Quick unix command to display specific lines in the middle of a file?

Trying to debug an issue with a server and my only log file is a 20GB log file (with no timestamps even! Why do people use System.out.println() as logging? In production?!)

Using grep, I've found an area of the file that I'd like to take a look at, line 347340107.

Other than doing something like

head -<$LINENUM + 10> filename | tail -20 

... which would require head to read through the first 347 million lines of the log file, is there a quick and easy command that would dump lines 347340100 - 347340200 (for example) to the console?

update I totally forgot that grep can print the context around a match ... this works well. Thanks!

I found two other solutions if you know the line number but nothing else (no grep possible):

Assuming you need lines 20 to 40,

sed -n '20,40p;41q' file_name

or

awk 'FNR>=20 && FNR<=40' file_name

When using sed it is more efficient to quit processing after having printed the last line than continue processing until the end of the file. This is especially important in the case of large files and printing lines at the beginning. In order to do so, the sed command above introduces the instruction 41q in order to stop processing after line 41 because in the example we are interested in lines 20-40 only. You will need to change the 41 to whatever the last line you are interested in is, plus one.

# print line number 52
sed -n '52p' # method 1
sed '52!d' # method 2
sed '52q;d' # method 3,  efficient on large files 

method 3 efficient on large files

fastest way to display specific lines

with GNU-grep you could just say

grep --context=10 ...

No there isn't, files are not line-addressable.

There is no constant-time way to find the start of line n in a text file. You must stream through the file and count newlines.

Use the simplest/fastest tool you have to do the job. To me, using head makes much more sense than grep, since the latter is way more complicated. I'm not saying "grep is slow", it really isn't, but I would be surprised if it's faster than head for this case. That'd be a bug in head, basically.

What about:

tail -n +347340107 filename | head -n 100

I didn't test it, but I think that would work.

I prefer just going into less and

• typing 50% to goto halfway the file,
• 43210G to go to line 43210
• :43210 to do the same

and stuff like that.

Even better: hit v to start editing (in vim, of course!), at that location. Now, note that vim has the same key bindings!

You can use the ex command, a standard Unix editor (part of Vim now), e.g.

• display a single line (e.g. 2nd one):

    ex +2p -scq file.txt
  

  corresponding sed syntax: sed -n '2p' file.txt

• range of lines (e.g. 2-5 lines):

    ex +2,5p -scq file.txt
  

  sed syntax: sed -n '2,5p' file.txt

• from the given line till the end (e.g. 5th to the end of the file):

    ex +5,p -scq file.txt
  

  sed syntax: sed -n '2,$p' file.txt

• multiple line ranges (e.g. 2-4 and 6-8 lines):

    ex +2,4p +6,8p -scq file.txt
  

  sed syntax: sed -n '2,4p;6,8p' file.txt

Above commands can be tested with the following test file:

seq 1 20 > file.txt

---

Explanation:

• + or -c followed by the command - execute the (vi/vim) command after file has been read,

• -s - silent mode, also uses current terminal as a default output,

• q followed by -c is the command to quit editor (add ! to do force quit, e.g. -scq!).

I'd first split the file into few smaller ones like this

$ split --lines=50000 /path/to/large/file /path/to/output/file/prefix

and then grep on the resulting files.

If your line number is 100 to read

head -100 filename | tail -1

Get ack

Ubuntu/Debian install:

$ sudo apt-get install ack-grep

Then run:

$ ack --lines=$START-$END filename

Example:

$ ack --lines=10-20 filename

From $ man ack:

--lines=NUM
    Only print line NUM of each file. Multiple lines can be given with multiple --lines options or as a comma separated list (--lines=3,5,7). --lines=4-7 also works. 
    The lines are always output in ascending order, no matter the order given on the command line.

sed will need to read the data too to count the lines. The only way a shortcut would be possible would there to be context/order in the file to operate on. For example if there were log lines prepended with a fixed width time/date etc. you could use the look unix utility to binary search through the files for particular dates/times

Use

x=`cat -n <file> | grep <match> | awk '{print $1}'`

Here you will get the line number where the match occurred.

