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I need to continually read from and write to a (virtual) file in bash, without closing the file in between. I'm looking for a program which will open the file for read and write, read from stdin and write what it reads to the file AND read from the file and write it to stdout. A bit like netcat but for files.

The file in question is a virtual file in /sys (a custom thing in something I'm working on) to which one writes a command and reads to receive the response - where the write and subsequent read must be done in the same session, i.e. on the same file descriptor without closing it in between.

Easy to do in C - open("file", O_RDWR), write(), read() - note: no need to seek().

Is there a standard GNU tool or do I need to write one?

  • Maybe you can used tee to achieve what you need? It will not read from the file though, but will copy to stdout what it writes to the file. – Bjorn Munch Mar 20 '16 at 20:29
  • If the suggestions that you've gotten so far aren't satisfactory to you, try explaining what you want to do a bit more clearly.  For example, are you starting with an empty file (or maybe a non-existent file), reading from stdin, and writing the data to both the file and stdout?  Or is there something already in the file when you start?  If so, do you want the new data to overwrite the old data?  In this case, do you want data from stdin to end up on stdout (after being written to the file), or do you want only the original contents of the file to be written to stdout? – G-Man Mar 21 '16 at 7:26
  • Please do not respond in comments; edit your question to make it clearer and more complete. – G-Man Mar 21 '16 at 7:27
  • Is that file behaving like a regular file (seekable, and read() return 0 bytes (EOF) when the end is reached), or like a bi-directional pipe or socket (where what you write is not what you may read again after seeking and where read() blocks when there's no available input)? – Stéphane Chazelas Mar 21 '16 at 11:48
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The redirection operator to open a file in read+write mode without truncation is <> in all Bourne-like shells (that maps to open(file, O_RDWR|O_CREAT) (though zsh also throws in a O_NOCTTY) or fopen(file, "w+")):

exec 3<> "$file"

opens the $file on file descriptor 3 in read+write mode (without truncating it and creating it if it didn't exist).

However, only ksh93 and zsh have seeking operators. dd can seek, but not backwards. And note no shell except zsh can have NUL bytes in their variables.

In zsh:

zmodload zsh/system
exec 3<> $file
sysread -i 3 -c 2 var # a read() of 2 bytes
sysseek -u 3 0 # seek back to beginning
# or sysseek -u 3 -w current -2 # to seek back 2 bytes
syswrite -o 3 something-else
exec 3<&- # close

In ksh93:

exec 3<> "$file"
var=$(dd bs=2 count=1 <&3 2>/dev/null; echo .)
var=${var%?}
exec 3<#((0)) # seek to beginning
# or exec 3<#((CUR-2)) # to seek back 2 bytes
print -ru3 something-else

Portably, you could still open the file several times, for each offset you want, like here to read and write 2 bytes at offset 2 (provided they're not bytes with value 0 if not using zsh):

var=$(dd bs=2 count=1 skip=1 < "$file"; echo .)
var=${var%?}
printf %s something-else | dd bs=2 seek=1 1<> "$file"

Or:

printf %s something-else | dd bs=2 seek=1 of="$file" conv=notrunc

To read and write to the same file, ksh93 has two other interesting redirection operators:

tr 01 10 < file >; file

Would store the output of tr in a temporary file and if tr is successful, rename that to file (beware the file is created anew, so with possibly different permissions and ownership).

tr -d 0 < file 1<>; file

Same as the standard/Bourne tr -d 0 < file 1<> file except that if tr succeeds, file is truncated where tr finished writing. You can use that for filter commands that produce less output than they read input, or more precisely commands that would not read data that they've previously written.

And zsh has the =(...) form of process substitution which you can use as:

 mv =(tr 01 10 < file) file

(with similar effect and caveats as ksh93's >;). Or:

 cp =(tr 01 10 < file) file

which would preserve attributes of file but means an extra copy.


Now if you need to read and write at the same offset using the same file descriptor and neither zsh nor ksh93 are available, you could always revert to perl/python/ruby...

perl -e '
  open F, "<>", "file" or die "open: $!";
  read F, $var, 1;
  seek F, 0, 0;
  print F "something-else"'

Now, after re-reading the updated version of your question, it looks like your file is behaving more like a socket or bidirectional pipe, and not like a regular, seekable file.

In which case, it could be just a matter of:

socat - file:your-file

or:

(cat >&3 3>&- & cat <&3 3<&-) 3<> your-file

to feed data from and to that file as read from/to stdin/stdout.

Note that each cat reads/writes to its own copy of the file descriptor 3 open by the shell, but they share the same open file description so it should be equivalent.

  • Thanks for the reply. I can't open the file several times: the write and read must be on the same file descriptor. I'll have a play with the exec 3<> /sys/blah technique and see if I can get anywhere, thanks. – Mark Smith Mar 21 '16 at 9:57
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    It's worth to note that >; and <>; can not be used with exec. – cuonglm Mar 21 '16 at 10:04
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Unless you are doing translation only, you can't read and write at the same time safely (and even with translation, it's possible that the file may end up "half translated" if the program exits with an error).

It sounds like you really want some combination of tee and sponge. You can use sponge from moreutils to soak up standard input and then write it out to a file. You can use tee to duplicate input to multiple outputs.

The end result would look something like this:

command < filename | tee >(sponge filename)

>() is an example of process substitution. In this case, >(sponge filename) is replaced with the path to a FIFO that goes to the stdin of sponge, which tee writes to. tee also writes its input to stdout independently.

Here's an example. You can see that the output is both written to stdout, and written to the file.

$ echo 'foo bar' > file
$ sed 's/foo/qux/' < file | tee >(sponge file)
qux bar
$ cat file
qux bar
  • Thanks for the reply. I've edited my question to make it more explicit. Firstly, I need to write to the file before reading. (Before AND after would be OK too.) Secondly the reads and writes need to be done on the same file descriptor. – Mark Smith Mar 21 '16 at 9:37
  • How is command < filename | tee >(sponge filename) solving the half translated problem? – Stéphane Chazelas Mar 21 '16 at 9:51
  • @StéphaneChazelas It doesn't. :-) That why I said "even with translation", not "with translation". – Chris Down Mar 21 '16 at 10:16
  • Note that if $TMPDIR is on the same filesystem as filename, then filename is created anew. While sponge will try and reproduce permissions, that will not be the case for other file attributes (ownership, ACLs, extended attributes...). – Stéphane Chazelas Mar 21 '16 at 10:32

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