We can determine existence of an program with "which", it gives full path to program queried, or nothing is program not exist, also it gives exit status 0 when program found, and 1 if not found, so comparing exit status is propably what we want to use to determine existence of a program.

Example output:

$ which dig
/usr/bin/dig

$ printf '%d\n' $?
0

$ which ifconfig

$ printf '%d\n' $?
1

So I wrote litte test.sh to compare and tell if a program exist:

#!/bin/bash

DIG=$(which dig)
[ ${PIPESTATUS[0]} -eq 0 ] && printf "DIG exist, path: $DIG\n" || printf "DIG unavailable\n"

IFCONFIG=$(which ifconfig)
[ ${PIPESTATUS[0]} -eq 0 ] && printf "IFCONFIG exist, path: $IFCONFIG\n" || printf "IFCONFIG unavailable\n"

Which outputs correctly on mine machine:

$ ./test.sh 
DIG exist, path: /usr/bin/dig
IFCONFIG unavailable

from synth-shell.

Snippets to get an IP-number (and only the number) as result, both IPv4 and IPv6 variants:

dig:

IPv4:

dig TXT -4 +short o-o.myaddr.l.google.com @ns1.google.com | awk -F'"' '{print $2}'

IPv6:

dig TXT -6 +short o-o.myaddr.l.google.com @ns1.google.com | awk -F'"' '{print $2}'

nslookup:

IPv4:

nslookup -q=txt o-o.myaddr.l.google.com 216.239.32.10 | awk -F \" 'BEGIN{RS="\r\n"}{print $2}END{RS="\r\n"}'

IPv6:

nslookup -q=txt o-o.myaddr.l.google.com 2001:4860:4802:32::a | awk -F \" 'BEGIN{RS="\r\n"}{print $2}END{RS="\r\n"}'

curl:

IPv4:

curl -s https://api.ipify.org

IPv6:

curl -s https://api6.ipify.org

wget:

IPv4:

wget -q -O - https://api.ipify.org

IPv6:

wget -q -O - https://api6.ipify.org

from synth-shell.

Implemented this in 40f9261

It check available programs with whichand then uses first found program and only tries next program if result is empty. Optimally we'd use exit result but so far we rely on result having anything in it and supressing any runtime errors from saving to result.

from synth-shell.