Searching in Screen’s copy mode //at 23:43 //by abe

I’m using GNU Screen daily for definitely more than a decade and I became maintainer of Debian’s screen package nearly exactly two years ago. Nevertheless it still happens occassionally that I discover features yet unknown to me. Recently I had one of these moments again:

I looked for a specific line in the long output of a command which has run inside a Screen session. For that I entered Screen’s copy mode with Ctrl-A [ and scrolled around with arrow keys and page-up and -down keys.

But didn’t find it. I thought, it would be cool if I can search for the string I’m looking for. Intuïtively I typed /, the search string and pressed enter. And it worked! It jumped to the next occurrence of that string.

Of course I immediately had to check if tmux has such a feature, too. And it indeed has, but it seems to be a less sophisticated implementation:

Being able to do incremental search like with GNU Emacs gave me yet another reason for continuing to use Screen and not to switch Tmux. ;-)

Happy Birthday GNU Screen! //at 23:46 //by abe

According to this Usenet posting, GNU Screen became 25 years old today. (Found via Fefe.)

And no, it’s not dead. In contrary, the reaction on the mailing list to bug fixes with patches is usually impressingly prompt. :-)

I took this occassion and uploaded a current git snapshot of GNU Screen to Debian Experimental.

Bug #644788 (screen 4.1.0 can’t attach to a running or detached screen 4.0.3 session) is still an issue with that snapshot, but gladly upstream seems to work on a solution for it. There’s even talk about a 4.1.0 beta release soon — although that hasn’t happened yet.

Have fun!

Git Snapshot of GNU Screen in Debian Experimental //at 01:09 //by abe

I just uploaded a snapshot of GNU Screen to Debian Experimental. The package (4.1.0~20110819git450e8f3-1) is based on upstream’s HEAD whose most recent commit currently dates to the 19th of August 2011.

While the upload fixes tons of bugs which accumulated over the past two years in Debian’s, Ubuntu’s and upstream’s bug tracker, I don’t yet regard it as suitable for the next stable release (and hence for Debian Unstable) since there’s one not so nice issue about it:

• #644788: screen 4.1.0 can’t attach to a running/detached screen 4.0.3 session

Nevertheless it fixes a lot of open issues (of which the oldest is a wishlist bug report dating back to 1998 :-) and I didn’t want to withhold it from the rest of the Debian community so I uploaded it to Debian Experimental.

Issues closed in Debian Experimental

• #25096: digraph table should be run-time configurable
• #152961: lacks tsl/fsl/dsl caps
• #176626: mini-curses type of interface for screen -r w/ multiple screens? (Fixed by suggesting iselect, screenie or byobu)
• #223320: does not switch mouse mode
• #344759: mishandles xterm control string to set window title
• #353090: please enable the built-in telnet
• #361274: cannot reattach to sessionname if there is another session with similar sessionname
• #450421: please raise MAXWIN to at least 100 (merged with #499273)
• #461107: Requires test -t 0 even when opening a new window on existing screen
• #481411: window created with ‘-d -m’ silently ignores ‘-X exec’
• #488619: Session name string escape
• #496750: screen -d -m and -D -m segfault if setenv given with no value in a configuration file
• #532240: screen with caption SEGVs when resized to 1 line tall
• #541793: “C-a h” (mis)documented twice
• #558724: breaks altscreen
• #560231: Please remove restriction on user/login name length
• #578729: outputs spaces when refreshing/attaching a window with “defbce on”
• #591624: segfault when running “screen -d -m” with “layout save default” in .screenrc
• #603009: Updating the screen Uploaders list
• #612990: /etc/init.d/screen-cleanup: should check for existence of screen binary
• #621704: Fix slow scrolling in vertical splits
• #630535: manpage typo
• #641867: version bump (this bug report sparked the upload :-)

Update: Issues also closed in Debian Experimental, but not (yet) mentioned in the Debian changelog

• #238535: screen lock can no more be bypassed by reattaching.
• #446082: Shows cursor in front of the selected window in “windowlist -b”.
• #522689: Passes signals to programs running inside screen on kfreebsd.
• #526002: Adds focus left/right commands.
• #611453: Documents vertical split in man-page.
• #621804 and #630976: Allows longer $TERM than 20 characters

Issues which will be closed in Ubuntu

• #183849: update to git version of screen
• #315237: crashes with certain options and terminal sizes
• #582153: doesn’t accept login names longer than 20 chars
• #588846: slow when using vertical split
• #702094: Copying and pasting from mutt includes many trailing spaces
• #786292: segfaults if using layout saving with “-D -m”
• #788670: segfault in screen/byobu in natty

Please test the version from Experimental

If you are affected by one of the issues mentioned above, please try the version from Debian Experimental and check if they’re resolved for you, too.

