How to extract date from filename with extenstion using shell script - shell

Question

I tried to extract date from filenames for first two rows only with extension .log
ex: filenames are as follows
my_logFile.txt contains
abc20140916_1.log
abhgg20140914_1.log
abf20140910_1.log
log.abc_abc20140909_1
The code I tried:
awk '{print substr($1,length($1)-3,4)}' my_logFile.txt
But getting op as:
.log
.log
.log
Need op as:
20140916
20140914
*****revised query*
I have a txt file containing n number of log files. Each line in txt file is like this.
-rw-rw-rw- 1 abchost abchost 241315175 Apr 16 10:45 abc20140405_1.log
-rw-rw-rw- 1 abchost abchost 241315175 Apr 16 10:45 aghtff20140404_1.log
-rw-rw-rw- 1 abchost abchost 241315175 Apr 16 10:45 log.pqrs20140403_1
I need to extract date out of file names from only first two rows. Here the filename has varying number of char before date.
The op should beL
20140405
20140404

Answer 1

Will this work to you?
$ head -2 file | grep -Po ' [a-z]+\K[0-9]+(?=.*\.log$)'
20140405
20140404
Explanation
head -2 file gets the first two lines of the file.
grep -Po ' [a-z]+\K[0-9]+(?=.*\.log$)' gets the set of digits in between a block of (space + a-z letters) and (.log + end of line).

Answer 2

try this,
cut -f9 -d " " <file> | grep -o -E "[0-9]{8}"
worked on my machine,
[root#giam20 ~]# cat sample.txt
-rw-rw-rw- 1 abchost abchost 241315175 Apr 16 10:45 abc20140405_1.log
-rw-rw-rw- 1 abchost abchost 241315175 Apr 16 10:45 aghtff20140404_1.log
-rw-rw-rw- 1 abchost abchost 241315175 Apr 16 10:45 log.pqrs20140403_1
[root#giam20 ~]# cut -f9 -d " " sample.txt | grep -o -E "[0-9]{8}"
20140405
20140404
20140403