5

I just noticed that I've seen different behavior across different systems when creating a quick test file (e.g. with sample input) via cat and redirection.

Here are the steps to see what I'm talking about:

Run cat > testfile.

Type in helli, then press backspace, then type o.

Press Enter.

Type Ctrl-D to end the input.

Run od -a testfile.

On some systems, e.g. Mac, you will get:

0000000    h   e   l   l   o  nl                                        
0000006

On other systems, e.g. a RHEL 5.7 host that I logged into via MobaXterm on Windows, you may get:

0000000    h   e   l   l   i  bs   o  nl                                        
0000010

Where is the code that makes for this difference? For example, in the example above, should I suspect MobaXterm, or the RHEL 5 system? Or which layer between my keyboard and the filesystem?

This is more a question out of curiosity than a real problem; I can obviously create files without the backspace characters by just using a text editor, but this has tripped me up in the past. One time during a casual presentation/training, I mentioned that creating files via cat has this problem with backspace characters being taken literally, only to be shown wrong when it didn't have that effect after all, on the Macs the students were using. So it got me curious.

AdminBee
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2 Answers2

4

That's determined by a) what characters does the terminal emulator send when the BackSpace key is pressed (^H/BS or ^?/DEL) and b) what character is used as VERASE by the tty driver (see and change the latter with stty(1) -- stty erase ^H).

If the terminal emulator is sending ^H but the tty isn't recognizing it as a special character, presssing the BackSpace key will visually "erase" the last character on the screen[1], but the character will be sent as-is (together with the character before it) to the process reading from the tty (cat).

Editors and interactive programs with line-editing capabilities set the tty to raw mode and do their own handling of special characters, and may treat ^H and ^? the same; cat isn't one of those ;-)

[1] this is subject to the echoctl stty setting -- if that is set, the terminal will echo control characters back in the "^"+chr(char^0x40) format (^H for BS = 8, etc).

1

You can reproduce the behavior you are seeing with cat by manipulating the terminal characteristics of your shell with the stty command.

When the terminal is in canonical mode, input is gathered into lines, line editing is enabled and optionally stdin is echoed to the screen (IEXTEN flag).

In non-canonical mode some control characters are passed on to the shell.

Example:

stty -icanon
cat > testfile # end with ctrl-c
#typed ls | space | backspace | ctrl-h | al | ctrl-d | ctrl-c
od -a testfile
0000000   l   s  sp del  bs   a   l  nl eot
0000011

To resume canonical mode:

stty icanon

To interrogate the current terminal device:

stty
# OR for more info
stty -a
speed 9600 baud; rows 62; columns 255; line = 0;
intr = ^C; quit = ^\; erase = ^?; kill = ^U; eof = ^D; eol = M-^?; eol2 = M-^?; swtch = <undef>; start = ^Q; stop = ^S; susp = ^Z; rprnt = ^R; werase = ^W; lnext = ^V; discard = ^O; min = 1; time = 0;
-parenb -parodd -cmspar 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 -flusho -extproc

The output of the flags section of this command is such that a preceeding - indicates that a flag is unset, so icanon is on and -icanon is off for the canonical flag.

From the man page:

Optional - before SETTING indicates negation. An * marks non-POSIX settings.

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