(Useful) Stupid Unix Tricks?

So the other day I messaged another admin from the console using the regular old 'write' command (as I've been doing for over 10 years). To my surprise he didn't know how to respond back to me (he had to call me on the phone) and had never even known you could do that. That got me thinking that there's probably lots of things like that, and likely things I've never heard of. What sorts of things do you take for granted as a natural part of Unix that other people are surprised at?

rm -rf /

[Anonymous Coward]

rm -rf /

Bah, subtlety:

[FooAtWFU]

:(){ :|:& };:

Re:Bah, subtlety:

[jkiol]

@$**& well we know it works in cygwin too.

Re:Bah, subtlety:

[Nathanbp]

:(){ :|:& };:

:() defines a function named : with no arguments. { :|:& } is what the function does. :|: calls itself twice (with a pipe between the two), and the & at the end runs it in the background as a new process. The ; finishes off that command, then the last : runs the function, starting the fork bomb (as each run starts 2 new processes, each of which starts 2 new processes...).

Re:rm -rf /

[acidreverb]

Mediocre minds think alike. Great minds are unique.

Re:rm -rf /

[4D6963]

Mediocre minds think alike. Great minds are unique.

My thoughts exactly.

Re:rm -rf /

[Anonymous Coward]

OMG OMG What did I do. What did I do. I am so fired. I am so EFFIN fired.

Re:rm -rf /

[Anonymous Coward]

OMG OMG What did I do. What did I do. I am so fired. I am so EFFIN fired.

Ha, I didn't get fired when I did it. It was about ten years ago now, I had a linux dualboot setup on a test PC and didn't need the linux install any more.

I thought, "I've always wanted to try rm -r /" so I did. About four seconds later it dawned, with an "oh shit" that I still had the dos/windows 3.11 partition still mounted read write.

Fortunately, I didn't permanently lose anything between good backups and Norton Disk Doctor.

Re:rm -rf /

[j79zlr]

Fortunately, I didn't permanently lose anything between good backups and Norton Disk Doctor.

Yes, you do need good backups whenever you are running Norton products. Good idea.

Re:rm -rf /

[butalearner]

I got something similar to these from some website that I have long forgotten: alias AvadaKedavra kill -9 alias Obliviate rm -rf alias Alohomora chmod -Rf ug+w alias Accio grep -Ir \!:1

Re:rm -rf /

[ArhcAngel]

I keep getting Windows cannot find 'rm' when I type that in. Am I doing it wrong?

what the hell. I had karma to burn.

Useful tricks.

[ElizabethGreene]

I can't live without svn. Svn is a revision control repository, usually used for source code. What makes it really powerful is that you can _easily_ have a history of everything that has changed in a file and when. On my systems, I keep /etc in svn, plus bind's zone files, plus all the non-image web content, and the "Network Documentation" folder.

Second trick, rsync. I use it to backup my home directory to another box. Very nice when you go through a hard drive/year.

Screen -x was my next pick, but somebody already mentioned it.

"echo ProtocolKeepAlives 120 >> /.ssh/config" No more dropped ssh sessions because of stupid nat boxes.

su -u Username -s .. become Username, but keep the current shell. Good for diagnosing permissions problems when the user has a /bin/false shell. (named/www-user/backup/etc).

A little awk goes a long way. Not the big-bad-I-am-a-programming-language-awk, but the smaller-friendlier extract one or two columns of text from something awk. ex. awk '{print $2}' prints the thing in the second column. Add -F the field separator tool and it gets really useful.
Better example. Here is a postfix log line.
      Nov 5 16:27:19 pdc postfix/smtpd[13601]: 92B3F499C25F: client=exprod5mx254.postini.com[64.18.0.49]
Here is the awk to extract just the message id. awk -F': ' '{print $2}'

And here is the "I didn't get this message your mailserver must have eaten it" disprover. It searches the maillog for every message from or to a given address and extracts the full email transaction for that message id.

grep -i [e-mail] /var/log/maillog | grep smtpd | awk -F': ' '{print $2}' | sort -un > temp.fil && grep maillog -f temp.fil

Next trick, back ticks. `` Back ticks substitute the output of a command within a command.
Ex. Name a file after the date. echo "hi" > `date +%Y%M%d`.txt

On the subject of dates. date -d'yesterday' or 'last week' or '-4 hours' can be handy.

