Consider the following scenario:
a FIFO named test is created. In one terminal window (A) I run cat <test and in another (B) cat >test. It is now possible to write in window B and get the output in window A. It is also possible to terminate the process A and relaunch it and still be able to use this setup as suspected. However if you terminate the process in window B, B will (as far as I know) send an EOF through the FIFO to process A and terminate that as well.
In fact, if you run a process that does not terminate on EOF, you'll still not be able to use your FIFO you redirected to the process. Which I think is because this FIFO is considered closed.
Is there anyway to work around this problem?
The reason to why I ran into this problem is because I'd like to send commands to my minecraft server running in a screen session. For example: echo "command" >FIFO_to_server. This is problably possible to do by using screen by itself but I'm not very comfortable with screen I think a solution using only pipes would be a simpler and cleaner one.
A is reading from a file. When it reaches the end of the file, it stops reading. This is normal behavior, even if the file happens to be a fifo. You now have four approaches.
Change the code of the reader to make it keep reading after the end of the file. That's saying the input file is infinite, and reaching the end of the file is just an illusion. Not practical for you, because you'd have to change the minecraft server code.
Apply unix philosophy. You have a writer and a reader who don't agree on protocol, so you interpose a tool that connects them. As it happens, there is such a tool in the unix toolbox: tail -f. tail -f keeps reading from its input file even after it sees the end of the file. Make all your clients talk to the pipe, and connect tail -f to the minecraft server:
tail -n +1 -f client_pipe | minecraft_server &
As mentioned by jilles, use a trick: pipes support multiple writers, and only become closed when the last writer goes away. So make sure there's a client that never goes away.
while true; do sleep 999999999; done >client_pipe &
The problem is that the server is fundamentally designed to handle a single client. To handle multiple clients, you should change to using a socket. Think of sockets as “meta-pipes”: connecting to a socket creates a pipe, and once the client disconnects, that particular pipe is closed, but the server can accept more connections. This is the clean approach, because it also ensures that you won't have mixed up data if two clients happen to connect at the same time (using pipes, their commands could be interspersed). However, it require changing the minecraft server.
Start a process that keeps the fifo open for writing and keeps running indefinitely. This will prevent readers from seeing an end-of-file condition.
From this answer -
On some systems like Linux, <> on a named pipe (FIFO) opens the named pipe without blocking (without waiting for some other process to open the other end), and ensures the pipe structure is left alive. For instance in:
So you could do:
cat <>up_stream >down_stream
# the `cat pipeline keeps running
echo 1 > up_stream
echo 2 > up_stream
echo 3 > up_stream
However, I can't find documentation about this behavior. So this could be implementation detail which is specific to some systems. I tried the above on MacOS and it works.
You can add multiple inputs ino a pipe by adding what you require in brackets with semi-colons in your 'mkfifo yourpipe':
(cat file1; cat file2; ls -l;) > yourpipe
Related
I have a setup such that sometimes I use xterm and sometimes I use putty. The command
xmodmap ~/.Xmodmap
takes longer to run when I am on putty because there is no xserver at DISPLAY.
Without getting into a heated discussion of whether or not my setup is right (because I can't change it), or whether the time difference is significant (no, it's not, but if you don't ask, you'll never learn), is there a way to ping the supposed xserver at DISPLAY to that it comes back instantaneously if there is no xserver there? That way I could set a flag and skip further X client calls, instead of calling xmodmap (or xterm or any other X client) and waiting for the inevitable timeout and 'unable to open display at' message.
xmodmap 1>/dev/null 2>/dev/null
if (($?))
then
## There is no xserver. Do not set any of this up.
return 0 ## return, not exit because this script is meant to be 'dotted in'
fi
I have a program already written in gawk that downloads a lot of small bits of info from the internet. (A media scanner and indexer)
At present it launches wget to get the information. This is fine, but I'd like to simply reuse the connection between invocations. Its possible a run of the program might make between 200-2000 calls to the same api service.
I've just discovered that gawk can do networking and found geturl
However the advice at the bottom of that page is well heeded, I can't find an easy way to read the last line and keep the connection open.
