I’m reading from a serial port connected to a gps device sending nmea strings.
A simplified invocation to illustrate my point:
$ awk '{ print $0 }' /dev/ttyPSC9
GPGGA,073651.000,6310.1043,N,01436.1539,E,1,07,1.0,340.2,M,33.3,M,,0000*56
$GPGSA,A,3,28,22,09,27,01,19,17,,,,,,2.3,1.0,2.0*39
$GPRMC,073651.000,A,6310.1043,N,01436.1539,E,0.42,163.42,070312,,,A*67
GPGGA,073652.000,6310.1043,N,01436.1540,E,1,07,1.0,339.2,M,33.3,M,,0000*55
$GPGSA,A,3,28,22,09,27,01,19,17,,,,,,2.3,1.0,2.0*39
If I instead try to read from a pipe, awk buffers the input before sending it to stdout.
$ cat /dev/ttyPSC9 | awk '{ print $0 }'
<long pause>
GPGGA,073651.000,6310.1043,N,01436.1539,E,1,07,1.0,340.2,M,33.3,M,,0000*56
$GPGSA,A,3,28,22,09,27,01,19,17,,,,,,2.3,1.0,2.0*39
$GPRMC,073651.000,A,6310.1043,N,01436.1539,E,0.42,163.42,070312,,,A*67
GPGGA,073652.000,6310.1043,N,01436.1540,E,1,07,1.0,339.2,M,33.3,M,,0000*55
$GPGSA,A,3,28,22,09,27,01,19,17,,,,,,2.3,1.0,2.0*39
How can I avoid the buffering?
Edit: Kyle Jones suggested that cat is buffering it’s output but that doesn’t appear to be happening:
$ strace cat /dev/ttyPSC9 | awk '{ print $0 }'
write(1, "2,"..., 2) = 2
read(3, "E"..., 4096) = 1
write(1, "E"..., 1) = 1
read(3, ",0"..., 4096) = 2
When I think about it: I thought that a program used line buffering when writing to a terminal and “regular buffering” for all other cases. Then, why is cat not buffering more? Is the serial port signaling EOF? Then why is cat not terminated?
Answers:
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Method 1
I know it is an old question, but a one-liner may help those who come here searching:
cat /dev/ttyPSC9 | awk '{ print $0; system("")}'
system("") does the trick, and is POSIX compliant. Non-posix systems: beware.
There exists a more specific function fflush() that does the same, but is not available in older versions of awk.
An important piece of information from the docs regarding the use of system(""):
gawk treats this use of the system() function as a special case and is
smart enough not to run a shell (or other command interpreter) with
the empty command. Therefore, with gawk, this idiom is not only
useful, it is also efficient.
Method 2
It is likely to be buffering in awk, not cat. In the first case, awk believes it is interactive because its input and output are TTYs (even though they’re different TTYs – I’m guessing that awk is not checking that). In the second, the input is a pipe so it runs non-interactively.
You will need to explicitly flush in your awk program. This is not portable, though.
For more background and details on how to flush output, read: http://www.gnu.org/software/gawk/manual/html_node/I_002fO-Functions.html
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