Last one. Loopback nat with Iptables, so you can access local hosts by their external ip. (Instead of setting up split dns.)
iptables -I POSTROUTING -s $local_network -d $local_network -j SNAT --to $lan_interface

My local network is 192.168.0.0/24 and the netfilter lan ip is 192.168.0.1, so that becomes...
iptables -t nat -A POSTROUTING -d 192.168.0.0/24 -s 192.168.0.0/24 -j SNAT --to 192.168.0.1

-ellie

Re:Useful tricks.

[cowens]

Next trick, back ticks. `` Back ticks substitute the output of a command within a command.
Ex. Name a file after the date. echo "hi" > `date +%Y%M%d`.txt

Don't use backticks unless you are stuck with bourne shell. Use $() instead:

echo hi > $(date +%Y%M%d).txt

They have two benefits over backticks: you can nest them and they are easier to see.

Re:rm -rf /

[Frymaster]

or the "unix koan" grep "" /dev/null

Re:rm -rf /

[jsolan]

puercopop@localhost ~ $ which which
which: no which in (/usr/bin:/usr/local/bin:/usr/bin:/bin:/opt/bin:/usr/x86_64-pc-linux-gnu/gcc-bin/4.1.2:/usr/qt/3/bin:/usr/games/bin)

Burn her anyway!

Re:rm -rf /

[MikeDirnt69]

I prefer this one:

$ touch woman

But I always get this back:

$ touch: woman: Permission denied

:(

Re:Show attached block devices

[duguk]

Some more common ones I've thought of:

screen - too useful, run apps in a virtual console which you can attach, deattach and share

cd `pwd -P` - Jump into the real directory (from a linked directory).

history - use it with grep if you forgot what you did

strings - just show the printable strings from a file

tail and head - tail -f is a lifesaver

sftp - i really shouldn't need to explain this.

file - do magic stuff

Hope that's some help.

Re:Show attached block devices

[MrMunkey]

cd -

Change to your previous directory. It's great for going from dev to test environments from time to time.

Re:Show attached block devices

[EvanED]

Along that line are pushd and popd. pushd <dir> changes to the specified directory and pushes it onto a stack of directories; popd changes to the directory at the top of the stack and removes it. There are commands for manipulating the directory stack but I don't know or use them.

With zsh, and I think with Bash as well, you can setopt AUTO_PUSHD and setopt PUSHD_SILENT and then cd behaves like pushd.

(Both of these commands, along with cd -, work in the Windows command interpreter too.)

Re:Show attached block devices

[aniefer]

ctrl+r (in bash?): reverse incremental search through history.
pushd/popd , change directory saving the old one on a stack.

Re:Show attached block devices

[Isomer]

More awesomely, if you have found something in your history with ^R or up arrow or whatever, then you can press ^O to "execute this line and put the next line in the history onto the command line". Thus:
vi foo.c
make
./foo
^Rvi^O^O^O^O^O^O^O^O^O^O^O^O^O^O
and so on.

Re:Show attached block devices

[jonaskoelker]

sftp - i really shouldn't need to explain this.

I much prefer sshfs. Diff doesn't work so well over ftp ;)

Re:Show attached block devices

[cain]

tail and head - tail -f is a lifesaver

I use tail -F, which is the same as tail -f, but works on non-existent files. Useful when tailing log files from programs that start a new log file every time it runs. Using tail -F in this case, you can just leave tail running while you start and restart the program overwriting the log file.

Re:Show attached block devices

[mollymoo]

du -cks

OK, it's not a trick or very obscure, but it is a useful set of flags and it spells the name of an animal. Which is cool, if you need to get out more. I need to get out more.

Re:Show attached block devices

[the_B0fh]

Oh my ghod. This is considered informative? Who let all the PFYs in?

Re:Show attached block devices

[ip_fired]

I actually like less better than tail -f. If you less a file, and then hit SHIFT-F it will tail the file, but you can break out of it and scroll around and search for terms. Very handy while looking at log files.

Re:Show attached block devices

[geminidomino]

Organizing them all is left as an exercise for the reader.

