argument to cp in backticks doesn't work?

Question

I tried using a command like

cp `ls ~/temp/*.xyz | head -1` ./

But that does not work. If I echo the value of command inside back ticks and put it manually in cp command it works. Any ideas?

Answer 1

The text produced by the backticks (or the alternate syntax $(…)), like text resulting from substituting a variable ($foo), is expanded by the shell: it is broken into words, and the words are interpreted as glob patterns (i.e. \[?* operate to match files). To avoid this issue, always use double quotes around variable and command substitutions.

There's a second issue: parsing the output of ls is a bad idea for various reasons.

If you use zsh as your shell, an easy way to do what you're attempting is

cp ~/temp/*.xyz([1]) ./

On other shells, this is harder. On the command line, you might risk using ls, if you know your file doesn't contain any nonprintable character (for ls's definition of nonprintable). In a script, this is calling for disaster; a simple two-liner in ksh or bash is

tmp=(~/temp/*.xyz)
cp -- "${a[0]}" ./

On other shells, you can do it this way (note that this overwrites the positional parameters):

set -- ~/temp/*.xyz
cp -- "$1" ./

Answer 2

Maybe the matched file contains special characters (e.g. spaces) so you need to quote the back ticks expression:

cp "`ls ~/temp/*.xyz | head -1`" .