Extract lines that appear twice

Shell Programming and Scripting
1

I have a text file that looks like this :

root/user/usr1/0001/abab1*
root/user/usr1/0001/abab2*
root/user/usr1/0002/acac1*
root/user/usr1/0002/acac2*
root/user/usr1/0003/adad1*
root/user/usr1/0004/aeae1*
root/user/usr1/0004/aeae2*

How could I code this to extract just the subjects that appear twice? I originally thought each subject would appear twice, so I just wanted to use the awk 'NR % 2 == 0' command, but that is no longer the case and now I don't know where to start. Help is much appreciated!

2

Which field is subject?

3

The 000* series.

4 
awk -F'/' ' ++arr[$4] == 2 { print $4 } ' file
5

I tried and it didn't seem to work... It just says Unmatched ' ?

6

You might have missed a quote

$ awk -F'/' ' ++arr[$4] == 2 { print $4 } ' file
0001
0002
0004
7

Ok I realized I forgot to type an extra ' after F (dumb), but now it returns just usr1 printed once?

8

What did you try? Are you trying to get subjects or users?

9

Do the lines in your testfile actually begin with another slash? Because then $4 would be the /-delimited column with the usr1 and you would need /-delimited column 5.

On another note: do you need anything that appears at least 2 times, or exactly 2 times? Because the example code posted does the first.

10

See if this awk script works for you...

awk -F/ '{x[$4] = (x[$4] ? x[$4] RS $0 : $0); y[$4]++} END {for(i in y) if(y==2) print x}' file