getent

#!/bin/sh
# POSIX sh variant for getent.
#
# This is similar to NetBSD's C implementation for getent
# but without support for enumeration. There is simply no
# specification for getent. I have simply made functions
# for databases I have found to be common with the Glibc
# implementation and NetBSD implementation. So, enumeration
# is not supported. I am not sure why that is even a feature.
#
# Glibc implementation basically 'cat's a formatted output
# for everything if no arguments are given. NetBSD only does
# that for passwd and group. I will be sticking with NetBSD
# here.
#
#
# Copyright (c) 2020 Cem Keylan
#
# Permission is hereby granted, free of charge, to any
# person obtaining a copy of this software and associated
# documentation files (the "Software"), to deal in the
# Software without restriction, including without limitation
# the rights to use, copy, modify, merge, publish, distribute,
# sublicense, and/or sell copies of the Software, and to
# permit persons to whom the Software is furnished to do so,
# subject to the following conditions:
# 
# The above copyright notice and this permission notice
# shall be included in all copies or substantial portions
# of the Software.
# 
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY
# KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE
# WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
# PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS
# OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR
# OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
# OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
# SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

out() { printf '%s\n' "$@" ;}
usage() {
    out "usage: ${0##*/} [database] [key ...]" \
        "       database may be one of:" \
        "             ethers group hosts networks passwd"
    exit "${1:-0}"
}

# We will be using this for all databases, so that we can
# remove comments and empty lines. Removing lines make it
# faster to parse the lines.
rm_comment() { sed 's/#.*//g;/^$/d' "$@" ;}

hosts() {
    while read -r ip canon aliases; do
        case "$1" in "$ip"|"$canon")
            printf '%-16s %s %s\n' "$ip" "$canon" "$aliases"
            return 0
        esac
    done
}

passwd() {
    while read -r line; do
        # The specification only checks for user name and UID.
        case "$line" in "$1:"*|*:*:$1:*:*:*:*)
            printf '%s\n' "$line"; return 0
        esac
    done
}

group() {
    while read -r line; do
        # The specification only checks for group name and GID.
        case "$line" in "$1:"*|*:*:"$1":*)
            printf '%s\n' "$line"; return 0
        esac
    done
}

ethers() {
    while read -r mac host; do
        [ "$1" ] && case "$1" in "$mac"|"$host")
            printf '%-16s %s\n' "$mac" "$host"
            return 0
        esac
    done
}

networks() {
    while read -r alias ip; do
        case "$1" in "$alias"|"$ip")
            printf '%-16s %s\n' "$alias" "$ip"
            return 0
        esac
    done
}


[ "$1" ] || usage
database=$1; shift
case "$database" in
    passwd|group) [ "$1" ] || rm_comment "/etc/$database" ;;
    ethers|networks|hosts) ;;
    *) out "error: Unknown database '$database'"; usage 1 ;;
esac
for key do rm_comment "/etc/$database" | "$database" "$key"; done