On Wed, Sep 17, 2025 at 20:48:13 +0700, Robert Elz wrote:
> g...@wooledge.org said:
> | I'm not sure exactly what bash is doing here, but I'm guessing it has
> | something to do with switching the terminal from canonical mode to raw
> | mode...?
>
> I don't look at bash code, but the NetBSD shell sets the "eol" char in
> the terminal to the delimiter, so that a read will terminate when that
> character is input. My guess is that bash might do the same.
I tried
strace bash -c 'read -r'
strace bash -c 'read -d q -r'
and the difference is quite profound. The first one calls ioctl() once,
and then calls read(). The second one calls ioctl() five times before
the reading begins, and three times after.
Also, the first one ends up doing a single read() call, while the second
one does a separate read() for each byte.
For the first one, I typed 'a', backspace, 'b', 'q', Enter. For the
second, I typed 'a', backspace, 'b', 'q'.
Here's the first:
ioctl(0, TCGETS, {c_iflag=BRKINT|IGNPAR|ICRNL|IXON|IMAXBEL|IUTF8,
c_oflag=NL0|CR0|TAB0|BS0|VT0|FF0|OPOST|ONLCR, c_cflag=B38400|CS8|CREAD,
c_lflag=ISIG|ICANON|ECHO|ECHOE|ECHOK|IEXTEN|ECHOCTL|ECHOKE, ...}) = 0
read(0, bq
"bq\n", 4096) = 3
(What you're not seeing is that I had an "a", then backspaced it, and
overwrote it with "b".)
Here's the second:
ioctl(0, TCGETS, {c_iflag=BRKINT|IGNPAR|ICRNL|IXON|IMAXBEL|IUTF8,
c_oflag=NL0|CR0|TAB0|BS0|VT0|FF0|OPOST|ONLCR, c_cflag=B38400|CS8|CREAD,
c_lflag=ISIG|ICANON|ECHO|ECHOE|ECHOK|IEXTEN|ECHOCTL|ECHOKE, ...}) = 0
ioctl(0, TCGETS, {c_iflag=BRKINT|IGNPAR|ICRNL|IXON|IMAXBEL|IUTF8,
c_oflag=NL0|CR0|TAB0|BS0|VT0|FF0|OPOST|ONLCR, c_cflag=B38400|CS8|CREAD,
c_lflag=ISIG|ICANON|ECHO|ECHOE|ECHOK|IEXTEN|ECHOCTL|ECHOKE, ...}) = 0
ioctl(0, TCGETS, {c_iflag=BRKINT|IGNPAR|ICRNL|IXON|IMAXBEL|IUTF8,
c_oflag=NL0|CR0|TAB0|BS0|VT0|FF0|OPOST|ONLCR, c_cflag=B38400|CS8|CREAD,
c_lflag=ISIG|ICANON|ECHO|ECHOE|ECHOK|IEXTEN|ECHOCTL|ECHOKE, ...}) = 0
ioctl(0, TCSETSW, {c_iflag=BRKINT|IGNPAR|ICRNL|IXON|IMAXBEL|IUTF8,
c_oflag=NL0|CR0|TAB0|BS0|VT0|FF0|OPOST|ONLCR, c_cflag=B38400|CS8|CREAD,
c_lflag=ISIG|ECHO|ECHOE|ECHOK|IEXTEN|ECHOCTL|ECHOKE, ...}) = 0
ioctl(0, TCGETS, {c_iflag=BRKINT|IGNPAR|ICRNL|IXON|IMAXBEL|IUTF8,
c_oflag=NL0|CR0|TAB0|BS0|VT0|FF0|OPOST|ONLCR, c_cflag=B38400|CS8|CREAD,
c_lflag=ISIG|ECHO|ECHOE|ECHOK|IEXTEN|ECHOCTL|ECHOKE, ...}) = 0
*snip dozens of rt_sigaction calls*
read(0, a"a", 1) = 1
read(0, ^?"\177", 1) = 1
read(0, b"b", 1) = 1
read(0, q"q", 1) = 1
ioctl(0, TCGETS, {c_iflag=BRKINT|IGNPAR|ICRNL|IXON|IMAXBEL|IUTF8,
c_oflag=NL0|CR0|TAB0|BS0|VT0|FF0|OPOST|ONLCR, c_cflag=B38400|CS8|CREAD,
c_lflag=ISIG|ECHO|ECHOE|ECHOK|IEXTEN|ECHOCTL|ECHOKE, ...}) = 0
ioctl(0, TCSETSW, {c_iflag=BRKINT|IGNPAR|ICRNL|IXON|IMAXBEL|IUTF8,
c_oflag=NL0|CR0|TAB0|BS0|VT0|FF0|OPOST|ONLCR, c_cflag=B38400|CS8|CREAD,
c_lflag=ISIG|ICANON|ECHO|ECHOE|ECHOK|IEXTEN|ECHOCTL|ECHOKE, ...}) = 0
ioctl(0, TCGETS, {c_iflag=BRKINT|IGNPAR|ICRNL|IXON|IMAXBEL|IUTF8,
c_oflag=NL0|CR0|TAB0|BS0|VT0|FF0|OPOST|ONLCR, c_cflag=B38400|CS8|CREAD,
c_lflag=ISIG|ICANON|ECHO|ECHOE|ECHOK|IEXTEN|ECHOCTL|ECHOKE, ...}) = 0
I can't tell what those ioctl() calls are doing. Why call TCGETS three
times in a row? And sadly, the special characters (c_cc) being passed
on the TCSETSW call are not shown by strace.
