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Top Forums Shell Programming and Scripting Grab exactly one byte from a FIFO, at random intervals Post 302748295 by vomv1988 on Tuesday 25th of December 2012 06:23:43 AM
Old 12-25-2012
Quote:
Originally Posted by bipinajith
Here is the code:
Code:
#!/bin/bash

MY_INPUT=INPUT
MY_FIFO=FIFO

(
tail -f ${MY_INPUT} | while IFS= read -n 1 CHAR ; do
        printf "${CHAR}" > ${MY_FIFO}
done
) &
READPID=$!

PROMPT=n
while [ "${PROMPT}" != "q" ]
do
        printf 'y: print next byte\nn: don'"'"'t print next byte\nq: quit\n'
        read -n 1 PROMPT
        printf '\n'
        if [ "${PROMPT}" = "y" ]
        then
# This is 'grab1byte':
                echo The input byte is:
                xxd -cols 1 ${MY_FIFO} | sed 's/^.*: //'
        fi
done

kill $READPID

I tried this very code and I am still getting the undesired behavior from my original post. It's funny how the amount of bytes I get from FIFO seems to be system-dependent; more specifically, it seems hardware-dependent. When I tried the code in a very fast mac, with bash, cat-ing FIFO dumped the whole string 'Hello, world!' to stdout. On my (slower) yeeloong notebook, cat-ing FIFO only dumps the first few bytes of the string, and then a few more bytes, etc. Maybe the software in a macbook is designed differently, although the shell is the same (bash), I don't know. I have no clue as to why this happens. This is just so strange.
 

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TAIL(1) 						    BSD General Commands Manual 						   TAIL(1)

NAME
tail -- display the last part of a file SYNOPSIS
tail [-F | -f | -r] [-b number | -c number | -n number] [file ...] DESCRIPTION
The tail utility displays the contents of file or, by default, its standard input, to the standard output. The display begins at a byte, line or 512-byte block location in the input. Numbers having a leading plus (``+'') sign are relative to the beginning of the input, for example, ``-c +2'' starts the display at the second byte of the input. Numbers having a leading minus (``-'') sign or no explicit sign are relative to the end of the input, for example, ``-n 2'' displays the last two lines of the input. The default starting location is ``-n 10'', or the last 10 lines of the input. The options are as follows: -b number The location is number 512-byte blocks. -c number The location is number bytes. -f The -f option causes tail to not stop when end of file is reached, but rather to wait for additional data to be appended to the input. The -f option is ignored if the standard input is a pipe, but not if it is a FIFO. -F The -F option implies the -f option, but tail will also check to see if the file being followed has been renamed or rotated. The file is closed and reopened when tail detects that the filename being read from has a new inode number. The -F option is ignored if reading from standard input rather than a file. -n number The location is number lines. -r The -r option causes the input to be displayed in reverse order, by line. Additionally, this option changes the meaning of the -b, -c and -n options. When the -r option is specified, these options specify the number of bytes, lines or 512-byte blocks to display, instead of the bytes, lines or blocks from the beginning or end of the input from which to begin the display. The default for the -r option is to display all of the input. If more than a single file is specified, each file is preceded by a header consisting of the string ``==> XXX <=='' where ``XXX'' is the name of the file. DIAGNOSTICS
The tail utility exits 0 on success, and >0 if an error occurs. SEE ALSO
cat(1), head(1), sed(1) STANDARDS
The tail utility is expected to be a superset of the IEEE Std 1003.2-1992 (``POSIX.2'') specification. In particular, the -F, -b and -r options are extensions to that standard. The historic command line syntax of tail is supported by this implementation. The only difference between this implementation and historic versions of tail, once the command line syntax translation has been done, is that the -b, -c and -n options modify the -r option, i.e. ``-r -c 4'' displays the last 4 characters of the last line of the input, while the historic tail (using the historic syntax ``-4cr'') would ignore the -c option and display the last 4 lines of the input. HISTORY
A tail command appeared in PWB UNIX. BSD
June 6, 1993 BSD
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