How to send a command with arguments without spaces?

If only there was a variable whose value is a space… Or more generally, contains a space.

cat${IFS}file.txt

The default value of IFS is space, tab, newline. All of these characters are whitespace. If you need a single space, you can use ${IFS%??}.

More precisely, the reason this works has to do with how word splitting works. Critically, it's applied after substituting the value of variables. And word splitting treats each character in the value of IFS as a separator, so by construction, as long as IFS is set to a non-empty value, ${IFS} separates words. If IFS is more than one character long, each character is a word separator. Consecutive separator characters that are whitespace are treated as a single separator, so the result of the expansion of cat${IFS}file.txt is two words: cat and file.txt. Non-whitespace separators are treated separately, with something like IFS=',.'; cat${IFS}file.txt, cat would receive two arguments: an empty argument and file.txt.

I found a way assuming a shell that supports csh-like brace expansion like ksh, bash or yash -o brace-expand (zsh supports brace expansion, but not as the first argument like that as that conflicts with command grouping):

{cat,file.txt}

with this way you don't have to use whitespaces in your argument.

One alternative is to use the value of IFS with the expansion of a variable:

$ echo Hello! > file.txt

$ IFS=:
$ a=cat:file.txt
$ $a
Hello!