how to convert from one date format to another format in linux ?

 how to convert from one date format to another format in linux ?

echo 04-11-2021 | { IFS=- read d m y && echo "$y$m$d"; }

20211104

echo 04-11-2021 | { IFS=- read d m y && echo "$y/$m/$d"; }

2021/11/04

 echo 04-11-2021 | { IFS=- read d m y && echo "$d$m$y"; }

04112021

 echo 04-11-2021 | awk -F- '{print $3$2$1}'

20211104

echo 04-11-2021 |gawk -F, '{split($1, a, "-"); print a[3] a[2] a[1]}' 

20211104

echo 04/11/2021 | { IFS=/ read d m y && echo "$y$m$d"; }

20211104

date -d "20211104" "+%Y-%m-%d"

2021-11-04

date -d 04/11/2021 +%F

2021-04-11

 %F   full date; same as %Y-%m-%d

how to check If the file Is Empty or not using shell script?

 how to check If the file Is Empty or not using shell script?

-s FILE

              FILE exists and has a size greater than zero

Example 1:

touch /$HOME/f1

echo "data" >/$HOME/f2

ls -l /$HOME/f{1,2}

[ -s /$HOME/f1 ] 

echo $?

Sample outputs:

1

The non-zero output indicates that the file is empty.

[ -s /$HOME/f2 ]

echo $?

Sample outputs:

0

$? is the return code of the last executed command.

Example 2:

$ ls

$ touch file_1

$ dd if=/dev/zero of=$HOME/file_2 bs=1 count=100

100+0 records in

100+0 records out

$ ls -l file_1 file_2 file_3

ls: file_3: No such file or directory

-rw-rw-r--  1 foo bar   0 Nov 20 20:28 file_1

-rw-rw-r--  1 foo bar 100 Nov 20 20:28 file_2

$ [[ -s file_1 ]]

$ echo $?

1

$ [[ -s file_2 ]]

$ echo $?

0

$ [[ -s file_3 ]]

$ echo $?

1