UNIX for Dummies Questions & Answers

6281271467892|2015-01-07
5555571618533|2015-01-01
5555571618533|2015-01-02
5555571800645|2015-01-12
5555567476389|2015-01-04
5555567972749|2015-01-12
5555567972749|2015-01-01
5555583587323|2015-01-15
5555583587323|2015-01-15

5555571618533|2015-01-02
5555571800645|2015-01-12
5555567476389|2015-01-04
5555567972749|2015-01-12
5555583587323|2015-01-15

awk -F'|' '{print $1"|"$2}' input.txt | sort -k2,2nr | awk -F'|' '!a[$1]++'

$ awk -F"|" ' ! arr[$1]++ ' OFS="|" file
6281271467892|2015-01-07
5555571618533|2015-01-01
5555571800645|2015-01-12
5555567476389|2015-01-04
5555567972749|2015-01-12
5555583587323|2015-01-15

$ sort -t'|' -kn1 -k2r file | awk -F"|" ' ! arr[$1]++ ' OFS="|"
5555567476389|2015-01-04
5555567972749|2015-01-12
5555571618533|2015-01-02
5555571800645|2015-01-12
5555583587323|2015-01-15
6281271467892|2015-01-07

 { action }

 awk -F"|" ' ! arr[$1]++ ' OFS="|" file

awk -F"|" '                     ######## Making field seprator as |
!arr[$1]++                      ######## creating an array named arr whose index is $1's value so ! arr[$1] means if any occurrence of $1 which is NOT present in 
                                        array arr then do it will make entry of that particular item into array an make it's value as one so next time it can't be taken 
                                        as we have put condition as ! arr[$1] 
                                        so if this condition is TRUE means when an entry is being added for $1 then no action is mentioned so it will print that line.
' OFS="|" file                  ######## mentioning Output field separator as | , reading Input_file as named file too