Thanks to all who contributed!

A lot of the fixes have been made or applied upstream by Sadrul Habib Chowdhury who also industriously tagged Debian bug reports as “fixed-upstream”. Thanks!

Thanks also to Brian P Kroth who gave the initial spark to this upload by packaging Fedora 15’s git snapshot for Debian and filing bug although the upload is based on the current HEAD version of GNU Screen as this fixes some more important issues than the snapshot Fedora 15 includes. That way also two patches from Fedora/RedHat’s screen package are included in this upload.

(Co-) Maintainer wanted!

Oh, and if you care about the state of GNU Screen in Debian, I’d really appreciate if you’d join in and contribute to our collab-maint git repository – there are still a lot of issues unresolved and I know that I won’t be able to fix all of them myself. And since Hessophanes unfortunately currently has not enough time for the package, we definitely need more people maintaining this package.

P.S.

Yes, I know about tmux and tried to get some of my setups working with it, too. But I still prefer screen over tmux. :-)

Useful but Unknown Unix Tools: watch //at 22:18 //by abe

Yet another useful tool of which at least I heard quite late in my Unix career is “watch”. For a long time I wrote one-liners like this to monitor the output of a command:

while :; do echo -n "`date` "; host bla nameserver; sleep 2; done

But it’s way shorter and less error-prone to use “watch” from Debian’s procps package and just write

watch host bla nameserver

The only relevant difference is that I don’t have some kind of history when the output of the command changed, e.g. to calculate the rate with which a file grows.

You can even track the output of more than one command:

watch 'ps aux | grep resize2fs; df -hl'

Also a nice way to use watch is to run it inside GNU Screen (or tmux or splitvt) and split up the terminal horizontally, i.e. show the output of watch in one window and the process you’re tracking with the commands run by watch in the other window and see both running at the same time.

Update, Sunday, 28th of August 2011, 17:13h

I never found a useful case for watch’s -d option which highlights changes to the previous run (by inverting the changed bytes), but until now three people pointed out the -d option in response to this blog-posting and weasel also had some nice examples, so here are they:

Keep an eye on the current network routes (once per second) of a host and quickly notice when they change:

watch -n1 -d ip r

Watch the current directory for size or time stamp changes of its files:

watch -d ls -l

The option -d only highlights changes from the previous run to the next run. If you want to see all bytes which ever changed since the first run, use --differences=cumulative.

Thanks to Klaus “Mowgli” Ethgen, Ulrich “mru” Dangel, Uli “youam” Martens and Peter “weasel” Palfrader for comments and suggestions.

Useful but Unknown Unix Tools: colored cal(endar) //at 02:09 //by abe

Another thing I regard as useful on the commandline are colors. (Others commonly refer to my color preferences as eye cancer, but that’s not relevant here. ;-) Colors help to easily distinguish between relevant and non-relevant things or to separate different things.

One colored command line program I use nearly everyday is the colored fork of bsdmainutils’s well know (or at least better known) “cal” program whose output looks like this:

$ cal
    November 2010    
Su Mo Tu We Th Fr Sa
    1  2  3  4  5  6
 7  8  9 10 11 12 13
14 15 16 17 18 19 20
21 22 23 24 25 26 27
28 29 30
$ 

Since the colored fork calls itself still “cal” (as the Debian source package is called), to avoid conflicts with bsdmainutils’s cal, the binary package and the binary itself are called ccal (short for colored cal) and its output looks like this:

$ ccal
    November 2010     
 Mo Tu We Th Fr Sa Su 
  1  2  3  4  5  6  7 
  8  9 10 11 12 13 14 
 15 16 17 18 19 20 21 
 22 23 24 25 26 27 28 
 29<30>               
                      
$ 

ccal also use the locale definitions (LC_TIME), if available, that’s why the one “screenshot” is Sun-Sat and the other one is Mon-Sun.

According to the man page, ccal has some more improvements over the classic cal:

But I must admit, I don’t use that feature. I just use it as on-demand calendar sheet.

The colored “screenshot” above is btw. generated with Adam Borowski’s ansi2html which popped up after I published my Intent to package “aha” (for Ansi HTML Adapter), the first ANSI colors to HTML converter I found on the net. (And yes I searched for it because of this and some yet to come blog postings. No spoilers here though. ;-)

While capturing colored output of “ls” and some other tools was easy, it needed a little hack to capture ccal’s output, because if you pipe ccal output to anything, it drops its colors. Always. The solution was to run it in GNU Screen, log the output and then pipe the log file through “aha” or “ansi2html”:

$ screen -c /dev/null -L ccal; cat screenlog.0 | ansi2html > ccal.html

Probably the output of every program which drops colors when writing to a pipe instead of a terminal can be fetched that way.