Cakewalk. Put the following in ~/.bash_profile or ~/.profile

For ubuntu:
SHELLID=(echo `tty` | sed 's!/!.!g')
HISTFILE=$HISTFILE$SHELLID

Logout. Log back in. Bada-bing.

Re:Show attached block devices

[FrangoAssado]

I think you missed a '$' before the open parenthesis, it should be:

SHELLID=$(echo `tty` | sed 's!/!.!g')

Also, you could replace "echo `tty`" with simply "tty":

SHELLID=$(tty | sed 's!/!.!g')

Re:Show attached block devices

[BorgCopyeditor]

Your data is corrupt: only the current sig is ever shown.

Re:Show attached block devices

[HTH NE1]

This shows all attached block devices (it also errors like crazy, hence the | more)

blockdev --report /dev/* | more

Redirect stderr much?

blockdev --report /dev/* 2> /dev/null

Re:Show attached block devices

[duguk]

On the contrary, shorter IS something to be proud of.

Cool! Thanks! I'll tell my boyfriend that next time!

Crashed My Laptop!

[TheNinjaroach]

I have never, ever seen a hard crash on my IBM Thinkpad T40. So I ran that command as root, looked at the output and thought "Huh that's neat." Thirty seconds later, my screens go black and I'm looking at a disoriented IBM POST screen, mostly black with a broken progress bar.

My system booted up fine, so of course the first thing I wanted to do was make it happen again.

`blockdev --report /dev/* | more `

Thirty seconds after the output finished and I'm looking at the garbled POST screen again. My laptop finished booting, I ran the command a third time before coming to tell Slashdot and

Using the "right" interpreter with env

[dwheeler]

If you begin your scripts with:

#!/bin/env python

and replace "python" with whatever your script interpreter is, then you can have the script automatically use whatever interpreter is first on your PATH. This is especially nice if you're "not sure where the interpreter executable is", e.g., it might not be in "/usr/bin" - so this helps portability. (The POSIX standards GUARANTEE that "env" is in /bin, so this is VERY portable.) This also makes it easy to try out new interpreters (load a test version's binaries in ~/python-beta, add that first to that PATH, and now the test version's interpreter is used.) This does have the extra cost of starting up /bin/env first, but often that's not a big deal.

Yes, this is a bad idea if the attacker can control the PATH & this is security-relevant. But you can't securely run most interpreters directly anyway, so that's usually not relevant.

Re:rm -rf /

[orclevegam]

rm -rf /

wtf??? (do not try this at home)

Really? What does it do? Think I'll try it and s
CARRIER LOST

Re:rm -rf /

[carrier lost]

CARRIER LOST

Hello?

Re:rm -rf /

[macshome]

Pshaw! All 1337 sysadmins just live as root!

Re:rm -rf /

[VShael]

Pshaw! All 1337 sysadmins just live as root!

Yes, but all the really stupid ones do as well.

Re:rm -rf /

[Omega996]

sudo is for ubuntu wannabes - real UNIX admins don't sudo - they su - .

Re:rm -rf /

[Teilo]

No. People who run as root all the time are either n00bs or morons.

N00bs if they have never spoken the words, "Oh sh*t!" after running a command;

Morons if they have.

Re:rm -rf /

[Omega996]

if you haven't said "oh shit" while doing something as root, you haven't done UNIX administration in a busy production environment.

Re:rm -rf /

[geminidomino]

Thank you.

There's two kinds of real UNIX Admins.

1. Those who have yelled "Oh Shit!"
2. Those who will.

Best Dilbert

[theshowmecanuck]

PHB: Do we have Eunuchs here?.. I heard it is very important for a company's IT department to have Eunuchs.
Dilbert: I think you mean Unix. Yes we have a lot of Unix machines here.
PHB: Oh... [pauses and thinks]... If the company nurse comes by, tell her I said, "never mind."

Re:rm -rf /

[Curtman]

when I told a guy over the phone to 'rm -rf /bin' but unknowingly he was at "/". Oooops

I don't get it. You told him to give rm an absolute path, it doesn't matter where he was.

