LinuxQuestions.org - display one line from multiple files

I started with nine files.

Code:

test$ ls

file_1  file_2  file_3  file_4  file_5  file_6  file_7  file_8  file_9

test$

Each file is formatted as shown.

Code:

test$ cat file_1

file_1 line_1

file_1 line_2

file_1 line_3

file_1 line_4

file_1 line_5

file_1 line_6

file_1 line_7

file_1 line_8

file_1 line_9

file_1 line_10

file_1 line_11

file_1 line_12

file_1 line_13

file_1 line_14

file_1 line_15

file_1 line_16

file_1 line_17

file_1 line_18

file_1 line_19

file_1 line_20

(... continues ...)

file_1 line_50

Then I tested my script to print line 20 from each file.

Code:

test$ awk 'FNR == 20' file_*

file_1 line_20

file_2 line_20

file_3 line_20

file_4 line_20

file_5 line_20

file_6 line_20

file_7 line_20

file_8 line_20

file_9 line_20

test$

You say you're working with 500 or more files, so my pattern file_* would expand into quite a long list. I can imagine such a long command line having negative consequences. With that in mind, it may be wise to execute the script on one file at a time. This is easily automated with the find command.

Code:

test$ find -maxdepth 1 -name 'file_*' -exec awk 'NR == 20 {print FILENAME ":" $0}' {} \;

./file_9:file_9 line_20

./file_5:file_5 line_20

./file_4:file_4 line_20

./file_1:file_1 line_20

./file_2:file_2 line_20

./file_6:file_6 line_20

./file_7:file_7 line_20

./file_8:file_8 line_20

./file_3:file_3 line_20

test$

If you would prefer the list be sorted by filename, then simply pipe the output of find into sort.

Code:

test$ find -maxdepth 1 -name 'file_*' -exec awk 'NR == 20 {print FILENAME ":" $0}' {} \; | sort

./file_1:file_1 line_20

./file_2:file_2 line_20

./file_3:file_3 line_20

./file_4:file_4 line_20

./file_5:file_5 line_20

./file_6:file_6 line_20

./file_7:file_7 line_20

./file_8:file_8 line_20

./file_9:file_9 line_20

test$

If you also want to print line 20 of files from subdirectories then specify how many directories deep in the -maxdepth clause. If you want to recurse infinitely deep into all subdirectories then the -maxdepth clause may be omitted.

Code:

test$ find -name 'file_*' -exec awk 'NR == 20 {print FILENAME ":" $0}' {} \; | sort

./file_1:file_1 line_20

./file_2:file_2 line_20

./file_3:file_3 line_20

./file_4:file_4 line_20

./file_5:file_5 line_20

./file_6:file_6 line_20

./file_7:file_7 line_20

./file_8:file_8 line_20

./file_9:file_9 line_20

./subdir/file_10:file_10 line_20

test$

References

HTH :)

EDIT
colucix made a good point about accelerating awk by using nextfile to ignore all lines after line 20. If I incorporate that into my last example I get this.

Code:

test$ find -name 'file_*' -exec awk 'NR == 20 {print FILENAME ":" $0; nextfile}' {} \; | sort

./file_1:file_1 line_20

./file_2:file_2 line_20

./file_3:file_3 line_20

./file_4:file_4 line_20

./file_5:file_5 line_20

./file_6:file_6 line_20

./file_7:file_7 line_20

./file_8:file_8 line_20

./file_9:file_9 line_20

./subdir/file_10:file_10 line_20

test$