How to rename a file with a known name to a new name which is stored in a variable?

Question

[UPDATE-1]: If possible, give me cons for motivation to learn the simplest commands.

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So, I have one file in directory:

ls | grep *.mp4 

video.mp4

I use this command to randomly rename & move file in another directory:

for a in *.mp4; do mv -n "$a" "${a/*/$RANDOM.mp4}"; done;  mv *.mp4 /home/misdeed/Documents

Is ok, I get randomly name for video.mp4

cd Documents; ls | grep *.mp4

19889.mp4

Qestion:

How to use is:

a="This is name file"
echo $a

This is name file

for rename *.mp4 (not randomly) and for get

This is name file.mp4

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I need something like it:

• a="This is name file"
• COMMAND FOR RENAME BUT WITHOUT $RANDOM. I WANT USE $a in renamed file

Answer 1

Let's say the original name is known in advance and it's video.mp4. You rename it with

mv video.mp4 "$a".mp4

Notes:

• If the variable is empty (or unset), the final name will be .mp4. Names starting with dot (.) are "hidden", plain ls skips them, you need ls -a to see them.
• If there is already a file with the target name, it will be lost and the former video.mp4 file will take its name. To prevent mishaps consider mv -i; it will prompt for confirmation if the destination path exists.

Answer 2

I assume you want the new random file name printed? In that case, the easiest is to reuse the value of your random variable.

for file in *.mp4; do
  RANDOM_NAME="${file/*/$RANDOM.mp4}";
  mv -n "$file" /home/misdeed/Documents/"$RANDOM_NAME";
  echo "This is name $RANDOM_NAME.mp4";
done