Diff for "BashFAQ/011"

Differences between revisions 28 and 33 (spanning 5 versions)

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"

Another method is to grab the last line from a listing of the first n lines. However, this is only applicable if you know the file has at least n lines.

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

Another approach, using AWK:

   1 awk "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 -n "$x,${y}p;${y}q;" "$file"                # Print lines $x to $y; quit after $y.
   3 head -n "$y" "$file" | tail -n "$((y - x + 1))"   # Same
   4 head -n "$y" "$file" | tail -n "+$x"            # If your tail supports it
   5 awk "NR>=$x{print} NR==$y{exit}" "$file"        # Same

Or a counter with a simple read loop:

   1 # Bash/ksh
   2 m=0
   3 while ((m++ < n)) && read -r _; do
   4     :
   5 done
   6 
   7 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:

   1 # Bash4
   2 mapfile -ts "$((n - 1))" -n 1 x <"$file"
   3 printf '%s\n' "$x"

-  ⇤ ← Revision 28 as of 2013-03-12 07:38:28 → 
  Size: 1793
  Editor: ormaaj
  Comment: Quotes
+   ← Revision 33 as of 2016-09-22 07:18:13 → ⇥
  Size: 1732
  Editor: geirha
  Comment: note about the head|tail approach
-Deletions are marked like this.
+Additions are marked like this.
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-{{{
+{{{#!highlight bash
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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:
+Another method is to grab the last line from a listing of the first `n` lines. However, this is only applicable if you know the file has at least `n` lines.
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-{{{
+{{{#!highlight bash
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-{{{
+{{{#!highlight bash
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-{{{
+{{{#!highlight bash
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-head -n "$y" "$file" | tail -n $((y - x + 1))   # Same
+head -n "$y" "$file" | tail -n "$((y - x + 1))"   # Same
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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:

{{{
# Bash4
{ mapfile -n "$n"; head -n 1; } <"$file"
}}}
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+Line 39:
-{{{
+{{{#!highlight bash
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+Line 51:
-{{{
+{{{#!highlight bash
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+Line 53:
-mapfile -ts $((n-1)) -n 1 x <"$file"
+mapfile -ts "$((n - 1))" -n 1 x <"$file"

Diff for "BashFAQ/011"

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

See Also