Little Known Linux commands

I tried to get this topic off the ground on a couple other sites and it didn't quite grab, so let's see what you people have to say. I want this thread to be one that others can come to to find...well..just what the title is...little known linux commands. Do you have a easier way of doing something? Do you have a work around for a service? Have you found some way of stoppping something from happening that would otherwise take 10 lines in the console? Or vise versa. Whatever it is, share with us. I know you guys have some. With out further adue, here is my first one to share.


Let's say you have to work on a nix system that doesn't have alot of rc.d scripts. Many daemons like to put scripts in "/var/run/". You'll prolly find something like "/var/run/sshd.pid". Other daemons may have similary named files there ie. daemonname.pid.
Let's say you want to HUP sshd to have it re-read it's config (This will NOT kill your current connection) You can do something like this:

kill -HUP `cat /var/run/sshd.pid`

This results in sending the sshd process a HUP signal. The stuff between the backticks will be executed, so this is the equivlent of 'Cat'ing' the /var/run/sshd.pid file, reading the result (which is just the process id of the parent sshd process) and then typing "kill -HUP xxx".
Now let's say you've got some daemon process that is just running away. It's spawning more and more processes and "service blah stop" is not doing anything for you. Here's a cute way to kill all of those processes with the "big hammer":

ps -auxwww | grep "daemonname" | awk '{print $2}' | xargs kill -9


That will seek out all processes running named "daemonname", awk is snatching the second column that has the pid number, and xargs passes that as an argument to the big hammer called "kill -9". Use that one wisely, or start with a "-1". You can match on anything in the ps output as well, such as a particular username. I use a lot of "w's" with ps in case the program is started with an insanely long path.Now note to self that using the -aux switch for ps will choke on a solaris system. Try using -efa for that.

Quote:

---

ps -auxwww | grep "daemonname" | awk '{print $2}' | xargs kill -9

---

i'd normally use killall instead of doing this sort of thing.

Here's one I use regularly:

Code:

---

ssh username@remotehost tar -C /usr -cf - src | tar -C /usr -xvf -

---

This will securely copy /usr/src from the remotehost to the host you're on.

Sirdice, what are the advantages of your way above

Code:

---

scp -rp username@remotehost:/usr/src .

---

I myselve like to do multiline stuff, for example to unrar all .rar files in a folder, bash lets you enter it just the way shown, the >'s are the way bash is shoing you are still working on the ame line.. and abreviates it when you look at the bash command cahce..

Code:

---

for rar in *.rar
>do
>unrar x "$rar"
>rm "$rar"
>done

---

this gets compressed to

Code:

---

for rar in *.rar; do unrar x "$rar"; rm "$rar"; done

---

I think it's easier to enter it multiline, no need to worry about the ;'s

it would be safer to check error code ( $? ) before the rm part, that would make it

Code:

---

for rar in *.rar
> do
> unrar x "$rar"
> if [[ $? == 0 ]]
> then
> rm "$rar"
> fi
> done

---

or

Code:

---

for rar in *.rar; do unrar x "$rar"; if [[ $? == 0 ]]; then rm "$rar"; fi; done

---

Quote:

---

Originally posted here by the_JinX
Sirdice, what are the advantages of your way above

Code:

---

scp -rp username@remotehost:/usr/src .

---

---

Tar has more options. Things like exclude and such.. I also believe mine is faster..but i'm not sure.. I had some trouble once with scp, after that I used ssh+tar. Worked like a charm. I don't remember what exactly went wrong..

A variation on the same theme:

Code:

---

ssh username@remotehost tar -C /usr -cf - src | gzip > src.tgz

---

This will create a src.tgz from the remotehost's /usr/src. Great for remote backups.
It also splits the load a bit, tar on remotehost and gzip on the localhost.

Redirection rulez! :D

It sure does..

a cool little oneliner to read last ten lines of logfiles in a "more natural" way

Code:

---

tac /var/log/messages | head -n 10

---

I use it on loads of log files..

A good one for packing up a Gentoo installation should you happen to forget to emerge your system using the -b (create a binary package as well as compile and install source) flag to emerge.


emerge sync ; emerge -up world

for i in `qpkg -I -nc`
do
quickpkg $i
done

tar -cvf ~/portage-`date +%y%m%d`.tar /usr/portage


This will make a binary package of every package you've compiled and installed. Using this instead of a tarfile from / allows you to reinstall any single package or even use them elsewhere on another system.

