Diff for "BashFAQ/011"

Differences between revisions 2 and 36 (spanning 34 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"

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.

   1 sed -n "4p" "$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 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.
-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.
+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.
 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 $file | tail -n 1
+{{{#!highlight bash
sed -n "4p" "$file"
-Line 22:
+Line 21:
-Another approach, using ["AWK"]:
{{{
   awk "NR==$n{print;exit}" file
+Another method is to grab lines starting at `n`, then get the first line of that.

{{{#!highlight bash
tail -n "+$n" "$file" | head -n 1
 Line 26:
+Another approach, using [[AWK]]:

{{{#!highlight bash
awk "NR==$n{print;exit}" "$file"
}}}

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