Diff for "BashFAQ/011"

Differences between revisions 2 and 38 (spanning 36 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 2 as of 2007-05-07 19:21:58 → 
  Size: 618
  Editor: GreyCat
  Comment: small clean-up
+   ← Revision 38 as of 2018-12-23 22:43:52 → ⇥
  Size: 1658
  Editor: geirha
  Comment: That is just a minor variation of the very first example
-Deletions are marked like this.
+Additions are marked like this.
 Line 1:
-[[Anchor(faq11)]]
+<<Anchor(faq11)>>
 Line 3:
-The dirty (but not quick) way would be:
{{{
    sed -n ${n}p "$file"
+One dirty (but not quick) way is:

{{{#!highlight bash
sed -n "${n}p" "$file"
-Line 8:
+Line 9:
-but this reads the whole input file, even if you only wanted the third line.
+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 `$n`, and deleting all other lines with the `d` command:
-Line 10:
+Line 11:
-This one avoids that problem:
{{{
    sed -n "$n{p;q;}" "$file"
+{{{#!highlight bash
sed "${n}q;d" "$file"
 Line 15:
-At line $n the command "p" is run, printing it, with a "q" afterwards: quit the program.
+Another method is to grab lines starting at `n`, then get the first line of that.
 Line 17:
-Another way, more obvious to some, is to grab the last line from a listing of the first ''n'' lines:
{{{
   head -n $n $file | tail -n 1
+{{{#!highlight bash
tail -n "+$n" "$file" | head -n 1
-Line 22:
+Line 21:
-Another approach, using ["AWK"]:
{{{
   awk "NR==$n{print;exit}" file
+Another approach, using [[AWK]]:

{{{#!highlight bash
awk "NR==$n{print;exit}" "$file"
 Line 26:
+If more than one line is needed, it's easy to adapt any of the previous methods:

{{{#!highlight bash
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
}}}

Or a counter with a simple `read` loop:

{{{#!highlight bash
# 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
# Bash4
mapfile -ts "$((n - 1))" -n 1 x <"$file"
printf '%s\n' "$x"
}}}

=== See Also ===
 * [[BashFAQ/001]]
 * http://wiki.bash-hackers.org/commands/builtin/mapfile
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CategoryShell