Re:rm -rf /

[TheLink]

I wonder if anyone has had problems with guiding people to uninstallin stuff like Asterisk over the phone ;).

e.g.
A: "Type rm -rf /etc/asterisk
B: "OK"
A: "Next..."
B: "Wait it's not done yet"
A: "?"
A: "!"

Seems like people should be more careful about product directory names ;).

Don't call your directories stuff like "star" or "slashdot" if you ever might need to get people to remove them over the phone.

Re:rm -rf /

[mr_mischief]

cat /dev/urandom > /dev/dsp

Re:rm -rf /

[Anonymous Coward]

sleep 8h; cat /dev/urandom > /dev/dsp

That's my alarm clock.

Re:rm -rf /

[subStance]

/dev/random to the dsp device ? I prefer to the midi device, then it sounds like someone throwing a drumkit and grand piano down the stairs at the same time.

Re:rm -rf /

[Dtyst]

I prefer the Russian roulette version:

[ $[ $RANDOM % 6 ] == 0 ] && rm -rf / || echo "You live"

Re:rm -f /lib/libc*

[feargal]

You want subtle?

ln -f /bin/rm /usr/bin/diff

Well

[Anonymous Coward]

Well.

dd if=/dev/zero of=/dev/sda1

Re:Well

[FooAtWFU]

Not quite the same, but in a similar vein, cat /dev/urandom > /dev/dsp

Sometimes a quick white-noise machine is relaxing. Heck, I used that command in combination with 'at' to act as a makeshift alarm clock when I was just moving into my first apartment and had forgotten my only other electronic device with an alarm (my cell phone) at the office.

Re:Well

[Technonotice_Dom]

Heck, I used that command in combination with 'at' to act as a makeshift alarm clock[..]

You mentioned it only in passing, so I thought I'd draw a little more attention to it. The 'at' command is a really handy way to automate one-off tasks that many people seem to miss. The interface is neat too, understanding plain English time specifications.

I've often seen people add a one-off task to a crontab, then try and forget to remove the entry once it's run!

Re:Well

[Hatta]

You only need a tiny bit of entropy to seed the pseudorandom number generator in /dev/urandom. Once it's seeded you get all the pseudorandom numbers you want. /dev/random gives you truly random numbers, and is highly dependent on the amount of entropy the system has. It will block if you run out of entropy, urandom will not.

session-sharing with screen -x

[FooAtWFU]

(used in my company for doing the agile/extreme "pair programming" think with a remote devloper, among other things).

screen is awesome.

Re:session-sharing with screen -x

[mollymoo]

In a similar vein, back in the days of floppies you could have endless fun ejecting other people's disks from Sun workstations. They put it in, you eject it. They put it in, you eject it. Repeat till you get bored or it looks like they're about to do a 'who'.

Re:session-sharing with screen -x

[quarterbuck]

eject is a useful tool, If you have a rack of servers all alike and you need to identify one of them . Some servers have blinking lights etc. but mine had no audio nor lights. But it had a CD tray.
I simply put eject and "eject -t" in a loop and go look in the server room -- the hyperactive server is the one I was looking for.

There is this part ...

[140Mandak262Jamuna]

... Generally people are surprised by the fact that you could type some strange incantations into a black window like awk grep etc and make the computer do things without touching the mouse. Yeah, some are surprised by that thing.

Re:There is this part ...

[Forty Two Tenfold]

I once made my friends' jaws hit the floor when I burned a cd for them - from console.

And once I had this strange feeling that something was wrong with the CD drive of a machine I was working at in the console until I realized I was opening and closing the CD tray on a machine in another room!

Re:There is this part ...

[sydneyfong]

True story.

A friend and I help admin the computers in my (former) high school. Due to security the doors were locked during off hours, and I'm not a frequent helper so I don't have the keys, but my friend did.

There was once when I was basically stranded in the computer lab, and my friend was in the server room (where the cell phone signals didn't reach). I don't know what he was doing at that time, but "walls" (on the linux machines) and "net sends" (to the Windows servers) didn't seem to work, so I ran a script to open and close the CD tray hoping to catch his attention.

I got a message asking "wtf are you doing?" a few minutes later :)

Re:There is this part ...

[Timex]

True story:

I was in a class learning how to install and run a network management program (this was mid-1990s), and the class had a set of IRIX systems to work from. There were six systems to work on, so some of the students had to pair-up.

