Sponsored Content
Top Forums Shell Programming and Scripting Grab exactly one byte from a FIFO, at random intervals Post 302747893 by vomv1988 on Sunday 23rd of December 2012 03:52:53 PM
Old 12-23-2012
Quote:
Originally Posted by bipinajith
This might sound stupid, but can you replace this section:
Code:
# This is 'grab1byte':
                INBYTE=`cat ${MY_FIFO}`
                echo The input byte is:
                printf "${INBYTE}" | xxd -cols 1 | sed 's/^.*: //'

with
Code:
# This is 'grab1byte':
                echo The input byte is:
                xxd -cols 1 ${MY_FIFO} | sed 's/^.*: //'

and give it another try? Let us know the result please.
The output is the same with that change, but thanks for trying anyway. After removing the sed filter from the original script I posted, the output looks something like:

Code:
y: print next byte
n: don't print next byte
q: quit
y
The input byte is:
0000000: 48  H
0000001: 65  e
y: print next byte
n: don't print next byte
q: quit
y
The input byte is:
0000000: 6c  l
0000001: 6c  l
0000002: 6f  o
0000003: 2c  ,
0000004: 20   
0000005: 77  w
y: print next byte
n: don't print next byte
q: quit

So this means that the FIFO buffer contains several bytes instead of just one... Which is weird, because, I thought 'printf "${CHAR}" > ${MY_FIFO}"' was supposed to pause the loop execution, UNTIL FIFO was emptied by something like 'cat ${MY_FIFO}'. After cat-ing FIFO, I thought the loop would freeze again at 'printf "${CHAR}" > ${MY_FIFO}"', until another instance of 'cat ${MY_FIFO}', but, apparently it doesn't. Apparently, it just feeds FIFO a random amount of bytes... WHY???

---------- Post updated at 04:52 PM ---------- Previous update was at 04:44 PM ----------

I'm thinking, maybe, cat opens up the FIFO for a longer time than it takes the loop to iterate, so the loop iterates several times, spitting several bytes into FIFO, until cat stops reading from FIFO (that is, until FIFO's output is closed)... Does this make any sense to you? And, if that is the case: How would I prevent that from happening?
 

10 More Discussions You Might Find Interesting

1. Shell Programming and Scripting

Date Intervals

I posted a question on date intervals about a month back asking about how I could be able to go about a user entering the starting year/month/day and an ending year/month/day and then the script automatically cycling through each day of each month of each year that the user has specified. I... (7 Replies)
Discussion started by: yongho
7 Replies

2. Shell Programming and Scripting

Check if 2 files are identical byte-to-byte?

In my server migration requirement, I need to compare if one file on old server is exactly the same as the corresponding file on the new server. For diff and comm, the inputs need to be sorted. But I do not want to disturb the content of the file and need to find byte-to-byte match. Please... (4 Replies)
Discussion started by: krishmaths
4 Replies

3. Shell Programming and Scripting

Remove a byte(Last byte from the last line)

Hi All Can anyone please suggest me how to remove the last byte from a falt file .This is from the last line's last BYTE. Please suggest me something. Thank's and regards Vinay (1 Reply)
Discussion started by: vinayrao
1 Replies

4. Shell Programming and Scripting

Divide numbers into intervals

divide input values into specified number (-100 or -200) according to the key (a1 or a2 ....) For ex: if we give -100 in the command line it would create 100 number intervals (1-100, 100-200, 200-300) untill it covers the value 300 in a1. Note: It should work the same even with huge numbers... (3 Replies)
Discussion started by: ruby_sgp
3 Replies

5. Ubuntu

expect script for random password and random commands

Hi I am new to expect. Please if any one can help on my issue its really appreciable. here is my issue: I want expect script for random passwords and random commands generation. please can anyone help me? Many Thanks in advance (0 Replies)
Discussion started by: vanid
0 Replies

6. Shell Programming and Scripting

Need to generate a file with random data. /dev/[u]random doesn't exist.

Need to use dd to generate a large file from a sample file of random data. This is because I don't have /dev/urandom. I create a named pipe then: dd if=mynamed.fifo do=myfile.fifo bs=1024 count=1024 but when I cat a file to the fifo that's 1024 random bytes: cat randomfile.txt >... (7 Replies)
Discussion started by: Devyn
7 Replies

7. UNIX for Dummies Questions & Answers

A crude random byte generator...

There was an upload recently on generating a pseudo-random file when /dev/random does NOT exist. This does not need /dev/random, /dev/urandom or $RANDOM either... (I assume $RANDOM relies on the /dev/random device in some way.) This code uses hexdump just because I like hexdump for ease of... (2 Replies)
Discussion started by: wisecracker
2 Replies

8. UNIX for Dummies Questions & Answers

Building intervals

Hi all, I hope you can help me with the following question: I have multiple tables like this: Chr Start End Zygosity Gene chr1 153233510 153233510 het LOR chr1 153233615 153233615 hom LOR chr1 153233701 153233701 hom LOR chr1 ... (5 Replies)
Discussion started by: lsantome
5 Replies

9. UNIX for Dummies Questions & Answers

Changing a special line and Byte in a random file

Hello I created 3 files by: dd if=/dev/urandom bs=1024 count=1000000 of=./testfile1 dd if=/dev/urandom bs=1024 count=5000000 of=./testfile2 dd if=/dev/urandom bs=1024 count=10000000 of=./testfile3 Now I want to know how to make a change in a specific byte and/or line of theses files? (2 Replies)
Discussion started by: frhling
2 Replies

10. UNIX for Dummies Questions & Answers

Gap length between intervals

hi all, I wish to calculate the length between intervals whose are defined by a starting and an end possition. The data looks like this: 1 10 23 30 45 60 70 100... The desired output should be: 13 # (23-10) 15 # (45-30) 10 # (70-60)... I donīt know how to operate with different... (2 Replies)
Discussion started by: lsantome
2 Replies
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 is the same as the -f option, except that every five seconds tail will check to see if the file named on the command line has been shortened or moved (it is considered moved if the inode or device number changes) and, if so, it will close the current file, open the filename given, print out the entire contents, and continue to wait for more data to be appended. This option is used to follow log files though rotation by newsyslog(8) or similar programs. -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. 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 -b, -r and -F 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 Version 7 AT&T UNIX. BUGS
When using the -F option, tail will not detect a file truncation if, between the truncation and the next check of the file size, data written to the file make it larger than the last known file size. BSD
June 6, 1993 BSD
All times are GMT -4. The time now is 06:55 AM.
Unix & Linux Forums Content Copyright 1993-2022. All Rights Reserved.
Privacy Policy