Replace the first line in a text file by a string

sed is the right tool, try doing :

var="movie.MOV"
sed -i "1s/.*/$var/" file.txt

explanations

• 1 mean first line
• the rest is the substitution s/// : we substitute everything (.*) by the $var variable
• the double shell quotation is mandatory here

Learn how to quote properly in shell, it’s very important :

“Double quote” every literal that contains spaces/metacharacters and every expansion: "$var", "$(command "$var")", "${array[@]}", "a & b". Use 'single quotes' for code or literal $'s: 'Costs $5 US', ssh host 'echo "$HOSTNAME"'. See
http://mywiki.wooledge.org/Quotes
http://mywiki.wooledge.org/Arguments
http://wiki.bash-hackers.org/syntax/words