The class was pretty-much done, and we were waiting to be dismissed.

Suddenly, I get a message on the console of my system stating that the system would reboot in five minutes. (This was the system default.) I went in, found and killed the shutdown process, then checked the logs to see who kicked it off. It was the two clowns in front of me. I went into their system (there was no real security here, if you haven't figured that out. The systems weren't on the Internet, so it wasn't a big deal) and set the system to init state 0 (this reboots the system immediately, for you guys that are oblivious to init states).

They looked at each other, they looked at me (I was just sitting there, looking at their reaction), and they looked at each other again. One said, "See? I TOLD you we shouldn't have f*d with him!"

Heh. :)

Re:There is this part ...

[infinite9]

I do both microsoft and unix development. This has led to some interesting situations. I wrote a mathematical parser in c#/.net that could process math expressions at runtime using Regex to get tokens from the expression. The regex wasn't too bad. But after a code review, my pointy-haired manager made me comment each symbol in the regex. 40 lines of comments to describe 1 line of code.

As a consultant, VI is my absolute favorite tool. Not on unix projects, on microsoft projects. It always happens eventually. Someone needs to modify a file in a way that screams for regex search with replace, but is a nightmare in visual studio or some other windowy editor. So I have them stand behind me while I write an long, arcane-looking regex line in VI. When I press enter, the entire file instantly morphs into exactly what they want. I can think of no better way to justify my exorbitant bill rate. lol

Re:There is this part ...

[Trailer Trash]

Years ago in college we had a few Sun 3 workstations, and all of us CS types had logins on them of course. I also worked for the university computing department as did one of my fellow CS students. Often I'd be working on the Sun 3 when he'd log in and run his "xeyes from hell script", which would essentially open a bunch of xeyes with random geometries.

Usually, when he was doing this he was sitting in another office working on a DOS machine running NCSA Telnet, which had the awesome feature that it would run an FTP server when you were using telnet.

So, I would go to another machine, figure out his IP address using "w", and ftp to his desktop. Then I'd type the killer: "cd a:".

With that, it would use the old BIOS call, which in the absence of a floppy would literally lock up the machine and prompt you to stick a disk in.

It gets better (or, worse if you're Colin). After he'd scrape up a floppy and stick it in, I would get the prompt back letting me know he'd found one. No problem: "cd b:".

There was no second floppy on that particular machine, so the BIOS would emulate two floppies and keep track of which was which. So he had to run and grab another to make the machine usable again. At that point, I could lock the machine up at will simply by cd'ing to the other floppy. And he would find me, and use "write" to ask for a truce.

Funny thing is, it was really easy to turn off the ftp server. I guess I can let that cat out of the bag now.

grep and awk

[yakmans_dad]

So much easier for me to use than perl. I presume the modern unix user prefers perl.

On to the pranks!

[Anonymous Coward]

I once worked with an admin that wrote a program that wrote directly to a users terminal and prompted in the same way "write" did. One notable exception is that he let you put whatever username@hostname you wanted.

I got quite a few requests from "[e-mail]" to chat...

Tab

[computersnstuff]

I'm sure everyone at some point is surprised of tabbed completion.

Re:Tab

[Anonymous Coward]

I'm sure everyone at some point is surprised of tabbed completion.

Woah! Got any more?



(yes, I'm being sarcastic)

Re:Tab

[jcam2]

You'd be surprised how often I have seen experienced programmers manually type out long commands or directory paths, instead of using tab completion. Sometimes I have to restrain myself from ripping the keyboard from their hands and using tab to enter the path myself in a 10th of the time.

Re:Tab

[jez9999]

Apparently in Unix you can connect to another computer - get this - *without having to be at the screen* to see it! I think the software was developer by a company called Shell or something. You use 'shhhh' to use it, which I think tells the computer to basically quieten down its TCP communications so it can get ready for the connection to the other machine. I'm probably not doing it justice but you have to see for yourself.

It's not as good as Windows though because you still have to use typing to do things, and then it's only very boring things like deleting files. :-( I haven't found Solitare or Pinball yet. Hopefully Shell will invent a remote desktop program soon though so we can replace Remote Desktop!

