Wildman via Python-list <python-list@python.org> writes: > On Mon, 31 Oct 2016 15:44:13 +1100, Ben Finney wrote: > > > One immediate difference I see is that you specify different > > arguments to ‘grep’. You have a different pattern for each command. > > > > * The ‘^user\:’ pattern matches “user\:” at the start of a line. > > > > * The ‘^$USER\:’ pattern I think won't match anything, since “$” matches > > end-of-line and then you expect further characters *past* the end of > > the line. I think that will always fail to match any line. > > Yes, the '^' indicates the start of the line and the ':' indicates > the character where to stop. The colon has a special meaning so it > has to be escaped, '\:'. The dollar sign precedes a variable. In > this case it is an environment variable.
The ‘grep’ program you're invoking knows nothing of such variables, and the ‘$’ sign means to ‘grep’ what I said above. > > Maybe you are expecting Bash to be involved somehow (and so “$USER” > > will be substituted by Bash with some other value). That's not what > > happens. > > No, the shell is already running. I don't know what you mean by this. If you mean that some *other* instances of the shell ar running: that isn't relevant to how your Python program invokes a subprocess. The shell is not involved in the command as you invoke it directly as a subprocess, without asking for a shell. > And $USER will be substituted by the name of the user that invoked the > shell. It will not, because there is no shell involved: your Python program invokes ‘sudo’, which invokes ‘grep’. The shell is never involved in that chain, so its substitutions rules are irrelevant. -- \ “Try adding “as long as you don't breach the terms of service – | `\ according to our sole judgement” to the end of any cloud | _o__) computing pitch.” —Simon Phipps, 2010-12-11 | Ben Finney -- https://mail.python.org/mailman/listinfo/python-list
