Termux print to txt

What you want to do is called "output redirection". By assuming the program whose output we need to be redirected is youtube-dl, the full command to issue becomes

youtube-dl --help &> /sdcard/help.txt

In the above string, youtube-dl --help is the program whose output you want to save, alongside any needed argument. &> means "redirect the standard input and standard error"; it's important to redirect both, as some programs output their help message to stderr. /sdcard/help.txt is a file in the internal storage, where the output will be saved.

Take care when using this method, as the target file will be recreated anew if it already exists, thus its contents will be lost. To concatenate the new input to an already existent file, use e.g.

youtube-dl --help &>> /sdcard/help.txt