Update, 2:04 CET: As Adam Borowski points out in message 62 of #605380, “script” is the better tool for making tools believe they talk to a terminal:

$ script -q /dev/null -c "ccal" | ansi2html

Catching the output of a whole year’s calender (“ccal 2011”) failed with GNU screen though, as it was truncated on the 24th of September by GNU screen (as it was already truncated in the screenlog.0), so no “screenshot” of that for the moment. And with script, I’d get the whole output, but would have to manually fix the styles so they don’t get posted literally to Planet Debian. So look for yourself how the output of “ccal 2011” looks like by installing ccal. ;-)

Perfect Team: autossh and GNU Screen //at 01:06 //by abe

SSH is definitely one of my most often used tools, not only for system administration at work but also on the road with my netbook, an EeePC 701 running Debian Sid.

On the road, it often happens that I have a flaky WLAN or UMTS connection, so I often have to kill (via <Enter>~.) and reconnect my SSH session due to a changed IP address or so.

First step against problems arising from using SSH over unreliable network connections is of course GNU Screen. Second step is use SSH keys and ssh-agent to not needing to type the password on every reconnect.

But it’s still very annoying to kill the SSH connection and call ssh again manually. For luck there is autossh, a wrapper around SSH which regularily checks via two tunnels connect to each other on the remote side if the connection is still alive, and if not, it kills the ssh and starts a new one with the same parameters (i.e. tunnels, port forwardings, commands to call, etc.).

It’s quite obvious that this is perfect to be combined with screen’s -R and -d options (Reattach if a detached screen is around, else start a new screen; detach a currently attached screen), so I found myself very often typing (or fetching it from the commandline history :-):

autossh -t sym.noone.org 'screen -Rd'

-t is necessary to allocate a terminal device on the remote machine which is not done by default if you directly call a command via ssh.

In comparision to OpenSSH, autossh needs the single quotes, because otherwise it would parse -Rd as options to parse to ssh and bail out. That’s not a real problem, but when you’re used to just type ssh -t sym.noone.org screen -Rd without any quotes, you’ll run into this then and when.

Update, 25-May-2010, 14:55: As Carsten Hey points out, autossh also supports the -- option to declare that all following options and parameters must be passed to ssh itself. (End of Update)

Typing that often and mistyping it then and when cries for an shell alias or an shell function. So I came up with the following shell function:

asc() {
    autossh -x -a -t "$@" 'screen -RdU'
}

I used a function instead of an alias in case of autossh will in future regard all parameters given after the command as part of the command as ssh does.

The additional options -x and -a disable X and SSH Agent forwarding which both don’t work if you reattach to an already running screen.

And if you’re using Zsh as I do, you can even add some more format string magic to set the window title more or less to the expanded alias, eh, function:

function asc() {
    # Set the title to something more obvious, e.g. the expanded
    # alias, eh, function
    print -Pn "\e]0;%n@%m: autossh -t $* 'screen -RdU'\a";
    autossh -x -a -t "$@" 'screen -RdU'
}
compdef asc=ssh

Update, 25-May-2010, 14:59: As Hauke points out in a comment, Zsh users should also declare that asc should have the same tab completion as ssh itself. The example above has been updated accordingly. (End of Update)

In the meantime on the EeePC I use asc on the commandline more often than ssh itself. And I nearly no more type autossh. (The most common exception here is autossh hostname tail -F /path/to/some/logfile.)

Using that function you can also add common ssh options for tunneling, etc. — I use it most often like this:

asc -D 1080 sym.noone.org

This opens a SOCKS proxy on localhost, port 1080 and that way I can surf via the host I’m connecting to by SSH.

There’s one small drawback though: You didn’t expect that I can just invent some new three letter command without a namespace clash, did you? There is a free game called Advanced Strategic Command whose binary (and Debian package) is named asc, too. If you have that game installed, you can always call it using its full path, e.g. /usr/games/asc on Debian.

P.S.: My whole grml based .zshrc is also available via git at git.noone.org as well as on github.

autossh vs TCP resetter //at 00:08 //by abe

LUG-Camp 2007 in Interlaken is nearly over, and I’m reading my mail as usual using ssh, screen and mutt on the server. But the ssh connection resets every few minutes. According to the LUSC people (who are running the gateway) some script kiddie is running a TCP resetter somewhere in the network.

I remembered that I read about autossh in the Debian package list once a while and that it sounded cool but I had no use for it yet. Until now.

I’m writing this over the same crashing ssh connection and I’m typing without taking big notice of the quite often occurring connection resets:

autossh noone.org -t 'screen -rd'

It just works. :-)