Diff for "BashFAQ/011"

Differences between revisions 29 and 35 (spanning 6 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 lines starting at n, then get the first line of that.

   1 tail -n "+$n" "$file" | head -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 29 as of 2015-03-05 00:26:49 → 
  Size: 1921
  Editor: izabera
  Comment: syntax hl
+   ← Revision 35 as of 2016-09-25 20:51:12 → ⇥
  Size: 1658
  Editor: tor2
  Comment: Switch order of head and tail to avoid problems if file contains less than n lines. Also requires less data to be copied.
-Deletions are marked like this.
+Additions are marked like this.
 Line 9:
-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:
 Line 12:
-sed -n "$n{p;q;}" "$file"
+sed "${n}q;d" "$file"
 Line 15:
-Another method is to grab the last line from a listing of the first `n` lines:
+Another method is to grab lines starting at `n`, then get the first line of that.
 Line 18:
-head -n "$n" "$file" | tail -n 1
+tail -n "+$n" "$file" | head -n 1
 Line 32:
-head -n "$y" "$file" | tail -n $((y - x + 1))   # Same
+head -n "$y" "$file" | tail -n "$((y - x + 1))"   # Same
 Line 35:
-}}}

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:

{{{#!highlight bash
# Bash4
{ mapfile -n "$n"; head -n 1; } <"$file"
-Line 60:
+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