I'm trying to figure out what the mode parameter of setfmode() which is called eventually by chmod() is.
When I print it out and do something like chmod +t test.txt "33700" gets printed out. When I do chmod +w test.txt "33252" gets printed out.
Is there a way to see which specific bits are being set using these numbers?
Refer to the FreeBSD manual page at https://www.freebsd.org/doc/handbook/permissions.html for a full description of the permission bits. Note that the illustrations of permission bits are in octal format.
Search engines are your friends - I searched for "FreeBSD permission bits" and instantly found the above link as the first returned result.
From chat:
"I'm trying to find out if the user is setting the sticky bit, so in setfmode() I think I'll bitwise AND the mode variable with the sticky bit constant, then check if that equals the sticky bit constant"
The sticky bit is octal 1000 - see the FreeBSD chmod man page
To write numbers in octal in C, precede the value with a 0, so assuming your mode value is in a variable named mode, do (mode & 01000). If that value is zero, the sticky bit is not set, if it is non-zero (i.e., 01000), the sticky bit is set.
Mark: "Ok my check seems to be working -
if ((mode & S_ISTXT) == S_ISTXT)"
Related
I'm writing an application for the non-GUI text terminal in Raspian Linux in C++ using the ncursesw library. I found out by accident that if you print anything with mvwaddch using A_ALTCHARSET (e.g. mvwaddch(stdscr, 1, 1, A_ALTCHARSET | 65);), then the entire Extended ASCII character set (0 - 255) becomes available to mvadd_wch starting from index 0xF000.
It doesn't look like 0xF000 is an official mapping for the UTF-8 locale that my terminal is configured for. Somehow the ncursesw library triggers the system to load up these characters.
How is this done? Is there a way to load up this character set without first writing junk data using A_ALTCHARSET?
It may be enough for my purposes to use a mvwaddch(stdscr, 0, 0, A_ALTCHARSET); to print a null character when the program initializes. But I would still like to know what is happening behind the scenes here.
For reference, this is the character set I am referring to:
ncurses has to support certain terminal descriptions which send "8-bit" codes for the alternate character set feature. The Linux console happens to be a commonly-used case. There's a few places in the source-code which you would find helpful to understand, e.g., in the update-code.
/*
* If the character falls into any of these special cases, do
* not force the result to a blank:
*
* a) it is printable (this works around a bug in wcwidth()).
* b) use_legacy_coding() has been called to modify the treatment
* of codes 128-255.
* c) the acs_map[] has been initialized to allow codes 0-31
* to be rendered. This supports Linux console's "PC"
* characters. Codes 128-255 are allowed though this is
* not checked.
*/
The Linux console has no special knowledge of PC-character sets (the console_codes manual page mentions only UTF-8 and ISO-8859-1), but the showconsolefont command uses a Linux ioctl to allow it to print a table of 256 codes, which depending upon the font which has been loaded, will match some PC code-page (see my answer to Why does showconsolefont have different output in tmux?).
I am looking to implement a remote client in golang which connects to Linux through nc and starts bash. So I need to tell bash what features I can parse from the stdout that it sends to me, and how I am going to send keycodes and other stuff to its stdin, so that it could parse them too.
This is done with TERM=something environment variable, which I need to set to some value. If I don't set it, then various programs start to complain:
$ mc
The TERM environment variable is unset!
I found that I can set TERM to dumb to say that my client is really limited. And still it seems that I am missing something.
$ export TERM=dumb
$ mc
Your terminal lacks the ability to clear the screen or position the cursor.
From here it looks like dumb terminal don't have these two abilities, but what abilities it is still expected to have? Is there a specification or some de-facto standard about it?
Going to the source can help. The terminal database has comments. Here is a slice from that:
#### Specials
#
# Special "terminals". These are used to label tty lines when you don't
# know what kind of terminal is on it. The characteristics of an unknown
# terminal are the lowest common denominator - they look about like a ti 700.
#
dumb|80-column dumb tty,
am,
cols#80,
bel=^G, cr=^M, cud1=^J, ind=^J,
unknown|unknown terminal type,
gn, use=dumb,
The "dumb" and "unknown" terminal types are assumed, but rarely used:
"dumb" has automargins (text "wraps" at the right margin), is assumed to have 80 columns, and an ASCII BEL and carriage return. For lack of something better, cud1 (cursor down) is an ASCII line-feed. The ind (index) value is the same, implying that text scrolls up when you reach the bottom of the screen.
There is no cursor-addressing (cup) nor alternates (such as moving along a row or column arbitrarily).
"unknown" adds the "generic" flag, which marks it as unsuitable for use by curses applications. Think of it as a printer.
As for minimum requirements, that actually depends upon the individual application. ncurses can manage to move around the screen without actually having cup. It works with a half-dozen strategies. If you read the source for mvcur, you can get an idea of what it needs.
However, applications such as mc do not simply rely upon ncurses to decide if it works, since (in this case) it may link with slang (which doesn't check that closely). So mc does its own checks, which may add restrictions.
In practice, unless you choose a limited terminal description such as "dumb", most of the terminals you are likely to encounter will work.
Further reading:
terminfo - terminal capability data base
curses interfaces to terminfo database (including mvcur)
ncurses/tty/lib_mvcur.c
Your best source of information will be the terminfo entry, easily viewed with the infocmp tool:
infocmp dumb
# Reconstructed via infocmp from file: /lib/terminfo/d/dumb
dumb|80-column dumb tty,
am,
cols#80,
bel=^G, cr=^M, cud1=^J, ind=^J,
which makes it pretty clear that the dumb terminal is quite limited ...