Hi all,

I was wondering, me being a win user and all, if there's any way to display devices loaded on the system? Kind of like the equivalent of the hardware manager in win...

Thanks all, and keep em coming.. this thread is really cool -- kudos to hatebreed. :)

-ik

jinx, have you heard of tail ??? tail -n N filename where N = number of lines you want to see..

Quote:

---

Originally posted here by Iron-Kurton
Hi all,

I was wondering, me being a win user and all, if there's any way to display devices loaded on the system?

-ik

---

check out /proc or /sys. there are pseudo or virtual files in these directories. that is to say, the files don't exist, their content is created on the fly.

Code:

---

cat /proc/devices
cat /proc/bus/usb/devices | less

---

Quote:

---

jinx, have you heard of tail ??? tail -n N filename where N = number of lines you want to see..

---

tail puts the most recent entries at the bottom, the "more natural" way puts the newest entries at the top. tail -f is great for monitoring log files. it keeps the file open.

oh, whilst were adding scripts that do our job for us...

apt-get update <- put that in your cron.daily directory ... The amount of times ive forgotten to run that file installed a program only to realise there is an update .... :(

lsof is also good for listing open files (this includes devices etc very useful for programs hogging a device)

lsof | grep /dev/dsp

I have to close some programs via that method because they lock my DSP

*su - root - If you just type su, not everything is exported, i.e. shell aliases and ls colors

Here's a way to use already existing command features that I've discovered some are not aware of:

Doing just a cd without syntax behind it will automagically redirect you back to the home directory of whatever user you happen to be logged in as.

test@/etc/dvd/> cd

Logged in as test will bring you back to /home/test

Logged in as test and under su root, will bring you back to /root

Quote:

---

Originally posted here by Lansing_Banda
*su - root - If you just type su, not everything is exported, i.e. shell aliases and ls colors

---

or just,

[code]su -[code]

:D

the most annoying part about su is that it doesn't even export the PATH.

Anyway, since I like this thread so much, here's my contribution (to tack onto Guardian Alpha, if I may):

Code:

---

cd -

---

will get you back to your previous working directory

example:

Code:

---

[root@me /]cd /usr/bin
[root@me bin]# cd /etc
[root@me etc]# cd -
/usr/bin
[root@me bin]#

---

it's a real time saver.

ik

a few time savers:
pgrep
pkill
zcat
zmore
zgrep

also, someone mentionned lsof, which has fstat for equivalent in bsd...


Ammo

Something that I find very useful is the screen command.

It comes in handy when you want to SSH back to the same session on a box. For example if you're running some kind of client eg an IRC client, usually when you close the SSH session you won't be able to go back to your IRC screen. The program screen solves this. Here's an example:

Code:

---

screen -t irc Bitchx

---

Now you're running Bitchx on a new terminal. To get back to your original terminal you press ^ad (Control+A, then D). The D stands for Detach.
You can now close your SSH session while the Bitchx client still runs. When you SSH in again, run the command

Code:

---

screen -r

---

And voila you're looking at the same Bitchx client you left running before.

Backgrounding ssh port forwarders, ability to use same terminal setting up multiple forwarders. This way you don't accidently close your term and thus killing your forwarder.

ssh -f -N -L 8080:123.123.123.123:80 [email protected]

Just to get back on my own post :) Why use tar instead of move or copy?

mv/cp screws up symlinks, tar doesn't. That's the reason I had problems :rolleyes:

I would still have to tend to either use scp or rsync w/ ssh encryption when copying files/dirs between hosts. But again its all more of a personnal opinion. Theres 50 different ways of doing it on unix :cool:

Alright.. Consider this:

mkdir test && cd test
touch mytest
ln -s mytest mytest2
ln -h mytest mytest2

Now scp that test dir to another machine... See what you end up with.. 3 copies of the same file and no more symlinks.

If you wanna backup across the net using SSH (course assuming /etc as the source to backup and bkuphost is destination hostname or IP):

tar zvcf - /etc | ssh bkuphost "cat > destfile.tgz"

Another thing you could do is replace the destfile.tgz with /dev/tape to backup to tape.