Diff for "BashFAQ/011"

Differences between revisions 26 and 36 (spanning 10 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 26 as of 2012-02-27 01:42:40 → 
  Size: 1978
  Editor: ormaaj
  Comment: { mapfile -n $n; head -n 1; } <"$file"
+   ← 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 3:
-The dirty (but not quick) way would be:
+One dirty (but not quick) way is:
 Line 5:
-{{{
    sed -n ${n}p "$file"
+{{{#!highlight bash
sed -n "${n}p" "$file"
 Line 8:
-but this reads the whole input file, even if you only wanted the third line.
-Line 10:
+Line 9:
-This one avoids that problem:
+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 `$n`, and deleting all other lines with the `d` command:
-Line 12:
+Line 11:
-{{{
    sed -n "$n{p;q;}" "$file"
+{{{#!highlight bash
sed "${n}q;d" "$file"
-Line 15:
+Line 14:
-At line $n the command "p" is run, printing it, with a "q" afterwards: quit the program.
-Line 17:
+Line 15:
-Another way, more obvious to some, is to grab the last line from a listing of the first ''n'' lines:
+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 19:
+Line 17:
-{{{
   head -n $n $file | tail -n 1
+{{{#!highlight bash
sed -n "4p" "$file"
-Line 22:
+Line 20:
+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 24:
+Line 29:
-{{{
   awk "NR==$n{print;exit}" file
}}}
If you want more than one line, it's pretty easy to adapt any of the previous methods:

{{{
   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
}}}
In Bash 4, this can also be done with mapfile builtin. By changing the value of argument for -n, you can get more than one line in the array MAPFILE:

{{{
   mapfile -ts $((n-1)) -n 1 < "$file"
   echo "${MAPFILE[0]}"
+{{{#!highlight bash
awk "NR==$n{print;exit}" "$file"
-Line 43:
+Line 33:
-mapfile can also be used similarly to head while avoiding buffering issues in the event input is a pipe:
{{{
   { mapfile -n $n; head -n 1; } <"$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
-Line 48:
+Line 43:
-=== Note ===
In most cases, you should sanitize your variable n to be sure, that it's not containing any of non-digits, before feeding it to sed or awk. You can do it with such simple code:
+Or a counter with a simple `read` loop:
-Line 51:
+Line 45:
-{{{
   # Bash
   n=${n//[!0-9]/}
+{{{#!highlight bash
# Bash/ksh
m=0
while ((m++ < n)) && read -r _; do
    :
done
-Line 55:
+Line 52:
-   # POSIX
   n=$(printf "%s" "$n"|tr -cd '0-9')
+head -n 1
-Line 58:
+Line 54:
- . One can argue that an `n` value of `Five brown horses with 3 feet and 9 little piggies.` should yield an error rather than silently behave as though it was really `39`.  If you want to code safe, quote your expansions and be done with it. --[[Lhunath]]
-Line 60:
+Line 55:
+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