tramp-encoding-shell

tramp-encoding-shell is a variable defined in `tramp.el'.
Its value is

"/bin/sh"

Documentation:
Use this program for encoding and decoding commands on the local host.
This shell is used to execute the encoding and decoding command on the
local host, so if you want to use `~' in those commands, you should
choose a shell here which groks tilde expansion. `/bin/sh' normally
does not understand tilde expansion.

For encoding and decoding, commands like the following are executed:

/bin/sh -c COMMAND < INPUT > OUTPUT

This variable can be used to change the "/bin/sh" part. See the
variable `tramp-encoding-command-switch' for the "-c" part.

If the shell must be forced to be interactive, see
`tramp-encoding-command-interactive'.

Note that this variable is not used for remote commands. There are
mechanisms in tramp.el which automatically determine the right shell to
use for the remote host.

You can customize this variable.