<<Anchor(faq27)>>
== How can two unrelated processes communicate? ==
Two unrelated processes cannot use the arguments, the environment or
stdin/stdout to communicate; some form of inter-process communication (IPC)
is required.

=== A file ===
  
Process A writes in a file, and Process B reads the file. This method is not synchronized and therefore is not
safe if B can read the file while A writes in it.  A lockdir or a
signal can probably help.

=== A directory as a lock ===

`mkdir` can be used to test for the existence of a dir and create it in one
atomic operation; it thus can be used as a lock, although not a very
efficient one.

Script A:
{{{
    until mkdir /tmp/dir;do # wait until we can create the dir
      sleep 1
    done
    echo foo > file         # write in the file this section is critical
    rmdir /tmp/dir          # remove the lock
}}}

Script B:
{{{ 
    until mkdir /tmp/dir;do #wait until we can create the dir
      sleep 1
    done
    read var < file         # read in the file this section is, critical
    echo "$var"             # Script A cannot write in the file
    rmdir /tmp/dir          # remove the lock
  
}}}

See [[http://mywiki.wooledge.org/BashFAQ/045|Faq #45 ]] and
[[http://wiki.bash-hackers.org/howto/mutex | mutex]]
for more examples with a lock directory.


=== Signals ===

Signals are probably the simplest form of IPC:
  
ScriptA:
{{{
    trap 'flag=go' USR1 #set up the signal handler for the USR1 signal

    # echo $$ > /tmp/ScriptA.pid #if we want to save the pid in a file

    flag=""
    while [[ $flag != go ]]; do # wait for the green light from Script B
      sleep 1;
    done
    echo we received the signal
}}}

You must find or know the pid of the other script to send it a
signal using kill:
{{{
     #kill all the
     pkill -USR1 -f ScriptA 
     
     #if ScriptA saved its pid in a file
     kill -USR1 $(</var/run/ScriptA.pid)

     #if ScriptA is a child:
     ScriptA & pid=$!
     kill -USR1 $pid
}}}

The first 2 methods are not bullet proof and will cause trouble if
you run more than one instance of scriptA.

=== Named Pipes ===

Named pipes are a much richer form of IPC. They are described on their
own page: NamedPipes.

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CategoryShell