Re:Tab

[Bandman]

Seriously, I was giving a presentation at a LUG meeting a few years ago, and during a break, some guys came up to me and said "We know you can't type that fast. How do you do that?"

Re:Tab

[Craig Davison]

With bash, you can even get tab completion for hostnames. Try this:

ssh user@l[tab]

Everything after the @ is filled in from /etc/hosts.

Re:Tab

[gEvil (beta)]

I just tried using this in Word. Instead of finishing the word I was typing, it kept on moving the little "insertion line" thing to the right. I already filed a bug report, but do any of you have a quick fix?

I never knew that command

[PingXao]

And I've been administering Linux systems for awhile now. Step back for a moment and you'll find that "man pages" and "info" are actually a pretty awful way to distribute documentation. As a supplement they'd be fine, but as the main source of information on how to use many commands... not so much.

Re:I never knew that command

[PhilipPeake]

This is only true because people write such terrible and incomplete manual pages.

The original Bell Labs man pages completely described the system from the point of view of an administrator or user. The only better documentation was the source.

The current blight of wimpy, inaccurate and incomplete man pages seems to originate from the GNU developers who insist on using the terrible "info" crap, writing huge volumes of text with no real content, and the tradition is continued by Linux developers who generally provide little or no man page documentation -- presumably in the hope that users of their software will be tempted to ask questions on various mailing lists where they can be ritually disemboweled for displaying such a lack of understanding and disturbing the peace of the cognoscenti who have much more important things to do than answer questions of mere users of their software.

Re:I never knew that command

[Niten]

Pretty much anything in here: http://www.openbsd.org/cgi-bin/man.cgi [openbsd.org]

Re:I never knew that command

[pbhj]

[...] a gift from the dieties.

Are they the ultra-slim super humans I keep hearing about? Or am I confusing them with deadly cocktails??

X-forwarding

[mikeb]

I've seen Windows people go slack-jawed in astonishment as I ssh to the other side of the world and run X programs over forwarding.

Some refuse to believe it, others shake their heads and walk away.

Re:X-forwarding

[BigJClark]

... or even funnier, is how long (as in decades) we've been able to do that.

Re:X-forwarding

[Chris Pimlott]

You could easily have an entire Ask Slashdot just on ssh, perhaps the greatest unix command ever invented.

One of it's many great uses is creating secure tunnels:

ssh user@remotehost -L123:example.com:456

Open a tunnel on your local machine, port 123, to example.com, port 456, via the remote host

ssh -R lets you go in the opposite direction (tunnel from remote end to local end), but if your application supports SOCKS, it's even easier:

ssh user@remotehost -D8080

Creates a secure tunnel supporting the SOCKS protocol.

Talk / DD / Mount

[p14-lda]

People seem to be losing the ability to use all the older manual ways of doing things.

On the older systems, talk was a great utility.

dd, device duplicator / disk destroyer

mount, what I can't have a desktop icon?

also managing disk volumes and the old conventions of /opt, /u, /usr, /usr/local

This new fangled Linux craze with all of the UI tools is feeding it. Redhat is training admins that are dependent on a given release of their enterprise software (which I am a huge fan of) but not teaching them how it works under the hood.

How about slirp? scp?

The one ray of hope seems to be a new generation hacking their bsd and linux based (iPhone/Android) phones and having fun in a somewhat embedded (but full blown) *nix environment.

cd -

[JustinOpinion]

In terms of navigation directories efficiently, I find that "cd -" is often forgotten (changes directory to your previous directory). I personally find it very useful, and couldn't live without it!

Re:directory stack

[Chris Pimlott]

In tcsh (I don't know other shells), you can do directory stack substitution. =0 is current directory, =1 is one up, =2 is two up, and so on.

In bash, it's ~0, ~1, ~2, etc.

One word:

[MMC Monster]

Showers

Job control.

[Craig Davison]

fg, bg, kill, Ctrl-Z, &. Learn it. Know it. Live it.

Even if they do know about job control, I've seen people look for a background job with ps, and then kill it using the PID. In most shells you can just do kill %, e.g. kill %1

short list of shell tips

[James Youngman]

Assuming you already know the simple stuff like how to use shell quotes correctly, what you can do with ps and top, ...

