Diff for "BashFAQ/011"

How can I print the n'th line of a file?

One dirty (but not quick) way is:

   1 sed -n "${n}p" < "$file"

But this reads the entire file even if only the third line is desired, which can be avoided by using the q command to quit on line $n, and deleting all other lines with the d command:

   1 sed "${n}q;d" < "$file"

Or

   1 sed "$n!d;q" < "$file"

Which appears to be faster with all of GNU, busybox and ast-open sed implementations.

Another method is to grab lines starting at n, then get the first line of that.

   1 <"$file" tail -n "+$n" | head -n 1

As that uses more specialized tools, that's generally generally significantly faster.

Another approach, using AWK:

   1 awk -v n="$n" 'NR==n{print;exit}' < "$file"

If more than one line is needed, it's easy to adapt any of the previous methods:

   1 x=3 y=4
   2 sed "$x,$y!d;${y}q" < "$file"                          # Print lines $x to $y; quit after $y.
   3 tail -n "+$x" < "$file" | head -n "$(( y - x + 1 ))"   # Same, generally faster
   4 awk -v x="$x" -v y="$y" 'NR>=x; NR==y{exit}' < "$file" # Same, generally slower

Or a counter with a simple read loop, though that's going to be orders of magnitude slower for any input with more than a few hundred lines.

   1 # Bash/ksh/zsh
   2 {
   3     m=0
   4     while ((m++ < x - 1)) && read -r _; do
   5         :
   6     done
   7 
   8     head -n "$((y - x + 1))"
   9 } < "$file"

To read into a variable, it is preferable to use read or mapfile aka readarray rather than an external utility. More than one line can be read into the given array variable or the default array MAPFILE by adjusting the argument to mapfile's -n option:

   1 # Bash4
   2 {
   3   mapfile -s "$((x - 1))" -n "$((y - x + 1))" lines
   4   printf %s "${lines[@]}"
   5 } < "$file"

See Also

• BashFAQ/001

• http://wiki.bash-hackers.org/commands/builtin/mapfile

CategoryShell