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Unix / Linux EOL is LF, linefeed, ASCII 10, escape sequence \n.

Here's a Python snippet to get exactly one keypress:

import sys, tty, termios
fd = sys.stdin.fileno()
old_settings = termios.tcgetattr(fd)
try:
    tty.setraw(sys.stdin.fileno())
    ch = sys.stdin.read(1)
finally:
    termios.tcsetattr(fd, termios.TCSADRAIN, old_settings)
    return ch

When I press Enter on my keyboard in response to this snippet, it gives \r, carriage return, ASCII 13.

On Windows, Enter sends CR LF == 13 10. *nix is not Windows; why does Enter give 13 rather than 10?


It's known the Python snippet is correct.

share|improve this question
    
Try reading two bytes. – Michael Hampton Feb 22 at 4:01
    
@MichaelHampton Nope, there's nothing waiting on that file descriptor after one byte is read – cat Feb 22 at 4:02
up vote 8 down vote accepted

While Thomas Dickey's answer is quite correct, Stéphane Chazelas correctly mentioned in a comment to Dickey's answer that the conversion is not set in stone; it is part of the line discipline.

In fact, the translation is completely programmable.

The man 3 termios man page contains basically all the pertinent information. (The link takes to Linux man-pages project, which does mention which features are Linux-only, and which are common to POSIX or other systems; always check the Conforming to section on each page there.)

The iflag terminal attributes (old_settings[0] in the code shown in the question in Python) has three relevant flags on all POSIXy systems:

  • INLCR: If set, translate NL to CR on input
  • ICRNL: If set (and IGNCR is not set), translate CR to NL on input
  • IGNCR: Ignore CR on input

Similarly, there are related output settings (old_settings[1]), too:

  • OPOST: Enable output processing.
  • OCRNL: Map CR to NL on output.
  • ONLCR: Map NL to CR on output. (XSI; not available in all POSIX or Single-Unix-Specification systems.)
  • ONOCR: Skip (do not output) CR in the first column.
  • ONLRET: Skip (do not output) CR.

For example, you could avoid relying on the tty module. The "makeraw" operation just clears a set of flags (and sets the CS8 oflag):

import sys
import termios

fd = sys.stdin.fileno()
old_settings = termios.tcgetattr(fd)
ch = None

try:
    new_settings = termios.tcgetattr(fd)
    new_settings[0] = new_settings[0] & ~termios.IGNBRK
    new_settings[0] = new_settings[0] & ~termios.BRKINT
    new_settings[0] = new_settings[0] & ~termios.PARMRK
    new_settings[0] = new_settings[0] & ~termios.ISTRIP
    new_settings[0] = new_settings[0] & ~termios.INLCR
    new_settings[0] = new_settings[0] & ~termios.IGNCR
    new_settings[0] = new_settings[0] & ~termios.ICRNL
    new_settings[0] = new_settings[0] & ~termios.IXON
    new_settings[1] = new_settings[1] & ~termios.OPOST
    new_settings[2] = new_settings[2] & ~termios.CSIZE
    new_settings[2] = new_settings[2] | termios.CS8
    new_settings[2] = new_settings[2] & ~termios.PARENB
    new_settings[3] = new_settings[3] & ~termios.ECHO
    new_settings[3] = new_settings[3] & ~termios.ECHONL
    new_settings[3] = new_settings[3] & ~termios.ICANON
    new_settings[3] = new_settings[3] & ~termios.ISIG
    new_settings[3] = new_settings[3] & ~termios.IEXTEN
    termios.tcsetattr(fd, termios.TCSANOW, new_settings)
finally:
    termios.tcsetattr(fd, termios.TCSADRAIN, old_settings)

return ch

although for compatibility's sake, you might wish to check if all those constants exist in the termios module first (if you run on non-POSIX systems). You can also use new_settings[6][termios.VMIN] and new_settings[6][termios.VTIME] to set whether a read will block if there is no pending data, and how long (in integer number of deciseconds). (Typically VMIN is set to 0, and VTIME to 0 if reads should return immediately, or to a positive number (tenth of seconds) how long the read should wait at most.)

As you can see, the above (and "makeraw" in general) disables all translation on input, which explains the behaviour cat is seeing:

    new_settings[0] = new_settings[0] & ~termios.INLCR
    new_settings[0] = new_settings[0] & ~termios.ICRNL
    new_settings[0] = new_settings[0] & ~termios.IGNCR

To get normal behaviour, just omit the lines clearing those three lines, and the input translation is unchanged even when "raw".

The new_settings[1] = new_settings[1] & ~termios.OPOST line disables all output processing, regardless what the other output flags say. You can just omit it to keep output processing intact. This keeps output "normal" even in raw mode. (It does not affect whether input is automatically echoed or not; that is controlled by the ECHO cflag in new_settings[3].)

Finally, when new attributes are set, the call will succeed if any of the new settings were set. If the settings are sensitive -- for example, if you are asking for a password on the command line --, you should get the new settings, and verify the important flags are correctly set/unset, to be sure.

