Diff for "BashFAQ/011"

Differences between revisions 26 and 39 (spanning 13 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 -v n="$n" 'NR==n{print;exit}' "$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 "+$x"            # If your tail supports it
   5 awk -v x="$x" -v y="$y" 'NR>=x{print} NR==y{exit}' "$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 39 as of 2020-05-07 08:35:17 → ⇥
  Size: 1685
  Editor: intranet
  Comment: fix awk code injection
-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:
+Another method is to grab lines starting at `n`, then get the first line of that.
-Line 19:
+Line 17:
-{{{
   head -n $n $file | tail -n 1
+{{{#!highlight bash
tail -n "+$n" "$file" | head -n 1
-Line 22:
+Line 20:
-Line 24:
+Line 23:
-{{{
   awk "NR==$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 "${MAPFILE[0]}"
+{{{#!highlight bash
awk -v n="$n" 'NR==n{print;exit}' "$file"
-Line 43:
+Line 27:
-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 "+$x"            # If your tail supports it
awk -v x="$x" -v y="$y" 'NR>=x{print} NR==y{exit}' "$file"        # Same
-Line 48:
+Line 37:
-=== 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 39:
-{{{
   # Bash
   n=${n//[!0-9]/}
+{{{#!highlight bash
# Bash/ksh
m=0
while ((m++ < n)) && read -r _; do
    :
done
-Line 55:
+Line 46:
-   # POSIX
   n=$(printf "%s" "$n"|tr -cd '0-9')
+head -n 1
-Line 58:
+Line 48:
- . 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 49:
+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