sorted by datewhile :; do...done - (Bash) replacement for watch if unavailableperl -e - run a Perl snippet on the command-linefree -kt - show memory information (kilobytes w/total)
ESC + .
'Pastes' the last parameter from the previous line into the current prompt
e.g.
ls -l /home/someuser/somedir/somefile
followed by
rm ESC + .
translates to
rm /home/someuser/somedir/somefile
Start with this article with some great Linux tricks. My favorite is pstree, which displays your processes in a tree format so you can see which process spawned which.
curl beats the hell out of wget for pretty much anything you would use wget for.
zgrep and zless are great for searching gzipped log files so you don't have to pipe them through gunzip or leave uncompressed stuff around in /var/log.
I see the yes command very nice:
yes | do_you_agree
From man page:
NAME
yes - output a string repeatedly until killed
SYNOPSIS
yes [STRING]...
yes OPTION
The concept is that everything in Linux is a file.
All configurations are in text files, and everything in Linux is treated as a file. This is a much simpler approach which makes it very easy to change things in Linux. In Linux even your filesystem itself can be viewed as a file.
The hidden feature is that there are no hidden features. The system provides a tremendous amount of power that you have complete access to. You have to understand every piece of it and every tool available to know what power is at your fingertips.
You should start by understanding every command in /bin, /sbin, /usr/bin, /usr/sbin, and every virtual file in /proc. Read the manpages, other documentation, and source as necessary.
If you aren't comfortable reading a man page or reading the source, that is your hidden feature.
od - dump files in octal and other formats. Useful to check for example if there's some BOM crap in the beginning of filefile - what might be the filetype of given file?lshw, lsusb, lspci - list hardwaretracepath - are we dealing with MTU problem here?netwox contains 200+ network related toolsip - for network config/infosysctl - filesystem/network/kernel infoebtables - iptables for bridgevconfig - VLAN configurationbrctl - Bridge configurationsocat - netcat on steroidsipgrab - tcpdump-like utility that prints detailed header informationdig - What's up with DNS server now?
Just found this one today:
dmidecode:
Reports information about your system's hardware as described in your system BIOS according to the SMBIOS/DMI standard. This information typically includes system manufacturer, model name, serial number, BIOS version, asset tag as well as a lot of other details of varying level of interest and reliability depending on the manufacturer. This will often include usage status for the CPU sockets, expansion slots (e.g. AGP, PCI, ISA) and memory module slots, and the list of I/O ports (e.g. serial, parallel, USB).
It definitely makes answering questions about somerandomserver27 at the colo facility a lot easier!
Not really a Linux thing, more of a Bash thing: process substitution,
diff some_local_file <(ssh somehost "cat some_remote_file")
This diffs a local and a remote file retrieved via SSH in one line.
history - shows last used commands
!<number> - executes the command with that number in history
logger lets you write messages to syslog from the shell prompt or a script.
logger "See? There! It happened again!"
A post that I have made on Stack Overflow: How to list only subdirectories in the current one?
ls -d */
It's a simple trick, but you wouldn't know how much time I needed to find that one!
I'm not sure if I am just displaying my ignorance but I just found out about the "last" command for check who has been logging in. It is very useful.
Another good program is "expect". I makes it extremely easy to automate any ssh/telnet logins or anything that forces you to provide input to a program.
find <blah> -exec <blah> {} \;
e.g.:
find . -iname *20080[123456].log -a ! -iname *.bz2 -exec bzip2 \{\} \;
bash history - I normally have 10 or so xterms running. Here is the relavent part of my profile:
# Make history ignore dups, ls, and exit
export HISTIGNORE="&:ls:[bf]g:exit"
# Save 100000 history comamnds
export HISTSIZE=10000
# Make each terminal use a separate history file
HISTDIR=${HOME}/.history
SHELLID=$(tty | sed 's!/!.!g')
HISTFILE=${HISTDIR}/history${SHELLID}
touch ${HISTFILE}
# load last histfile as current history
history -r $(/bin/ls ${HISTDIR}/history${SHELLID} | /usr/bin/tail -n 1)
If I need to search for a command, I can just grep through my history directory.
aespipe - reads from standard input and writes to standard output. It can be used to create and restore encrypted tar or cpio archives.