As I'm mostly reading JSON data, I can set RS="}" and exit when body length reaches the expected content-length. This might break with any trailing white space though. I'd like a more robust approach. Does anyone have a nicer way to implement sporadic http requests in awk that keep the connection open. Currently I have the following structure...
con="/inet/tcp/0/host/80";
send_http_request(con);
RS="\r\n";
read_headers();
# now read the body - but do not close the connection...
RS="}"; # for JSON
while ( con |& getline bytes ) {
body = body bytes RS;
if (length(body) >= content_length) break;
print length(body);
}
# Do not close con here - keep open
Its a shame this one little thing seems to be spoiling all the potential here. Also in case anyone asks :) ..
awk was originally chosen for historical reasons - there were not many other language options on this embedded platform at the time.
Gathering up all of the URLs in advance and passing to wget will not be easy.
re-implementing in perl/python etc is not a quick solution.
I've looked at trying to pipe urls to a named pipe and into wget -i - , that doesn't work. Data gets buffered, and unbuffer not available - also I think wget gathers up all the URLS until EOF before processing.
The data is small so lack of compression is not an issue.
The problem with the connection reuse comes from the HTTP 1.0 standard, not gawk. To reuse the connection you must either use HTTP 1.1 or try some other non-standard solutions for HTTP 1.0. Don't forget to add the Host: header in your HTTP/1.1 request, as it is mandatory.
You're right about the lack of robustness when reading the response body. For line oriented protocols this is not an issue. Moreover, even when using HTTP 1.1, if your scripts locks waiting for more data when it shouldn't, the server will, again, close the connection due to inactivity.
As a last resort, you could write your own HTTP retriever in whatever langauage you like which reuses connections (all to the same remote host I presume) and also inserts a special record separator for you. Then, you could control it from the awk script.
Normally CouchDB communicates with a view server over STDIO via a simple line-based protocol.
What I want to do is, instead of a view server, have it read and write to some sort of pipe or pseudo terminal, to which I then connect, and play the view server.
I think one way to do it is with a lot of complicated use of cat and FIFO's.
But I found out that on my Mac echo "hi" > /dev/ttys000 comes back to my terminal, so I was thinking it should be possible to establish a connection this way, but I can't tell from manpages how to do it for real.
[update] I found write, which sends message from couch to my terminal, but couch is not attached, so I can't send messages in return.
nc -l 12345
And then a regular netcat to connect to it.
Is there a way to only close "one end" of a TCP socket to cleanly indicate one side of a connection is done writing to the connection? (Just like you do with a pipe in every Unix pipe tutorial ever.) Or should I use some in-band solution like a sentinel value or some such?
You can shutdown a socket for read or write using the second parameter to the method:
shutdown(sock, SHUT_RD)
shutdown(sock, SHUT_WR)
If the server is doing the writing, and does a shutdown() for write, the client should get an end of file when it tries to read (rather than blocking and waiting for data to arrive). It will however still be able to write to the socket.
Suppose I want to transfer just a portion of a file over FTP - is it possible using a standard FTP protocol?
In HTTP I could use a Range header in the request to specify the data range of the remote resource. If it's a 1mb file, I could ask for the bytes from 600k to 700k.
Is there anything like that in FTP? I am reading the FTP RFC, don't see anything, but want to make sure I'm not missing anything.
There's a Restart command in FTP - would that work?
Addendum
After getting Brian Bondy's answer below, I wrote a read-only Stream class that wraps FTP. It supports Seek() and Read() operations on a resource that is read via FTP, based on the REST verb.
Find it at http://cheeso.members.winisp.net/srcview.aspx?dir=streams&file=FtpReadStream.cs
It's pretty slow to Seek(), because setting up the data socket takes a long time. Best results come when you wrap that stream in a BufferedStream.
Yes you can use the REST command.
REST sets the point at which a subsequent file transfer should start. It is used usually for restarting interrupted transfers. The command must come right before a RETR or STOR and so come after a PORT or PASV.
From FTP's RFC 959:
RESTART (REST) The argument field
represents the server marker at which
file transfer is to be restarted. This
command does not cause file transfer
but skips over the file to the
specified data checkpoint. This
command shall be immediately followed
by the appropriate FTP service command
which shall cause file transfer to
resume.
Read more:
http://www.faqs.org/rfcs/rfc959.html#ixzz0jZp8azux
You should check out how GridFTP does parallel transfers. That's using the sort of techniques that you want (and might actually be code that it is better to borrow rather than implementing from scratch yourself).