1. Using awk '$3 ~ /foo/ { bar }' to grep just one column of a file
2. reset
3. find . -blah -exec quux \+
4. Adding : to the front of complex commands you just typed but realise you don't want to execute yet so that they get into your shell history
5. Meta-T in Bash for swapping arguments
6. find . -printf X | wc -c for counting files (since find |wc -l would miscount files with newlines in the name)
7. set, shift and implicit shell loops (for without in)
8. "${foo:-bar}" and similar
9. "${x%%.ext}.newext"
10. comm -3 <(sort long) <(sort short)
11. unalias rm

Share mouse and keyboard

[pieleric]

When I pop up with my laptop to discuss with a colleague, after a while I might do on their computer:
xhost +mylaptopname

and on my laptop I do:
x2x thecomputername:0 -west

Then suddenly my mouse can go over the two computers, my keyboard works on both as well, and I can even copy-paste between the two computers. It looks like the two computers got united. In a flash, newbies get a new idea of what means unix and X ;-)

My personal fave

[44BSD]

echo '[q]sa[ln0=aln256%Pln256/snlbx]sb3135071790101768542287578439snlbxq'|dc

grep --color

[krappie]

grep --color

For some reason, many people are greatly surprised when they figure out that grep will highlight matches for them.

lsof

[pak9rabid]

lsof is a LIFE SAVER for trying to find what's still using something in a mounted resource when trying to unmount something. For example:

lsof /mnt/myMount

That will list which processes have anything under /mnt/myMount open

It's also useful to find who's accessing what device. For example, say you're trying to listen to an mp3 and Amarok bitches about the sound device not being available. In that case, you could do something like this (assuming you're using ALSA):

lsof /dev/snd

That will list what processes are accessing any of your ALSA sound devices.

Shell history tricks

[steveha]

There are a whole bunch of "history" tricks, to recall old commands without using the mouse.

When I started college, I studied the shell's man page until I knew them all. Some are so obscure I have forgotten them.

Generally, these involve an '!' character in some way.

Here are a few I use:

!! # run again the last command that was run
!9 # run again the command with history number 9
!v # run again the last command that started with a 'v'
!vi # run the last command that started with "vi"
!?foo? # run the last command that had the string "foo" anywhere in it

diff oldfile newfile
mv !$ !^ # same as "mv newfile oldfile"
# !$ is last arg of previous command, !^ is first arg

ls foo bar baz
rm -f !!* # same as "rm -f foo bar baz"
# !!* repeats all arguments from previous command

There are actually some baroque tricks that recall a previous command and perform a search-and-replace on it, but for anything that complicated I just recall the line and edit it. The baroque tricks would have been pretty darn cool back in the paper teletype days, though.

By the way, the Bash shell can be configured to edit command lines using vi or Emacs commands. I described how to do it in an article I wrote for Linux Journal magazine. It's the last section, "vi or Emacs Mode in the Shell".

http://www.linuxjournal.com/article/8361 [linuxjournal.com]

Oh, not exactly a history trick, but here's something I use all the time:
ls -1 > /tmp/files
vi /tmp/files # edit list to include just the files I want
rm `cat /tmp/files`
# `cmd` inserts the standard output from cmd into the command line as if you typed it

ls -1 > /tmp/files
vi /tmp/files
# edit list to include just the files I want
# now run this command: :%s+.*+mv & /some/directory/path/&+
# save file and quit vi
source /tmp/files

This moves the chosen files to "/some/directory/path". The breakdown of the vi command is as so:
: # invoke "ex mode" for search and replace command
% # run the following command on every line of the file
s # do a search and replace
+ # use a '+' for the command delimiter, so I won't have to backslash escape '/' chars in the path

.* # all characters on the line
+ # end the match pattern, begin replace pattern
# & refers to the match pattern, thus all chars on the line
mv & /some/directory/path/& # replace "foo" with "mv foo /some/directory/path/foo"

Takes less time to do it than to explain it!

The above is perhaps overkill if all the files are going to the same place. It's great if you want to send some files one place, some to another, because you can just edit the destinations until it looks right.

steveha

Time warp

[SchmellsAngel]

cal 9 1752

Re:rev

[genner]

The rev command has got to be one of the most useless Unix commands I've ever come across. It's almost as if someone's first c program somehow got taken up as a part of standard Unix! Maybe in the days before sed and awk and perl it had some function in pipes that I can't grok, but nowadays other than making hints for video game websites I can't imagine what it's for.

