How can I substitute one command for another temporarily?

Question

I'd like to alias sed to gsed, but only while a particular script is running.

I have a script that invokes sed. On the Mac, BSD sed doesn't support the required features, but I don't want to replace it globally with the GNU version.

I can't set an alias in the invoker, as aliases aren't propagated, and I can't set it in a login script as then it will apply globally. The script can't be edited.

I'm posting my current solution, using a nasty symlink/path hack, as an answer, but I'm hoping that someone will come up with a better solution.

Answer 1

#!/bin/sh
# Runs a command with gsed substituting for sed

set -e

mkdir -p /tmp/temp-path
ln -fs `which gsed` /tmp/temp-path/sed
export PATH=/tmp/temp-path/:$PATH
eval "$@"

Answer 2

One alternative is to install the GNU versions of utilities in a specific directory, such as /usr/local or /usr/local/gnu

Then when you wish to use that specific app in a script you would refer to it explicitly as /usr/local/gnu/bin/sed

Another option would be to assign a SED variable while is exportable.

SED=`which gsed`
export SED

Thereafter call sed via ${SED}