Bash syntax error from valid command via ruby

Ruby
1

Hi,

OS: Ubuntu 14.04.1 LTS
ruby 1.9.3p484 (2013-11-22 revision 43786) [x86_64-linux]
ruby 2.0.0p384 (2014-01-12) [x86_64-linux-gnu]

$ grep ‘grep’ <(ps aux)
dave 17227 0.0 0.0 11744 652 pts/2 S+ 20:29 0:00 grep
[…]

This valid bash command [using bash process substitution** “<(cmd)”]
fails when trying to issue it through the usual ruby methods.
** http://tldp.org/LDP/abs/html/process-sub.html

I can work around it, easily, but can anyone identify the problem,
please?

$ ruby -e “%x{grep ‘grep’ <(ps aux)}”
sh: 1: Syntax error: “(” unexpected

$ ruby -e “system("grep ‘grep’ <(ps aux)")”
sh: 1: Syntax error: “(” unexpected

$ ruby -e “IO.popen("grep ‘grep’ <(ps aux)")”
sh: 1: Syntax error: “(” unexpected

$ ruby -e “Process.spawn("grep ‘grep’ <(ps aux)")”
sh: 1: Syntax error: “(” unexpected

$ ruby2.0 -e “%x{grep ‘grep’ <(ps aux)}”
sh: 1: Syntax error: “(” unexpected

Back to the old way, for now:
$ ruby -e “puts %x{ps aux | grep ‘grep’}”
dave 17876 0.0 0.0 30644 4860 pts/2 Sl+ 20:47 0:00 ruby -e
puts %x{ps aux | grep ‘grep’}
dave 17878 0.0 0.0 4444 636 pts/2 S+ 20:47 0:00 sh -c
ps aux | grep ‘grep’
dave 17881 0.0 0.0 11748 888 pts/2 S+ 20:47 0:00 grep
grep

daz

2

Seems I accidentally revealed the answer in my final example …

Ruby invokes sh -c (which is dash on my system - not bash).

$ sh -c “ps aux | grep ‘grep’”
dave 18746 0.0 0.0 4444 640 pts/2 S+ 21:12 0:00 sh -c
ps aux | grep ‘grep’
dave 18748 0.0 0.0 11748 888 pts/2 S+ 21:12 0:00 grep
grep

$ sh -c “grep ‘grep’ <(ps aux)”
sh: 1: Syntax error: “(” unexpected

Guess I should link “sh” to bash, or something.

Thanks.

daz

3

It surprised me that ruby already knows what shell I want …

$ ruby -e “p ENV[‘SHELL’]”
“/bin/bash”

… I was hoping I could just change that in my script because globally
changing “sh” to use bash would slow down the system (that’s why Ubuntu
changed the default).

daz

4

Dave Butcher wrote in post #1167185:

$ grep ‘grep’ <(ps aux)
dave 17227 0.0 0.0 11744 652 pts/2 S+ 20:29 0:00 grep
[…]

You can do

processes=%x{pgrep grep}.scan(/\d+/)

unless processes.empty?
$cmd=processes.join ’ ’
result=%x{ps aux #$cmd}
end

But do you want to accomplish?

Kind regards

robert

5

On my system, that would be %x{ps aux -p #$cmd} following pgrep.

But I just found %x{ps -C procxxx} which gives the right result if the
process name is known.

In reality, I might need to grep for (e.g.) “/usr/bin/procxxx args” in
the startup command.

Thanks, anyway. It was the syntax error I was puzzling about.

daz