How to validate a IPv6 address format with shell?

The code in the first link isn't particularly elegant, but modulo stylistic fixes, I don't think you can simplify much beyond that (and as indicated in a comment, it may already be too simple). The spec is complex and mandates a number of optional features, which is nice for the end user, but cumbersome for the implementor.

You could probably find a library for a common scripting language which properly encapsulates this logic in a library. My thoughts would go to Python, where indeed Python 3.3 includes a standard module called ipaddress; for older versions, try something like

#!/usr/bin/env pythonimport socketimport systry:    socket.inet_pton(socket.AF_INET6, sys.argv[1])    result=0except socket.error:    result=1sys.exit(result)

See also Checking for IP addresses

Here is a solution in POSIX compatible shell script that handles IPv4 and IPv6 addresses with an optional subnet mask. To test an IP that should not have a subnet mask just pass it a dummy one when performing the test. It seems like a lot of code but it should be significantly faster than using external programs like grep or scripts that are likely to fork.

Single IPv6 zero groups compressed to :: will be treated as invalid. The use of such representation is strongly discouraged, but technically correct. There is a note in the code explaining how to alter this behaivour if you wish to allow such addresses.

#!/bin/shset -e# return nonzero unless $1 contains only digits, leading zeroes not allowedis_numeric() {    case "$1" in        "" | *[![:digit:]]* | 0[[:digit:]]* ) return 1;;    esac}# return nonzero unless $1 contains only hexadecimal digitsis_hex() {    case "$1" in        "" | *[![:xdigit:]]* ) return 1;;    esac}# return nonzero unless $1 is a valid IPv4 address with optional trailing subnet mask in the format /<bits>is_ip4() {    # fail if $1 is not set, move it into a variable so we can mangle it     [ -n "$1" ] || return    IP4_ADDR="$1"    # handle subnet mask for any address containing a /    case "$IP4_ADDR" in        *"/"* ) # set $IP4_GROUP to the number of bits (the characters after the last /)                IP4_GROUP="${IP4_ADDR##*"/"}"                # return failure unless $IP4_GROUP is a positive integer less than or equal to 32                is_numeric "$IP4_GROUP" && [ "$IP4_GROUP" -le 32 ] || return                # remove the subnet mask from the address                IP4_ADDR="${IP4_ADDR%"/$IP4_GROUP"}";;    esac    # backup current $IFS, set $IFS to . as that's what separates digit groups (octets)    IP4_IFS="$IFS"; IFS="."    # initialize count    IP4_COUNT=0    # loop over digit groups    for IP4_GROUP in $IP4_ADDR ;do          # return failure if group is not numeric or if it is greater than 255        ! is_numeric "$IP4_GROUP" || [ "$IP4_GROUP" -gt 255 ] && IFS="$IP4_IFS" && return 1        # increment count        IP4_COUNT=$(( IP4_COUNT + 1 ))        # the following line will prevent the loop continuing to run for invalid addresses with many occurrences of .        # this makes no difference to the result, but may improve performance when validating many such invalid strings        [ "$IP4_COUNT" -le 4 ] || break    done    # restore $IFS    IFS="$IP4_IFS"    # return success if there are 4 digit groups, otherwise return failure    [ "$IP4_COUNT" -eq 4 ]}# return nonzero unless $1 is a valid IPv6 address with optional trailing subnet mask in the format /<bits>is_ip6() {    # fail if $1 is not set, move it into a variable so we can mangle it     [ -n "$1" ] || return    IP6_ADDR="$1"    # handle subnet mask for any address containing a /    case "$IP6_ADDR" in        *"/"* ) # set $IP6_GROUP to the number of bits (the characters after the last /)                IP6_GROUP="${IP6_ADDR##*"/"}"                # return failure unless $IP6_GROUP is a positive integer less than or equal to 128                is_numeric "$IP6_GROUP" && [ "$IP6_GROUP" -le 128 ] || return                # remove the subnet mask from the address                IP6_ADDR="${IP6_ADDR%"/$IP6_GROUP"}";;    esac    # perform some preliminary tests and check for the presence of ::    case "$IP6_ADDR" in        # failure cases        # *"::"*"::"*  matches multiple occurrences of ::        # *":::"*      matches three or more consecutive occurrences of :        # *[^:]":"     matches trailing single :        # *"."*":"*    matches : after .        *"::"*"::"* | *":::"* | *[^:]":" | *"."*":"* ) return 1;;        *"::"* ) # set flag $IP6_EXPANDED to true, to allow for a variable number of digit groups                 IP6_EXPANDED=0                 # because :: should not be used for remove a single zero group we start the group count at 1 when :: exists                 # NOTE This is a strict interpretation of the standard, applications should not generate such IP addresses but (I think)                 #      they are in fact technically valid. To allow addresses with single zero groups replaced by :: set $IP6_COUNT to                  #      zero after this case statement instead                 IP6_COUNT=1;;         *      ) # set flag $IP6_EXPANDED to false, to forbid a variable number of digit groups                 IP6_EXPANDED=""                 # initialize count                 IP6_COUNT=0;;    esac    # backup current $IFS, set $IFS to : to delimit digit groups    IP6_IFS="$IFS"; IFS=":"    # loop over digit groups    for IP6_GROUP in $IP6_ADDR ;do        # if this is an empty group then increment count and process next group        [ -z "$IP6_GROUP" ] && IP6_COUNT=$(( IP6_COUNT + 1 )) && continue        # handle dotted quad notation groups        case "$IP6_GROUP" in            *"."* ) # return failure if group is not a valid IPv4 address                    # NOTE a subnet mask is added to the group to ensure we are matching addresses only, not ranges                    ! is_ip4 "$IP6_GROUP/1" && IFS="$IP6_IFS" && return 1                    # a dotted quad refers to 32 bits, the same as two 16 bit digit groups, so we increment the count by 2                    IP6_COUNT=$(( IP6_COUNT + 2 ))                    # we can stop processing groups now as we can be certain this is the last group, : after . was caught as a failure case earlier                    break;;        esac        # if there are more than 4 characters or any character is not a hex digit then return failure        [ "${#IP6_GROUP}" -gt 4 ] || ! is_hex "$IP6_GROUP" && IFS="$IP6_IFS" && return 1        # increment count        IP6_COUNT=$(( IP6_COUNT + 1 ))        # the following line will prevent the loop continuing to run for invalid addresses with many occurrences of a single :        # this makes no difference to the result, but may improve performance when validating many such invalid strings        [ "$IP6_COUNT" -le 8 ] || break    done    # restore $IFS    IFS="$IP6_IFS"    # if this address contained a :: and it has less than or equal to 8 groups then return success     [ "$IP6_EXPANDED" = "0" ] && [ "$IP6_COUNT" -le 8 ] && return    # if this address contained exactly 8 groups then return success, otherwise return failure    [ "$IP6_COUNT" -eq 8 ]}

