Diff for "BashFAQ/011"

Differences between revisions 1 and 38 (spanning 37 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 1 as of 2007-05-02 22:55:48 → 
  Size: 620
  Editor: redondos
  Comment:
+   ← 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"}}} but this reads the whole input file, even if you only wanted the third line.
+One dirty (but not quick) way is:
 Line 5:
-The following {{{sed}}} command line reads a file printing nothing (-n). At line $n the command "p" is run, printing it, with a "q" afterwards: quit the program.

{{{
    sed -n "$n{p;q;}" "$file"
+{{{#!highlight bash
sed -n "${n}p" "$file"
-Line 11:
+Line 9:
-Another way, more obvious to some, is to grab the last line from a listing of the first n lines:
{{{
   head -n $n $file | tail -n 1
+But this reads the entire file even if only the third line is desired, which can be avoided by using the `q` command to quit on line `$n`, and deleting all other lines with the `d` command:

{{{#!highlight bash
sed "${n}q;d" "$file"
-Line 16:
+Line 15:
-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 20:
+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