$ ps aux | tee >(head -n1) | grep syslog
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
syslog 806 0.0 0.0 34600 824 ? Sl Sep07 0:00 rsyslogd -c4
The grep
and head
commands start at about the same time, and both receive the same input data at their own leisure, but generally, as data becomes available. There are some things that can introduce the 'unsynchronized' output which flips lines; for example:
The multiplexed data from tee
actually gets sent to one process before the other, depending primarily on the implementation of tee
. A simple tee
implementation will read
some amount of input, and then write
it twice: Once to stdout and once to its argument. This means that one of those destinations will get the data first.
However, pipes are all buffered. It is likely that these buffers are 1 line each, but they might be larger, which can cause one of the receiving commands to see everything it needs for output (ie. the grep
ped line) before the other command (head
) has received any data at all.
Notwithstanding the above, it's also possible that one of these commands receives the data but is unable to do anything with it in time, and then the other command receives more data and processes it quickly.
For example, even if head
and grep
are sent the data one line at a time, if head
doesn't know how to deal with it (or gets delayed by kernel scheduling), grep
can show its results before head
even gets a chance to. To demonstrate, try adding a delay: ps aux | tee >(sleep 1; head -n1) | grep syslog
This will almost certainly output the grep
output first.
$ ps aux | tee >(grep syslog) | head -n1
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
I believe you often only get one line here, because head
receives the first line of input and then closes its stdin and exits. When tee
sees that its stdout has been closed, it then closes its own stdin (output from ps
) and exits. This could be implementation-dependent.
Effectively, the only data that ps
gets to send is the first line (definitely, because head
is controlling this), and maybe some other lines before head
& tee
close their stdin descriptors.
The inconsistency with whether the second line appears is introduced by timing: head
closes stdin, but ps
is still sending data. These two events are not well-synchronized, so the line containing syslog
still has a chance of making it to tee
's argument (the grep
command). This is similar to the explanations above.
You can avoid this problem altogether by using commands that wait for all input before closing stdin/exiting. For example, use awk
instead of head
, which will read and process all its lines (even if they cause no output):
ps aux | tee >(grep syslog) | awk 'NR == 1'
But note that the lines can still appear out-of-order, as above, which can be demonstrated by:
ps aux | tee >(grep syslog) | (sleep 1; awk 'NR == 1')
Hope this wasn't too much detail, but there are a lot of simultaneous things interacting with each other. Separate processes run simultaneously without any synchronization, so their actions on any particular run can vary; sometimes it helps to dig deep into the underlying processes to explain why.
I suggest using pssh which was specifically designed for executing multiple parallel SSH sessions, instead of running several serial SSH commands concurrently.
E.g.,
pssh -i -H user@remote1 -H user@remote2 -H user@localhost '(python -u long_running_program.py &> ~/log$PSSH_NODENUM.txt)'
Best Answer
You don't need (or want) the parentheses there. Also, the
grep
syntax for logical OR isgrep 'foo\|bar
. You need to escape the|
unless you use-E
. So, any of these will work:or
or