Now you can use the following command to print 100 lines

awk -v var="$x" 'NR>=var && NR<=var+100{print}' <file>

or you can use "sed" as well

sed -n "${x},${x+100}p" <file>

With sed -e '1,N d; M q' you'll print lines N+1 through M. This is probably a bit better then grep -C as it doesn't try to match lines to a pattern.

Building on Sklivvz' answer, here's a nice function one can put in a .bash_aliases file. It is efficient on huge files when printing stuff from the front of the file.

function middle()
{
    startidx=$1
    len=$2
    endidx=$(($startidx+$len))
    filename=$3

    awk "FNR>=${startidx} && FNR<=${endidx} { print NR\" \"\$0 }; FNR>${endidx} { print \"END HERE\"; exit }" $filename
}

To display a line from a <textfile> by its <line#>, just do this:

perl -wne 'print if $. == <line#>' <textfile>

If you want a more powerful way to show a range of lines with regular expressions -- I won't say why grep is a bad idea for doing this, it should be fairly obvious -- this simple expression will show you your range in a single pass which is what you want when dealing with ~20GB text files:

perl -wne 'print if m/<regex1>/ .. m/<regex2>/' <filename>

(tip: if your regex has / in it, use something like m!<regex>! instead)

This would print out <filename> starting with the line that matches <regex1> up until (and including) the line that matches <regex2>.

It doesn't take a wizard to see how a few tweaks can make it even more powerful.

Last thing: perl, since it is a mature language, has many hidden enhancements to favor speed and performance. With this in mind, it makes it the obvious choice for such an operation since it was originally developed for handling large log files, text, databases, etc.

You could try this command:

egrep -n "*" <filename> | egrep "<line number>"

Easy with perl! If you want to get line 1, 3 and 5 from a file, say /etc/passwd:

perl -e 'while(<>){if(++$l~~[1,3,5]){print}}' < /etc/passwd

I am surprised only one other answer (by Ramana Reddy) suggested to add line numbers to the output. The following searches for the required line number and colours the output.

file=FILE
lineno=LINENO
wb="107"; bf="30;1"; rb="101"; yb="103"
cat -n ${file} | { GREP_COLORS="se=${wb};${bf}:cx=${wb};${bf}:ms=${rb};${bf}:sl=${yb};${bf}" grep --color -C 10 "^[[:space:]]\\+${lineno}[[:space:]]"; }

EDITED(just for fun): this was my only good answer?/dark joke comment click that vote! like i would be part of this forum anyway mainly its all ways same. question this way->answer something else multiple way, but not actually for question

Who hates when there is allways answer using some other LOL linux include wasted bytes that do same thing

print line 5

sed -n '5p' file.txt
sed '5q' file.txt

print everything else than line 5

`sed '5d' file.txt

and my creation using google

#!/bin/bash
#removeline.sh
#remove deleting it comes move line xD

usage() {                                 # Function: Print a help message.
  echo "Usage: $0 -l LINENUMBER -i INPUTFILE [ -o OUTPUTFILE ]"
  echo "line is removed from INPUTFILE"
  echo "line is appended to OUTPUTFILE"
}
exit_abnormal() {                         # Function: Exit with error.
  usage
  exit 1
}

while getopts l:i:o:b flag
do
    case "${flag}" in
        l) line=${OPTARG};;
        i) input=${OPTARG};;
        o) output=${OPTARG};;
    esac
done

if [ -f tmp ]; then
echo "Temp file:tmp exist. delete it yourself :)"
exit
fi

if [ -f "$input" ]; then
   re_isanum='^[0-9]+$'
   if ! [[ $line =~ $re_isanum ]] ; then
      echo "Error: LINENUMBER must be a positive, whole number."
      exit 1
   elif [ $line -eq "0" ]; then
      echo "Error: LINENUMBER must be greater than zero."
      exit_abnormal
   fi
   if [ ! -z $output ]; then
      sed -n "${line}p" $input >> $output
   fi
   if [ ! -z $input ]; then
      # remove this sed command and this comes move line to other file
      sed "${line}d" $input > tmp && cp tmp $input
   fi
fi

if [ -f tmp ]; then
rm tmp
fi