Replace a word with another set of strings in a UNIX file - sed

Question

When I try to replace a string using sed command it works perfectly fine.
For eg :
When i used the below sed command:
sed 's/DB_ALTER/DB_REPRISE/g' /product/dwhrec1/abc.ksh > /product/dwhrec1/abc1.ksh
This command works perfectly fine and replace all the "DB_ALTER" with "DB_REPRISE" and writes the result to abc1.ksh script.
But when I place all such values in a file. for eg:
cat Repla.txt
DB_ALTER
DB_CMD
DB_GEST_COMM
for i in `cat Repla.txt`
do
sed 's/$i/DB_REPRISE/g' /product/dwhrec1/abc.ksh > /product/dwhrec1/abc1.ksh
done
But this does not work. In my file Repla.txt is just an example. In actual it has many values.
Can anyone please help me on this command or suggest some alternative.
Thanks

Answer 1

There are two problems with your script. The first is that the $i variable appears within single quotes. That means that bash will not substitute for the value of i. It needs to be in double-quotes.
Secondly, every time that you run sed, it overwrites the previous abc1.ksh file. You should copy abc.ksh to abc1.ksh and then modify in place abc1.ksh as many times as needed:
cp abc.ksh abc1.ksh
for i in `cat Repla.txt`; do
sed -i'' "s/$i/DB_REPRISE/g" abc1.ksh
done
The -i flag to sed causes it to modify the file in place.
Also, bash will apply word splitting to cat Repla.txt. This can surprise people who were expecting it to work line-by-line, not word-by-word.
Workaround in case your sed does not support -i
The sed on both linux (GNU) and Mac OSX (BSD) support -i. If your sed does not, try:
cmd=
for i in `cat Repla.txt`; do
[ "$cmd" ] && cmd="$cmd;"
cmd="$cmd s/$i/DB_REPRISE/g"
done
sed "$cmd" abc.ksh >abc1.ksh
The above puts all the substitution commands that you need in a single shell variable. This way, sed only needs to be run once and -i is not used.
Another option
If it is acceptable to overwrite the source file, then:
for i in $(cat Repla.txt)
do
sed 's/'$i'/DB_REPRISE/g' abc.ksh >abc1.ksh
mv -f abc1.ksh abc.ksh
done
The above puts in single quotes all of the sed command except for the part that we want the shell to expand. This is not needed in this example but could be useful if your replacement text had shell-active characters. The above also uses the more modern $(...) in place of backquotes for command substitution.
If $i were to contain spaces (it doesn't here), we would need to enclose it in double-quotes to protect it against shell word splitting as in:
for i in $(cat Repla.txt)
do
sed 's/'"$i"'/DB_REPRISE/g' abc.ksh >abc1.ksh
mv -f abc1.ksh abc.ksh
done