Differences between revisions 27 and 40 (spanning 13 versions)
⇤ ← Revision 27 as of 2012-02-27 02:27:07 →
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Editor: ormaaj
Comment: Agree with Lhunath - rm useless note. Fix writing style.
← Revision 40 as of 2025-04-20 12:55:53 → ⇥
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Editor: StephaneChazelas
Comment:
Deletions are marked like this. Additions are marked like this.
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{{{
    sed -n ${n}p "$file"		 {{{#!highlight bash
sed -n "${n}p" < "$file"
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But this reads the entire file even if only the third line is desired, which can be avoided by printing line {{{$n}}} using the "p" command, followed by a "q" to exit the script:
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{{{
    sed -n "$n{p;q;}" "$file"		 But this reads the entire file even if only the third line is desired, which can be avoided by using the `q` command to quit on line `$n`, and deleting all other lines with the `d` command:

{{{#!highlight bash
sed "${n}q;d" < "$file"
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Another method is to grab the last line from a listing of the first {{{n}}} lines:
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{{{
   head -n $n $file | tail -n 1		 Or

{{{#!highlight bash
sed "$n!d;q" < "$file"
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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.

{{{#!highlight bash
<"$file" tail -n "+$n" | head -n 1
}}}

As that uses more specialized tools, that's generally generally significantly faster.
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{{{
   awk "NR==$n{print;exit}" file		 {{{#!highlight bash
awk -v n="$n" 'NR==n{print;exit}' < "$file"
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{{{
   x=3 y=4
   sed -n "$x,${y}p;${y}q;" "$file" # Print lines $x to $y; quit after $y.
   head -n $y "$file" | tail -n $((y - x + 1)) # Same
   head -n $y "$file" | tail -n +$x # If your tail supports it
   awk "NR>=$x{print} NR==$y{exit}" "$file" # Same		 {{{#!highlight bash
x=3 y=4
sed "$x,$y!d;${y}q" < "$file" # Print lines $x to $y; quit after $y.
tail -n "+$x" < "$file" | head -n "$(( y - x + 1 ))" # Same, generally faster
awk -v x="$x" -v y="$y" 'NR>=x; NR==y{exit}' < "$file" # Same, generally slower
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In Bash 4, a pure-bash solution can be achieved succinctly using the {{{mapfile}}} builtin. More than one line can be read into the array {{{MAPFILE}}} by adjusting the argument to mapfile's -n option:
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{{{
   mapfile -ts $((n-1)) -n 1 < "$file"
   echo "${MAPFILE[0]}"		 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.

{{{#!highlight bash
# Bash/ksh/zsh
{
    m=0
    while ((m++ < x - 1)) && read -r _; do
        :
    done

    head -n "$((y - x + 1))"
} < "$file"
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{{{mapfile}}} can also be used similarly to {{{head}}} while avoiding buffering issues in the event input is a pipe:
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{{{
   { mapfile -n $n; head -n 1; } <"$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:

{{{#!highlight bash
# Bash4
{
  mapfile -s "$((x - 1))" -n "$((y - x + 1))" lines
  printf %s "${lines[@]}"
} < "$file"

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

CategoryShell

BashFAQ/011 (last edited 2025-04-20 12:55:53 by StephaneChazelas)