If you want to see your current terminal settings, run

stty -a

The input flags are usually on the fourth line, and the output flags on the fifth line, with a - preceding the flag name if the flag is unset. For example, the output could be

speed 38400 baud; rows 58; columns 205; line = 0;
intr = ^C; quit = ^\; erase = ^?; kill = ^U; eof = ^D; eol = M-^?; eol2 = M-^?; swtch = M-^?; start = ^Q; stop = ^S; susp = ^Z; rprnt = ^R; werase = ^W; lnext = ^V; flush = ^O; min = 1; time = 0;
-parenb -parodd cs8 hupcl -cstopb cread -clocal -crtscts
-ignbrk brkint -ignpar -parmrk -inpck -istrip -inlcr -igncr icrnl ixon -ixoff -iuclc ixany imaxbel iutf8
opost -olcuc -ocrnl onlcr -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0 ff0
isig icanon iexten echo echoe echok -echonl -noflsh -xcase -tostop -echoprt echoctl echoke

On pseudoterminals, and USB TTY devices, the baud rate is irrelevant.

If you write Bash scripts that wish to read e.g. passwords, consider the following idiom:

#!/bin/bash
trap 'stty sane ; stty '"$(stty -g)" EXIT
stty -echo -echonl -imaxbel -isig -icanon min 1 time 0

The EXIT trap is executed whenever the shell exits. The stty -g reads the current settings of the terminal at the start of the script, so the current settings are restored when the script exits, automatically. You can even interrupt the script with Ctrl+C, and it'll do the right thing. (In some corner cases with signals, I've found that the terminal sometimes gets stuck with the raw/noncanonical settings (requiring one to type reset + Enter blindly at the terminal), but running stty sane before restoring the actual original settings has cured that every time for me. So that's why it's there; a sort of added safety.)

You can read input lines (unechoed to the terminal) using read bash built-in, or even read the input character-by-character using

IFS=$'\0'
input=""
while read -N 1 c ; do
    [[ "$c" == "" || "$c" == $'\n' || "$c" == $'\r' ]] && break
    input="$input$c"
done

If you don't set IFS to ASCII NUL, read built-in will consume the separators, so that c will be empty. Trap for young players.

share|improve this answer
1  
Oh, for gods' sake, nothing is ever simple :( – cat Feb 22 at 17:59
    
I'm accepting this answer because it's most helpful to me as a Python dev, even though the other one is great – cat Feb 22 at 18:01
2  
@cat: While this may be most helpful to you, I'd still say Thomas Dickey's answer is more correct. I'd rather you accept that instead. – Nominal Animal Feb 22 at 18:03
3  
While your willingness to forgo your +15 rep does you credit, @cat is quite right. Whether an answer is accepted or not is no indication that it is the "most correct" of the posted answers. It only means that's the one the OP preferred for whatever personal reasons. The "most correct" is usually the most highly upvoted. Accepting an answer is down to personal preference, if the OP prefers yours, there is no reason not to accept it. – terdon Feb 22 at 18:59
1  
@terdon: Okay, I stand corrected, then. – Nominal Animal Feb 22 at 21:20

Essentially "because it's been done that way since manual typewriters". Really.

A manual typewriter had a carriage on which the paper was fed, and it moved forward as you typed (loading a spring), and had a lever or key which would release the carriage, letting the spring return the carriage to the left-margin.

As electronic data entry (teletype, etc) were introduced, they carried that forward. So the Enter key on many terminals would be labeled Return.

Line feeds happened (in the manual process) after returning the carriage to the left margin. Again, the electronic devices imitated the manual devices, making a separate line-feed operation.

Both operations are encoded (to allow the teletype to be more than a standalone device creating a paper type), so we have CR (carriage-return) and LF (line-feed). This image from ASR 33 Teletype Information shows the keyboard, with Return on the right side, and Line-Feed just to the left. Being on the right, it was the main key:

enter image description here

Unix came along later. Its developers liked to shorten things (look at all of the abbreviations, even creat for "create"). Faced with a possibly two-part process, they decided that line-feeds only made sense if they were preceded by carriage-returns. So they dropped the explicit carriage returns from files, and translated the terminal's Return key to send the corresponding line-feed. Just to avoid confusion, they referred to line-feed as "newline".

When writing text on the terminal, Unix translates in the other direction: a line-feed becomes carriage-return / line-feed.

(That is, "normally": so-called "cooked mode", in contrast to "raw" mode where no translation is done).

Summary:

  • carriage-return / line-feed is the sequence 13 10
  • the device sends 13 (since "forever" in your terms)
  • Unix-like systems change that to 13 10
  • Other systems do not necessarily store just 10 (Windows largely accepts just 10 or 13 10, depending how important compatibility is).
share|improve this answer
1  
I looked for a nice picture to show the levers for a manual typewriter, but found only low-resolution images. – Thomas Dickey Feb 22 at 2:07
3  
If you had to type on one of those, you'd abbreviate everything too! – Michael Hampton Feb 22 at 4:02
2  
Regarding the history part: the manual typewriters I used in my use, similar to this one only had one lever. When you pulled it, it first cranked the roller (line feed) and then it would just pull the carriage along. And it was this pull that loaded the spring. Each letter typed, or tab pressed, would release the spring somewhat, moving the carriage back to the "unloaded" position, which was at the end of the line, not its start. – RealSkeptic Feb 22 at 10:47
2  
On input, CR is translated (by the tty line discipline) to LF, not CR LF. It's on output (including the echo of the input) that LF is translated to CR LF. When you type foo<Return> in cooked mode, the application reads foo\n and foo\r\n is sent back by the line discipline for echo to the terminal. – Stéphane Chazelas Feb 22 at 16:08
2  

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