Anno domini 2022 Thu, 13 Jan 15:43:29 +0100
Antony Stone scripsit:
> On Thursday 13 January 2022 at 15:07:22, Hendrik Boom wrote:
>
> > On Wed, Jan 12, 2022 at 05:45:08PM -0500, Steve Litt wrote:
>
> > > [slitt@mydesk ~]$ cat -n /etc/fstab | cut -b 1-20 | head -n5
> > >
> > > 1 UUID=730eaf92
> > > 2 UUID=41abb5fd
> > > 3 UUID=96cfdfb3
> > > 4 UUID=6F66-BF7
> > > 5 tmpfs /tmp tm
> > >
> > > [slitt@mydesk ~]$ "cat -n" /etc/fstab | cut -b 1-20 | head -n5
> > > bash: cat -n: command not found
> > >
> > > [slitt@mydesk ~]$ "cat -n /etc/fstab" | cut -b 1-20 | head -n5
> > > bash: cat -n /etc/fstab: No such file or directory
> >
> > So if it has parameters it's a command, and if it diesn't it's just
> > a file or directory?
>
> It looks a good deal more complicated than that...
>
> $ "cat /etc/fstab"
> bash: cat /etc/fstab: No such file or directory
>
> $ "cat fstab"
> bash: cat fstab: command not found
>
> I have no idea what's really going on here.
Your example misses a minor detail. "" and '' build strings with/without
variable substitution (e.g. A="cat /etc/fstab"). When passed as a not-quoted
variable (e.g. $A) to the current shell the whole string is broken up into
arguments at whitespaces (e.g. $A -> "cat" "/etc/fstab"), the first argument is
the command that gets passed all remaining arguments including pipe symbols
('|'). A quoted variable is passed as one argument ("$A" -> "car /etc/fstab") -
if it's the only argument then that programm/command/function is evaluated and
is most likely to fail. Note that the pipe symbol ("|") as part of a string is
passed as an argument or part of an argument to argument 0 (command) and does
not build a pipe.
Nik
>
>
> Antony.
>
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