How to change date format in linux/unix text file

I have sample.txt file in my linux system.all the data that is present in my file are in the following date format

            Mon Apr 24 16:14:06 PDT 2017
            Tue Apr 25 13:31:25 PDT 2017
            Mon Apr 03 13:53:03 PDT 2017
            Sun Feb 05 16:19:27 PST 2017
            Sun Feb 05 17:51:42 PST 2017
            Sun Feb 05 18:16:21 PST 2017
            Sun Feb 05 19:17:33 PST 2017
            Sun Feb 05 21:14:23 PST 2017
            Sat Feb 04 10:39:33 PST 2017
            Sat Feb 04 10:46:52 PST 2017
            Sat Feb 04 14:35:27 PST 2017

I want to convert all this date format in my sample.txt file to YYYY-MM-DD Format.

Answer

It’s quite simple with date -d, e.g.

while read -r line; do 
    echo $(date -d "$line" +%Y-%m-%d); 
done < file.txt

Input File

$ cat file.txt
            Mon Apr 24 16:14:06 PDT 2017
            Tue Apr 25 13:31:25 PDT 2017
            Mon Apr 03 13:53:03 PDT 2017
            Sun Feb 05 16:19:27 PST 2017
            Sun Feb 05 17:51:42 PST 2017
            Sun Feb 05 18:16:21 PST 2017
            Sun Feb 05 19:17:33 PST 2017
            Sun Feb 05 21:14:23 PST 2017
            Sat Feb 04 10:39:33 PST 2017
            Sat Feb 04 10:46:52 PST 2017
            Sat Feb 04 14:35:27 PST 2017

Example Use/Output

$ while read -r line; do echo $(date -d "$line" +%Y-%m-%d); done < file.txt
2017-04-24
2017-04-25
2017-04-03
2017-02-05
2017-02-05
2017-02-05
2017-02-05
2017-02-05
2017-02-04
2017-02-04
2017-02-04