Diff installed packages between nix generations

nix-env-diff.bash

#!/usr/bin/env bash
# From https://github.com/pinealan/dotfiles

nix-list() {
    nix-env --list-generations
}

get_current_gen() {
    if [ -z $1 ] ; then
        # default to current generation of nix-env
        nix-list | grep current | awk '{print $1}'
    else
        # get n-th last generation
        nix-list | grep -B $1 current | head -n 1 | awk '{print $1}'
    fi
}

verify_gen_exist() {
    if ! nix-list | awk '{print $1}' | grep $1 > /dev/null ; then
        echo "Error: generation $1 not found." >&2
        exit 1
    fi
}

# This function needs to be run one at a time.
# It make changes to filesystem (i.e. touches /nix/var/<user>/profile symlink),
# so running it multiple times in the same command could lead concurrent
# invocation that fails due to a lock on the file
echo_nix_gen_pkgs() {
    local original_gen=$(get_current_gen)
    nix-env -G $1 2> /dev/null
    nix-env -q
    nix-env -G $original_gen 2> /dev/null
}

main() {
    # Select diff command
    if hash icdiff 2>/dev/null ; then
        icdiff -L $from_gen -L $to_gen $1 $2
    elif ! hash diff 2>/dev/null ; then
        echo "Error: diff or icdiff not installed." >&2
        exit 1
    elif hash grc 2>/dev/null ; then
        grc diff $1 $2 || true
    else
        diff $1 $2 || true
    fi
}

# Select diff-ed generations
if [ -z $1 ] ; then
    from_gen=$(get_current_gen 1)
else
    verify_gen_exist $1
    from_gen=$1
fi

if [ -z $2 ] ; then
    to_gen=$(get_current_gen)
else
    verify_gen_exist $2
    to_gen=$2
fi


# see function comments on separating these invocations
old=$( echo_nix_gen_pkgs $from_gen )
new=$( echo_nix_gen_pkgs $to_gen )

# main
main <(echo $old | tr ' ' '\n') <(echo $new | tr ' ' '\n')

When icdiff is installed:

When invoked with no arguments, diff is between current and previous generation.

To diff between current and specific generations

nix-env-diff <from_gen>

Diff between 2 specific generations

nix-env-diff <from_gen> <to_gen>