How do I extract lines from a file using their line number on unix?

Question

Using sed or similar how would you extract lines from a file? If I wanted lines 1, 5, 1010, 20503 from a file, how would I get these 4 lines?

What if I have a fairly large number of lines I need to extract? If I had a file with 100 lines, each representing a line number that I wanted to extract from another file, how would I do that?

score 17 · Accepted Answer · answered Jan 06 '10 at 23:09

17

Something like "sed -n '1p;5p;1010p;20503p'. Execute the command "man sed" for details.

For your second question, I'd transform the input file into a bunch of sed(1) commands to print the lines I wanted.

answered Jan 06 '10 at 23:09

Steve Emmerson

7,702
5
33
59

1

+1, the thing to look up for the second part of the answer is `sed -f` – Michael Krelin - hacker Jan 06 '10 at 23:20
`sed -n '1p;5p;1010p;20503p inputFile.txt > outputFile.txt` – javaPlease42 Apr 15 '14 at 17:25

score 6 · Answer 2 · edited Mar 18 '10 at 23:15

6

with awk it's as simple as:

awk 'NR==1 || NR==5 || NR==1010' "file"

edited Mar 18 '10 at 23:15

Dennis Williamson

346,391
90
374
439

answered Jan 06 '10 at 23:10

ennuikiller

46,381
14
112
137

@michael, nonsense, awk can do that too. – ghostdog74 Jan 07 '10 at 00:39
2

ennuikiller, yes, I was mostly commenting on +1 for using awk in this context, ghostdog74, so can perl, python, pure bash, etc. It's a matter of opinion on the *right* tool for the job. – Michael Krelin - hacker Jan 07 '10 at 23:25

ghostdog74 · Answer 3 · 2010-01-07T01:07:25.553

3

@OP, you can do this easier and more efficiently with awk. so for your first question

awk 'NR~/^(1|2|5|1010)$/{print}' file

for 2nd question

awk 'FNR==NR{a[$1];next}(FNR in a){print}' file_with_linenr file

edited Jan 07 '10 at 01:07

answered Jan 07 '10 at 00:41

ghostdog74

327,991
56
259
343

The second response is a bit obfuscated. To explain: `FNR==NR` will occur only when reading `file_with_linenr`, not `file`. In this case, the text of the line is added to a set `a`, and execution skips to the next line of input. Thus when reading from `file`, only the `(FNR in a)` case applies, and prints the text of the relevant line if its number was put in `a` in parsing `file_with_linenr`. – joeln Jun 08 '14 at 11:32

score 1 · Answer 4 · answered Jan 07 '10 at 05:21

This ain't pretty and it could exceed command length limits under some circumstances^*:

sed -n "$(while read a; do echo "${a}p;"; done < line_num_file)" data_file

Or its much slower but more attractive, and possibly more well-behaved, sibling:

while read a; do echo "${a}p;"; done < line_num_file | xargs -I{} sed -n \{\} data_file

A variation:

xargs -a line_num_file -I{} sed -n \{\}p\; data_file

You can speed up the xarg versions a little bit by adding the -P option with some large argument like, say, 83 or maybe 419 or even 1177, but 10 seems as good as any.

_{*xargs --show-limits </dev/null can be instructive}

score 0 · Answer 5 · answered Jan 06 '10 at 23:08

0

I'd investigate Perl, since it has the regexp facilities of sed plus the programming model surrounding it to allow you to read a file line by line, count the lines and extract according to what you want (including from a file of line numbers).

my $row = 1
while (<STDIN>) {
   # capture the line in $_ and check $row against a suitable list.
   $row++;
}

answered Jan 06 '10 at 23:08

Brian Agnew

268,207
37
334
440

and you can use perl -e 'perlcode here' from the command prompt. Perl also has a range operator .. as in 3..12 which will allow you to create a list of numbers where needed. – Christian V Jan 06 '10 at 23:22
You should be using `$.`, which automagically contains the current line number – Hasturkun Jan 06 '10 at 23:35
Anybody interested in Perl command line techniques might want to look at Minimal Perl, from Manning... http://manning.com/maher/ – Joe Internet Jan 07 '10 at 06:36

score 0 · Answer 6 · answered Mar 17 '10 at 19:27

0

In Perl:

perl -ne 'print if $. =~ m/^(1|5|1010|20503)$/' file

answered Mar 17 '10 at 19:27

ire_and_curses

68,372
23
116
141

How do I extract lines from a file using their line number on unix?

6 Answers6