Differences between revisions 36 and 40 (spanning 4 versions)
⇤ ← Revision 36 as of 2018-12-21 20:47:27 →
Size: 1920
Editor: Valentin Bajrami
Comment: Another method of printing a line using -n and p flag with sed.
← Revision 40 as of 2025-04-20 12:55:53 → ⇥
Size: 2101
Editor: StephaneChazelas
Comment:
Deletions are marked like this. Additions are marked like this.
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sed -n "${n}p" "$file" sed -n "${n}p" < "$file"
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sed "${n}q;d" "$file" sed "${n}q;d" < "$file"
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There is yet another simple way of printing only the line you want using `sed`. The "-n" option will not print anything unless `p` flag is explicitly specified. For example to print the 4th line in a file use. Or
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sed -n "4p" "$file" sed "$n!d;q" < "$file"
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Which appears to be faster with all of GNU, busybox and ast-open sed implementations.
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tail -n "+$n" "$file" | head -n 1 <"$file" tail -n "+$n" | head -n 1
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As that uses more specialized tools, that's generally generally significantly faster.
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awk "NR==$n{print;exit}" "$file" awk -v n="$n" 'NR==n{print;exit}' < "$file"
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sed -n "$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}' < "$file" # Same, generally slower
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Or a counter with a simple `read` loop: Or a counter with a simple `read` loop, though that's going to be orders of magnitude slower for any input with more than a few hundred lines.
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# Bash/ksh
m=0
while ((m++ < n)) && read -r _; do
    :
done		 # Bash/ksh/zsh
{
    m=0
    while ((m++ < x - 1)) && read -r _; do
        :
    done
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head -n 1     head -n "$((y - x + 1))"
} < "$file"
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To read into a variable, it is preferable to use `read` or `mapfile` rather than an external utility. More than one line can be read into the given array variable or the default array `MAPFILE` by adjusting the argument to mapfile's -n option: To read into a variable, it is preferable to use `read` or `mapfile` aka `readarray` rather than an external utility. More than one line can be read into the given array variable or the default array `MAPFILE` by adjusting the argument to mapfile's -n option:
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mapfile -ts "$((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)