В awk:
[~]$ A="aaa"
[~]$ echo '' | awk -v a=${A} '{print a}'
aaa
[~]$ echo '' | awk -v a=${A} '{print a}'
aaa
В sed:
[~]$ A="old"
[~]$ B="oldstring"
[~]$ echo $B | sed 's/'$A'/new/g'
newstring
[~]$ B="oldstring"
[~]$ echo $B | sed 's/'$A'/new/g'
newstring