Difference between ls -l , ls -ltr and ll?

First of all, the mentioned commands do not generate the same output. Here are the details:

  1. ll : There is no command like ll, in Ubuntu it is an alias for the command ls -alF. you can find it by:

    $ type ll
    ll is aliased to `ls -alF'
    
    • -a option is to show hidden files (will show . and .. too)

    • -l option is to show the output as a long list along with various attributes e.g. permissions, file sizes, use, group, modification time etc.

    • -F will append one of */=>@| to the entries, it is basically used to differentiate files from directories as it will append / to the directory entries

    Example :

    $ ll
    total 24
    drwxrwxr-x 4 chayan chayan 4096 Jun 25 13:53 ./
    drwxrwxr-x 3 chayan chayan 4096 Jun 25 13:34 ../
    drwxrwxr-x 2 chayan chayan 4096 Jun 25 13:35 data/
    -rw-rw-r-- 1 chayan chayan   88 Jun 25 13:50 filenames.txt
    
  2. ls -l : As mentioned earlier -l will show the entries as a long list along with various attributes.

    Example:

    $ ls -l
    total 16
    drwxrwxr-x 2 chayan chayan 4096 Jun 25 13:35 data
    -rw-rw-r-- 1 chayan chayan   88 Jun 25 13:50 filenames.txt
    
  3. ls -ltr :

    • -t option will sort the entries by modification date (with newest first)

    • -r will reverse the sorting order.

    As -t will sort by modification time with newest first, -r will cause the reverse i.e. oldest entries will be shown first now.

    Example (Adding a file and a directory to make it clearer) :

    $ ls -lt
    total 16
    drwxrwxr-x 2 chayan chayan 4096 Jun 25 13:50 log
    -rw-rw-r-- 1 chayan chayan   88 Jun 25 13:50 filenames.txt
    -rw-rw-r-- 1 chayan chayan  208 Jun 25 13:49 move.sh
    drwxrwxr-x 2 chayan chayan 4096 Jun 25 13:35 data
    
    $ ls -ltr
    total 16
    drwxrwxr-x 2 chayan chayan 4096 Jun 25 13:35 data
    -rw-rw-r-- 1 chayan chayan  208 Jun 25 13:49 move.sh
    -rw-rw-r-- 1 chayan chayan   88 Jun 25 13:50 filenames.txt
    drwxrwxr-x 2 chayan chayan 4096 Jun 25 13:50 log
    

Check man ls to get more idea.