I want to know if anyone does know a way to dump or copy the whole lot of viewable messages in a xterm window on linux. The very important thing is I don't want to know how to send a command out and kapture its output for stream 1 and 2 as well as the input, as this is well known to me.
I may explain for what this is needed. You do something and expect not any complications but than you got pages of msg's als err msg or normal output. To be able to see later after it you should be able to get them in a file and as long as you are able to scroll that all back and forther with your mouse it is sure the data is there some where. But the time may be not to scroll and screenshot and scroll ....
I would be glad to help me out in such cases and it would be fine to have the full view including all your own typing and all the msg's in same order as you watch it when you scroll it back.
I don't really know where this is stored and how you could get that saved. I know that I could dump the whole lot of Memory and search it for a part of the xterm window, but that is a bit over the top I think.
There is a control sequence, which I had forgotten. This question reminded me. In XTerm Control Sequences, it is noted "print all pages":
CSI ? Pm i
Media Copy (MC, DEC-specific).
Ps = 1 -> Print line containing cursor.
Ps = 4 -> Turn off autoprint mode.
Ps = 5 -> Turn on autoprint mode.
Ps = 1 0 -> Print composed display, ignores DECPEX.
Ps = 1 1 -> Print all pages.
That dates from 1999 (patch #119), so you likely have it in your xterm. You could do this in a shell command like this:
printf '\033[?11i'
A comment mentions the page Hidden gems of xterm, which uses the corresponding action print-everything (something that can be executed via the translations resource). It is in the manual page, of course. The same comment points to Extra characters in XTerm printerCommand output, which mentions the resource printAttributes. By default, the control sequences for the printer tell xterm to send extra control characters (to reconstruct video attributes). The resource can be modified (set to 0) to suppress that. That is even older (patch #74).
Without that — Conceivably one could construct an application which used the X SendEvent protocol to construct a series of events which would be interpreted as xterm actions to scroll back, select text and copy it chunk-by-chunk via the clipboard. You could even write it in Perl (there is a module for X protocol). But seriously, no.
If you want to capture text which was written to xterm, you can do this by preparing before the text is needed by different methods (see manual):
turn on the xterm logging feature (not that user-friendly because it generates the filename). This can be enabled using the "Log to File (logging)" menu entry.
use the printer control sequences to write lines as they are written (again, not that friendly, though there is a menu entry to turn it on and off, "Redirect to Printer (print-redir)")
use script to capture all output to the terminal. I use this, because it works with any terminal on any POSIX-like system (even Cygwin).
Each of these methods produces a file containing escape/control sequences, which requires filtering out. The hypothetical program using SendEvent could in principle eliminate that.
Warning: you know how they say "there's not such thing as a stupid question"? Well, this one is, or, I suspect it's really minor, but wth, why not ask. Search engines didn't bring me anything remotely useful, though that could be bad searchterm-fu.
I recently downloaded sqlite3 onto Ubuntu 10 to start learning SQL commands. I un-tar'd 3.7.12.01 and make installed.
After creating a test.db with create table test (id) I decided to see what I'd get if I cat it. Just because.
The result is an EOT character (u+0004) which is sitting right over my prompt. Illustrated screenshot: http://imgur.com/omfMa
I realise this is not the type of file you would use cat on. I only want to know, before I go further,
does the strange placement of this character signal any future issues when actually playing around with SQL, or some issue with newlines, or fonts (this is monofur set at a high font size) or similar?
I've never seen a result character placed directly over my prompt before.
The character is placed over your prompt, because it is a double-width character, and terminals in general are not good at handling double-width characters. It does not mean anything.
There are some control codes which can do very funny things with your terminal, such as changing colors, fonts etc.
But none of them do really harm - you should be able to reset your terminal to a healthy state, or close it and open a new one.
So I just found out I can create log files of everything I do in screen (C-a H). Sounds like a nice way to keep track of potential goofs in a particular screen session. However, when I went to try it out the logfile is reported as being a binary file (and can't be viewed like a regular text as such). So am I missing something? A quick man page looksee and searching Google (and SO) turns up nothing about this.
So my question is: How do I generate plain text log files in screen?
Assuming the answer is "What a noob... how about you try making them? RTFM." my question becomes: How do I use less to view screen logfiles I've created (since less screenlog.0 does not work on a binary file)?
EDIT: So cat works fine but less complains that the file is binary... why?
SOLUTION: as jcomeau_ictx helpfully pointed out, you can view these logfiles fine with cat or more but with less you must add the -r flag less -r screenlog.0
I just found a screenlog.0 on the net; it is plain text, with some escape sequences. Just 'cat' the file, you should be able to view it just fine.
[after more checking]
Control-A H is what generates the screenlog on my system. And though 'cat' works, you'll miss a lot of data. Use 'more' instead of 'less' to interpolate the escape codes.
I found neither less nor more nor cat to be an ideal solution for viewing screenlog files. All "replay" some of the control character so that e.g. screen deletions as produced by "clear" (don't remember the corresponding control character) are beeing shown, hiding what has been cleared.
What i know works great is: use "view" or "vi", it just shows the control character in escaped notation. Probably any other text editor works, too (not tested).
-L logs to file,
tail -f 'logfilename' to monitor this file