How to print the longest line in a file?

Question

I'm looking for the simplest method to print the longest line in a file. I did some googling and surprisingly couldn't seem to find an answer. I frequently print the length of the longest line in a file, but I don't know how to actually print the longest line. Can anyone provide a solution to print the longest line in a file? Thanks in advance.

Answer 1

cat ./text | awk ' { if ( length > x ) { x = length; y = $0 } }END{ print y }'

UPD: summarizing all the advices in the comments

awk 'length > max_length { max_length = length; longest_line = $0 } END { print longest_line }' ./text 

Answer 2

sed -rn "/.{$(<file expand -t1 |wc -L)}/{p;q}" file

This first reads the file inside the command substitution and outputs the length of the longest line, (previously, expand converts tabs to spaces, to overcome the semantics of wc -L -- each tab in the line will add 8 instead of 1 to line length). This length is then used in a sed expression meaning "find a line this number of characters long, print it, then quit". So this actually can be as optimal as the longest line is near to the top of the file, heheh (thanks fered for the awesome and constructive comments).

Another, I had thought earlier than the sed one (in bash):

#!/bin/bash
while read -r line; do
    (( ${#line} > max )) && max=${#line} && longest="$line"
done
echo "$longest"

Answer 3

In pure bash:

#!/bin/bash

_max_length=0
while IFS= read -r _line; do
    _length="${#_line}"
    if (( _length > _max_length )); then
        _max_length=${_length}
        _max_line=( "${_line}" )
    elif (( _length == _max_length )); then
        _max_line+=( "${_line}" )
    fi
done

printf 'Max line length: %d\n' "${_max_length}"
printf 'Lines matched with that length: %d\n' "${#_max_line[@]}"
(( ${#_max_line[@]} )) && printf '%s\n' '----------------' "${_max_line[@]}"

Answer 4

The following example was going to be, and should have been, a comment to dmitry.malikov's answer, but because of the Useless Use of Visible Comment Space there, I've chosen to present it here, where it will at least be seen...

This is a simple variation of the dmitry's single-pass awk method.
It prints all "equal longest" lines. (Note. delete array is a gawk extension).

awk 'length >x { delete y; x=length }
     length==x { y[NR]=$0 } END{ for (z in y) print y[z] }' file

Answer 5

Here's a Perl solution:

perl -e 'while(<>){
           $l=length;  
           $l>$m && do {$c=$_; $m=$l}  
         } print $c' file.txt 

Or, if you want to print all the longest lines

perl -e 'while(<>){
           $l=length;
           push @{$k{$l}},$_;
           $m=$l if $l>$m;
         } print @{$k{$m}}' file.txt 

---

Since I had nothing better to do, I ran some benchmarks on a 625M text file. Surprisingly, my Perl solution was consistently faster than the others. Granted, the difference with the accepted awk solution is tiny, but it is there. Obviously, solutions that print multiple lines are slower so I have sorted by type, fastest to slowest.

Print only one of the longest lines:

$ time perl -e 'while(<>){
           $l=length;  
           $l>$m && do {$c=$_; $m=$l}  
         } print $c' file.txt 
real    0m3.837s
user    0m3.724s
sys     0m0.096s



$ time awk 'length > max_length { max_length = length; longest_line = $0 }
 END { print longest_line }' file.txt
real    0m5.835s
user    0m5.604s
sys     0m0.204s



$ time sed -rn "/.{$(<file.txt expand -t1 |wc -L)}/{p;q}" file.txt 
real    2m37.348s
user    2m39.990s
sys     0m1.868s

Print all longest lines :

$ time perl -e 'while(<>){
           $l=length;
           push @{$k{$l}},$_;
           $m=$l if $l>$m;
         } print @{$k{$m}}' file.txt 
real    0m9.263s
user    0m8.417s
sys     0m0.760s


$ time awk 'length >x { delete y; x=length }
     length==x { y[NR]=$0 } END{ for (z in y) print y[z] }' file.txt
real    0m10.220s
user    0m9.925s
sys     0m0.252s


## This is Chris Down's bash solution
$ time ./a.sh < file.txt 
Max line length: 254
Lines matched with that length: 2
real    8m36.975s
user    8m17.495s
sys     0m17.153s

Answer 6

You can use wc:

wc -L fileName