Re: [lmi] Shell scripting bafflement

[Vadim Zeitlin]

On Wed, 26 Feb 2020 22:27:11 +0000 Greg Chicares <address@hidden> wrote:

GC> Vadim--Sorry to bother you with this, but I just can't see why
GC> these two nearly-identical commands behave oppositely, and I'm
GC> sure you'll know why.

 I think I do, but I feel like I'm missing something crucial here: why do
you say that these commands are nearly-identical when it looks to me that
they're not identical at all?

GC> This does exactly what I expect, as [non]gnu.org is certainly
GC> reachable on my machine:
GC> 
GC> /home/greg[0]$if curl https://git.savannah.nongnu.org:443 >/dev/null 2>&1 ; 
then      
GC>   echo true
GC> else
GC>   echo false
GC> fi
GC> 
GC> true

 This checks the exit code of curl command, which is 0 in this case, and so
is considered to be "true".

GC> But if I enclose the command in "[ $(...) ]", it "fails":

 This tests... nothing. $(...) expands to nothing because there is no
output of curl, as you had taken care to suppress it, and so this is
equivalent to "[ ]". To be honest, I didn't know what does testing nothing
does, but it turns out that it always returns false. It can be seen by
doing

        $ [ ] || echo false
        false

or

        $ test || echo false
        false

GC> /home/greg[0]$
GC> /home/greg[0]$if [ $(curl https://git.savannah.nongnu.org:443 >/dev/null 
2>&1) ]; then
GC>   echo true
GC> else
GC>   echo false
GC> fi
GC> 
GC> false

 So this doesn't seem surprising to me at all. But I can't figure out what
was your intention here, i.e. _why_ do you use the command expansion shell
construct and what exactly do you expect to obtain from it?

 Please let me know if I'm missing something obvious here,
VZ

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