Usually people run their desktop system with X. But you can usually access true text-mode terminal, several of them actually. These are called virtual consoles. You can then normally login and use command-line, etc.
Just hit Ctrl + Alt + F1, Ctrl + Alt + F2, etc. Your X-server is generally running at Ctrl + Alt + F7.
This of course is distribution and configuration dependent.
Also this command to kill the X-server is sometimes useful: Ctrl + Alt + Backspace.
These key combinations work at least on i86 PC's.
scp
I'm a web dev and our development setup requires me to push files over to our dev server all the time. I wrote a quick wrapper around scp to handle this for me.
dd - convert and copy files
I use this very frequently to make copys of DVD's or HDD Patitions that I need to backup.
dd if=/dev/sda2 of=/tmp/copy_of_sda2
it's very handy and configurable, just have a look at it's man page
For "hidden" feature as in most often overlooked or missed by users new to Linux (/Unix).
man manman -k <blah> or apropos <blah>"Hidden" features
The power and flexibility of SSH never cease to amaze me. Also the ability to make a raid array out of anything (floppies, anyone?), all the filesystems available, cool stuff like LVM, the crypto tools, the possibility of crafting your own tools with the myriad of compilers, interpreters, languages... Oh, and not forgetting getting new stuff installed with apt-get or similar.
It is just so much brilliant than windows.
lsof -i - list all opened socket
htop - like top, but with more eye candy
rdiff-backup - for incremental backup
mc - midnight commander
ethstatus - ethernet statistics
netstat, nmap
iftop - display bandwidth usage on an interface by host
NX combines some of the benefits of screen and ssh -X with clever compression. Now I can run gnumeric remotely over a modem connection and resume it when the connection dies.
fgrep
e.g. fgrep -r * --include=*.rb
walks the dir structure from the current directory looking for in ruby source files
and is much easier to use than find
My top 5 "hidden" features (hopefully these aren't already up here)
cd - try it out :)sort, then to uniq -c and then to sort -n can really work wonders. You can use that to get your top talkers out of an Apache log file or the memory hogs from ps -o.proc/sys interface/filesystem. There is so much there that you can adjust/view in real time. Wrapping a cat or grep of statement in watch -d -n 0.1 is awesome and very handy.Notables are dd, netcat, screen, tcpdump and find but most have been mentioned already.
I find the whereis and which commands handy. Use these when you have alternative versions of the same application with the same name and want to use a specific one of them.
I simply can't do without these commands
Let's see ...
I use bash color codes in my /root/.bash_profile to make my root bash prompt a different color than my non-root standard user prompt. Just one more reminder that I'm a single command away from destroying an entire system. ;)
I use nc (netcat) a lot to test things. Nice utility with a lot of versatility.
For the longest time I never knew that chown could accept both username and groupname together as an argument, i.e.: chown user:group -R /some/directory. That fact has saved me a lot of typing (no more chown/chgrp pairs).
The df command's a quick shortcut to see all your mounted filesystems.
I use pgrep a lot in scripts to see if a process is running.
kexec is pretty neat, lets you reboot without going through BIOS, which shortens reboot times significantly. Don't really reboot a lot, though...
find is my greatest tool. I use it to locate things, and execute commands on files or directories found.
No system is complete without ssh too.
Never used script(1) to save a terminal session?
apg(1) to make random passwords
do you want to know how many processors/cores you have? nproc(1)
sfdisk(8) great partition tool
multitail(1) multiple tail at once...great tool
For me, the greatest feature is that nothing is really "hidden" - it's all there right under your eyes if you bother to read the documentation or code.
