Base64 decode purely via Bash builtins.

d64

#!/bin/sh

((V=N=0))
while :; do
    ((V<<=6,++N))
    IFS= read -n1 C && {
        printf -vC '%d' "'$C"
        ((C=C>64&&C<91?C-65:C>96&&C<123?C-71:C>47&&C<58?C+4:C==43?62:C==47?63:(V>>=6,--N,0),V|=C))
    }
    ((N==4)) && {
        for((N-=2;N>=0;--N)); do
            printf `printf '\\\\x%02X' $((V>>N*8&255))`
        done
        [ -z "$C" ] && break
        ((V=N=0))
    }
done

# one-liner:
#  d64(){ local N=0 V=0 C;while :;do((V<<=6,++N));IFS= read -n1 C&&{ printf -vC '%d' "'$C";((C=C>64&&C<91?C-65:C>96&&C<123?C-71:C>47&&C<58?C+4:C==43?62:C==47?63:(V>>=6,--N,0),V|=C));};((N==4))&&{ for((N-=2;N>=0;--N));do printf `printf '\\\\x%02X' $((V>>N*8&255))`;done;[ -z "$C" ]&&break;((V=N=0));};done;}
#
# tests:
#  echo 'aGVsbG8hCg==' | d64
#  echo 'aGVsbG8hIQo=' | d64
#  echo 'aGVsbG8hISEK' | d64
#  echo 'aGVsbG8hISEhCg==' | d64
#  echo 'aGVs  bG8h  Cg===' | d64
#  echo 'aGV sbG8 hCg' | d64
#  echo "YWE\@+\nYmI/:\[\{\!Cg" | d64