Rename a group of files with one command

If I have a group of files with a .htm extention, how can I rename them all to .html?

mv *.htm *.html

does not work.

Or, you could use pure bash... (except for mv, that is..)

for file in *.htm; do mv "$file" "${file%.htm}.html"; done

and avoid the nasty basename stuff. ;)

Bash has an extensive set of variable expansion options. The one used here, '%', removes the smallest matching suffix from the value of the variable. The pattern is a glob pattern, so ${file%.*} would also work. The '%%' operator removes the largest matching suffix, and is interchangeable in the example above, as the pattern is fixed, ${file%%.*}.html would turn a.b.htm into a.html though.

See the variable substition section of the bash manpage for more neat tricks. There's a lot that can be done within bash directly.

There shouldn't be spaces, newlines or other whitespace in the filenames, but this version of freiheit's answer handles those. It also uses $() instead of backticks for readability along with other benefits.

for file in *.htm
do
    mv "$file" "$(basename "$file" .htm).html"
done

Even better - for the special case of just adding on to the end:

for file in *.htm
do
    mv "$file" "${file}l"
done