CODEWITHSUNDEEP
linuxbashpipefifomkfifo
user3347114
user3347114

Reputation: 185

how to read chunks from named pipe in bash

i have a command that potentially outputs a lot of data to stdout and I need to upload that via ftp to a remote location.

I found this question Upload output of a program directly to a remote file by ftp and I really liked the idea of redirecting the output into a named pipe and then read junks from it. however as soon as I read the first chunk via dd the command inputing into the pipe just exits and there is no more data to read from the pipe.

to test this i created a fifo

#> mkfifo fifo

then I wrote into the fifo on one shell:

#> echo bla  > fifo

and on another shell i read from it:

#> dd if=fifo of=spool.1 bs=1 count=1

it outputs the first byte into spool.1 and then the command writing into the pipe exits and I can't read the remaining data from the pipe.

I would like to read the next chunk from that pipe but I can't figure out what I am doing wrong

any idea how to keep that pipe open until all data is read from it?

Upvotes: 1

Views: 1672

Answers (1)

chepner
chepner

Reputation: 531345

dd needs to read from standard input, rather than opening and closing the pipe itself, to keep the write end open for echo. Once the write end is closed, you can't open the read end again.

For example

{ 
  dd of=spool.1 bs=1 count=1
  dd of=spool.2 bs=2 count=2
  dd of=spool.2 bs=2 count=2
} < fifo

fifo is opened once for the compound command {...}, and each call to dd inherits the same open file descriptor without closing it.

Upvotes: 3

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