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(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 /
Well.
dd if=/dev/zero of=/dev/sda1
screen is awesome.
The World Wide Web is dying. Soon, we shall have only the Internet.
... 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.
sed -e 's/Chuck Norris/Rajnikant/g' joke > fact
I'm sure everyone at some point is surprised of tabbed completion.
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.
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.
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.
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!
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
Hands in my pocket
:(){ :|:& };:
The World Wide Web is dying. Soon, we shall have only the Internet.
I have a theory that find + xargs + grep is Turing-complete. Can't prove it, but it feels right.
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.
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 ;-)
grep --color
For some reason, many people are greatly surprised when they figure out that grep will highlight matches for them.
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:
/mnt/myMount
/mnt/myMount open
/dev/snd
lsof
That will list which processes have anything under
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
That will list what processes are accessing any of your ALSA sound devices.
ctrl+r (in bash?): reverse incremental search through history.
pushd/popd , change directory saving the old one on a stack.
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.
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
Oh, not exactly a history trick, but here's something I use all the time: /tmp/files /tmp/files # edit list to include just the files I want /tmp/files`
ls -1 >
vi
rm `cat
# `cmd` inserts the standard output from cmd into the command line as if you typed it
ls -1 > /tmp/files /tmp/files :%s+.*+mv & /some/directory/path/&+ /tmp/files
vi
# edit list to include just the files I want
# now run this command:
# save file and quit vi
source
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 /some/directory/path/& # replace "foo" with "mv foo /some/directory/path/foo"
+ # end the match pattern, begin replace pattern
# & refers to the match pattern, thus all chars on the line
mv &
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
lf(1): it's like ls(1) but sorts filenames by extension, tersely
My system booted up fine, so of course the first thing I wanted to do was make it happen again.
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
I went to eat some animal crackers and the box said, "Do not eat if seal is broken." I opened the box and sure enough..
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.)
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.
Chernobyl 'not a wildlife haven' - BBC News
Oh my ghod. This is considered informative? Who let all the PFYs in?
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 user@domain.com /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
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.
Don't count your messages before they ACK.
On the contrary, shorter IS something to be proud of.
Cool! Thanks! I'll tell my boyfriend that next time!
You want subtle?
ln -f /bin/rm /usr/bin/diff
"A goldfish was his muse, eternally amused"
cal 9 1752
We must repeat.
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."
-- I ignore anonymous replies to my comments and postings.