Linux – save output from a previous terminal command using only the keyboard

consolelinuxterminal

I just spent the last two hours running a dd command (or picture any similar "difficult to re-do" scenario) from a live CD without a GUI; all I have is my trusty "multi-window" (CTRL+ALT+F#) Bash terminal.

Alas, during the command dd threw out several nasty error messages and a bit more information that I would like to keep. I have a USB drive plugged in to which I can write data, but how do I get the previous output saved as a text file after the command has already been run?

If this had been a terminal emulator inside a nice GUI, I would have simply used my mouse to select the text, copy it, and paste it into a document. And had I known the command would have produced errors, I would have piped it out to a file to begin with, but alas, the additional output came as a surprise.

How do I save text output from my previous command to a file without re-running the command? Is this even possible?

Best Answer

A linux kernel should store an on-screen log for your vts in the corresponding /dev/vcsa*[ttynum] device.

It is why the following works:

echo hey >/dev/tty2
dd bs=10 count=1 </dev/vcs2

...which prints...

hey

The corresponding /dev/vcsa[ttynum] device will store an encoded version of the formatted text on-screen, whereas the /dev/vcs[ttynum] will be a plain dump. The vcsa[ttynum] devices will encode a pair of bytes which describe each on-screen char and its attributes, as well as a string at the head of each logical page that indicates the referenced tty's lines,columns count.

As @kasperd points out, I had it wrong before by assuming the \a BEL was encoded between every character, when in fact: The default color combination happens to coincide with the bell character.

For your purposes using the /dev/vcs[ttynum] is probably easiest. Here's a look at the differences:

echo hey >/dev/tty2
dd bs=10 count=1 </dev/vcs2 |
sed -n l

...prints...

hey       $

...and...

echo hey >/dev/tty2
dd bs=10 count=1 </dev/vcsa2 |
sed -n l

...prints...

0\200\000\004h\ae\ay\a$

Related Solutions

Console – How to Change the Font of the Text-Mode Console

If you use the Linux console, the best way I found is:

/etc/default/console-setup

put, for example

CHARMAP="UTF-8"
CODESET="Lat7"
FONTFACE="Terminus"
FONTSIZE="28x14"

Another way is to use setfont from the kbd package:

setfont /usr/share/consolefonts/Lat7-Terminus28x14.psf

This works for my Debian; it may be different for you.

In Debian, you can also run dpkg-reconfigure -plow console-setup to be prompted for the various console settings and pick them from menus.

Edit - I put together a small page how to setup the font colors. The section that is relevant for this post has the header "the Linux VTs" (= ttys, or "console").

Search text in command output on a PuTTY terminal

The Ctrl + a + [ is meant for use within the application screen (an app for multiplexing consoles).

less

Generally the easiest method to do this is to use tools such as less and to pipe the output from whatever application is generating the messages on the console, and search within the application less. You can do so using the forward slash (/) followed by whatever string you're searching for. Hit return to execute the search.

Example

$ less filename.log

...then in less, type a forward slash followed by string to search, foo

grep

In the same vain as above with using less, you can also use tools such as tail to print the lats few lines of a applications log file messages, and also use grep to search for only lines that contain a matching string/pattern.

$ grep "somestring" filename.log