Differences between revisions 28 and 40 (spanning 12 versions)
⇤ ← Revision 28 as of 2013-03-12 07:38:28 →
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Editor: ormaaj
Comment: Quotes
← 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 `$n` using the `p` command, followed by a `q` to exit the script: But this reads the entire file even if only the third line is desired, which can be avoided by using the `q` command to quit on line `$n`, and deleting all other lines with the `d` command:
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{{{
sed -n "$n{p;q;}" "$file"		 {{{#!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: Or
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{{{
head -n "$n" "$file" | tail -n 1		 {{{#!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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{{{ {{{#!highlight bash
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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}" "$file" # Same		 sed "$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}' < "$file" # Same, generally slower
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In Bash 4, `mapfile` can be used similarly to `head` while avoiding buffering issues in the event input is a pipe, because it guarantees the [[ FileDescriptor|fd ]] will be seeked to where you left it: Or a counter with a simple `read` loop, though that's going to be orders of magnitude slower for any input with more than a few hundred lines.
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{{{
# Bash4
{ mapfile -n "$n"; head -n 1; } <"$file"		 {{{#!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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Or a counter with a simple `read` loop: 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:
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{{{
# Bash/ksh
m=0
while ((m++ < n)) && read -r _; do
    :
done

head -n 1
}}}

To read into a variable, it is preferable to use `read` or `mapfile` 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
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mapfile -ts $((n-1)) -n 1 x <"$file"
printf '%s\n' "$x"		 {
  mapfile -s "$((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)