Unhackable encryption of course.

Re:rev

[Chris Pimlott]

I sometimes use rev to sort text by the end of the line, not the first. This is often useful when comparing two similar file structures.

For example:

$ wc -l foo/* bar/*
      6 foo/dead.letter
    86 foo/xorg.conf
      6 bar/dead.letter
    54 bar/xorg.conf

$ wc -l foo/* bar/* | rev | sort | rev
    86 foo/xorg.conf
    54 bar/xorg.conf
  152 total
      6 foo/dead.letter
      6 bar/dead.letter

(Yes, I'm aware you can use sort -k to specify the sort key, but this is quicker and easier)

Re:Listing directory contents without the ls comma

[marcansoft]

You're not giving echo an asterisk as a paratemer. You're giving the shell an asterisk, which it dutifully expands. echo (which in this case is a shell builtin, but it doesn't have to be then just echoes them back.

This isn't some echo peculiarity. It works for anything, even commands that don't normally take files, or even with files that look like switches (conversely, if you want to treat all subsequent arguments as files, not switches, most programs have a '--' switch):

$ ls
a -l b c
$ ls *
-rw-r--r-- 1 marcansoft users 0 2008-11-05 21:58 a
-rw-r--r-- 1 marcansoft users 0 2008-11-05 21:58 b
-rw-r--r-- 1 marcansoft users 0 2008-11-05 21:58 c
$ ls -- *
a -l b c

In the second example, ls sees "ls a -l b c" and takes -l as a switch instead of a filename.

Re:Listing directory contents without the ls comma

[elgatozorbas]

Maybe an ex-windows user who assumed "delete /bin" was the linux equivalent of "empty wastebasket" ?

Re:This one always surprises people for some reaso

[interiot]

• Optimized version of that: find / | grep -i $SOMETHING
• Even more optimized: find / -iname $SOMETHING
• However, most systems support locate/updatedb already, and that's much faster.

Re:grep -R

[multipartmixed]

Hell, I think it's probably a GNU extension, because it's still not in Solaris.

I think rgrep appeared around BSD 4.4, though.

Oh well. I still surprise people with backticks. *sigh*

-exec as a test

[Chris Pimlott]

One great feature of find that many people are unaware of is that you can use -exec as a test, not just as an action. For example, this is equivalent to your command above:

find . -exec grep -q {} \; -print

The "-print" action is only executed if the -exec command returns success.

You can do a lot of handy things with this. Here's a real-world example from earlier today. I wanted to change the mime-type of all the xml files in my svn repository from "application/xml" to "text/xml":

find . -name \*.xml -exec sh -c "svn propget svn:mime-type {} | grep -q application/xml" \; -exec svn propset svn:mime-type text/xml {} \;

Re:Find / Grep

[Anonymous Coward]

I have a theory that find + xargs + grep is Turing-complete. Can't prove it, but it feels right.

Re:A simple search

[AKAImBatman]

Xargs is much more fun with complex data processing. e.g.

Convert all PSDs to PNGs:

ls *.psd | cut -d . -f 1 | xargs -L1 -i convert {}.psd {}.png

Parse out and sort column 2 from a semicolon delimited file:

cat myfile.txt | cut -d \; -f 2 | sort > output.txt

Oh, I almost forgot about one of my favorite tricks. Count the number of items:

wc -l
(paste the list into the window and then type CTRL-D)

It even works when the list of items has oddities. e.g. I had a list where every other line was blank. So I needed to count n/2 the value. Except that one of the blank lines wouldn't copy, so I actually needed (n+1)/2.

echo $(($((`wc -l`+1))/2))

Want to make sure your sig is under 120 characters? Type "wc -c" in, paste it into your terminal, then press CTRL-D. Instant character count.

Ah, all the fun stuff you can do with Unix tools.

Re:A real time saver!

[Mish]

DISCLAIMER: Don't run this!
I didn't think I needed to say this, but I just showed someone this and they thought it was a legitemately helpful command...