How to use variables in sed command

Two things play into this:

1. You have to escape $ (i.e., use \$) in doubly-quoted shell strings if you want a literal $, and
2. \ does not retain its literal meaning when it comes before a $ inside backticks (that is to say, inside backticks, \$ becomes just $).

When you write

`sed -i "s/OU=$OU1/$$$\${OU1}/g" text_file1.txt`

because the command is in backticks, you spawn a subshell with the command

sed -i "s/OU=$OU1/$$$${OU1}/g" text_file1.txt

Since $$$$ is inside a doubly-quoted string, variable expansion takes place, and it is expanded as two occurrences of $$ (the process ID of the shell that's doing the expansion). This means that the code sed sees is ultimately

s/OU=QA/1234512345{OU1}/g

...if the process ID of the spawned subshell is 12345.

In this particular case, you don't need the command substitution (the backticks), so you could write

sed -i "s/OU=$OU1/\$\$\$${OU1}/g" text_file1.txt

However, using shell variables in sed code is always a problem. Consider, if you will, what would happen if OU1 had the value /; e rm -Rf * # (hint: GNU sed has an e instruction that runs shell commands). For this reason, I would always prefer awk to do substitutions that involve shell variables:

cp text_file1.txt text_file1.txt~
awk -v OU1="$OU1" '{ gsub("OU=" OU1, "$$$" OU1) } 1' text_file1.txt~ > text_file1.txt

This avoids code injection problems by not treating OU1 as code.

If you have GNU awk 4.1 or later,

awk -v OU1="$OU1" -i inplace  '{ gsub("OU=" OU1, "$$$" OU1) } 1' text_file1.txt

can do the whole thing without a (visible) temporary file.

You must be careful when putting $ inside double quotes.

sed -i "s/OU=$OU1/"'$$$'"${OU1}/g" text_file1.txt

Example:

$ OU1="QA"
$ echo 'OU=QA' | sed "s/OU=$OU1/"'$$$'"${OU1}/g"
$$$QA