Creating an alias that is an incomplete command

Question

The documentation from tldp.org doesn't really specify when or how the bash interpreter replaces an aliased string. I'm looking for information on how to create an alias that is an incomplete command.

alias quickscp="scp a.really.long.internal.domain.name:"
quickscp/data/testfile ./testfile

This fails - looks like bash doesn't expand testalias unless it is matching a string with whitespace (^/$ included I assume) surrounding it.

It would be nice if aliases could assume some of the functionality of the C preprocessor, such as macro expansion,

http://tldp.org/LDP/abs/html/aliases.html agrees with me here - "it would be nice if" implies that it does not behave like a C preprocessor. Is there any way to force it? Something specific to the scp case above would solve my problem, but I'd be interested to know if there's a way to get the logic I'm trying to implement as well (for future hackery).

I guess this can be tl;dr'd to: How do I get bash to expand an alias if it isn't surrounded by whitespace?

Answer 1

It's in the definition of a Bash alias that it is a separate word (cf. the manual). I think what you want instead is a shell function:

quickscp () {
  if [[ "$#" -ne 2 ]]; then echo "Need two arguments"; return 1; fi
  scp host.example.com:"$1" "$2"
  return "$?"
}

Call it like:

quickscp source target

As a bonus, you can do parameter checking and you'll get the return code of the scp command, so you can do quickscp foo bar && echo "yay" or similar chaining of commands.

Answer 2

I don't think you can do this with an alias. How about a function?

unalias quickscp
quickscp () (
   first="$1"
   shift
   exec scp "a.really.long.internal.domain.name:$first" "$@"
)

Notes:

• unalias because the portable definition of a function can collide with already defined alias.
• The function body is deliberately placed in a subshell to make the variable named first local, without a non-portable local first statement.
• exec because we no longer need the subshell after scp starts.

---

A completely different approach is to define a host in your SSH config (global: /etc/ssh/ssh_config; user-specific: ~/.ssh/config):

Host foo
Hostname a.really.long.internal.domain.name

Then proceed like this:

scp foo:/data/testfile ./testfile

This will work with ssh or scp. It has nothing to do with aliases in a shell.

Answer 3

One trick I know of is using en empty string as a delimiter for expansion:

quickscp""/data/testfile ./testfile