Here are some tests.

# testsTEST_PASSES=0TEST_FAILURES=0for TEST_IP in 0.0.0.0 255.255.255.255 1.2.3.4/1 1.2.3.4/32 12.12.12.12 123.123.123.123 101.201.201.109 ;do    ! is_ip4 "$TEST_IP" && printf "IP4 test failed, test case '%s' returned invalid\n" "$TEST_IP" && TEST_FAILURES=$(( TEST_FAILURES + 1 )) || TEST_PASSES=$(( TEST_PASSES + 1 )) donefor TEST_IP in ::1 ::1/128 ::1/0 ::1234 ::bad ::12 1:2:3:4:5:6:7:8 1234:5678:90ab:cdef:1234:5678:90ab:cdef \               1234:5678:90ab:cdef:1234:5678:90ab:cdef/127 1234:5678:90ab::5678:90ab:cdef/64 f:1234:c:ba:240::1 \               1:2:3:4:5:6:1.2.3.4 ::1.2.3.4 ::1.2.3.4/0 ::ffff:1.2.3.4 ;do    ! is_ip6 "$TEST_IP" && printf "IP6 test failed, test case '%s' returned invalid\n" "$TEST_IP" && TEST_FAILURES=$(( TEST_FAILURES + 1 )) || TEST_PASSES=$(( TEST_PASSES + 1 )) donefor TEST_IP in junk . / 0 -1.0.0.0 1.2.c.0 a.0.0.0 " 1.2.3.4" "1.2.3.4 " " " 01.0.0.0 09.0.0.0 0.0.0.01 \               0.0.0.09 0.09.0.0.0 0.01.0.0 0.0.01.0 0.0.0.a 0.0.0 .0.0.0.0 256.0.0.0 0.0.0.256 "" 0 1 12 \               123 1.2.3.4/s 1.2.3.4/33 1.2.3.4/1/1 ;do    is_ip4 "$TEST_IP" && printf "IP4 test failed, test case '%s' returned valid\n" "$TEST_IP" && TEST_FAILURES=$(( TEST_FAILURES + 1 )) || TEST_PASSES=$(( TEST_PASSES + 1 )) donefor TEST_IP in junk "" : / :1 ::1/ ::1/1/1 :::1 ::1/129 ::12345 ::bog ::1234:345.234.0.0 ::sdf.d ::1g2 \               1:2:3:44444:5:6:7:8 1:2:3:4:5:6:7 1:2:3:4:5:6:7:8/1c1 1234:5678:90ab:cdef:1234:5678:90ab:cdef:1234/64 \               1234:5678:90ab:cdef:1234:5678::cdef/64  ::1.2.3.4:1 1.2.3.4:: ::1.2.3.4j ::1.2.3.4/ ::1.2.3.4:junk ::1.2.3.4.junk ;do    is_ip6 "$TEST_IP" && printf "IP6 test failed, test case '%s' returned valid\n" "$TEST_IP" && TEST_FAILURES=$(( TEST_FAILURES + 1 )) || TEST_PASSES=$(( TEST_PASSES + 1 )) doneprintf "test complete, %s passes and %s failures\n" "$TEST_PASSES" "$TEST_FAILURES"

Most distros come with package iproute2 (name may vary) preinstalled. So you can rely on the command ip for querying the routing table:

ip -6 route get <probe_addr>/128 >/dev/null 2>&1 

Even on a machine without appropriate route this delivers rc=0 when the probe is in valid v6-syntax.