Diff for "BashFAQ/011"

Differences between revisions 17 and 37 (spanning 20 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 17 as of 2011-02-13 08:03:13 → 
  Size: 1329
  Editor: Lhunath
  Comment:
+   ← Revision 37 as of 2018-12-21 20:48:17 → ⇥
  Size: 1920
  Editor: Valentin Bajrami
  Comment:
-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 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 `$n`, and deleting all other lines with the `d` command:
 Line 11:
-This one avoids that problem:
{{{
    sed -n "$n{p;q;}" "$file"
+{{{#!highlight bash
sed "${n}q;d" "$file"
-Line 16:
+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 18:
+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"
}}}

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 28:
-{{{
   awk "NR==$n{print;exit}" file
+{{{#!highlight bash
awk "NR==$n{print;exit}" "$file"
-Line 28:
+Line 33:
-If you want more than one line, it's pretty easy to adapt any of the previous methods:
+If more than one line is needed, it's easy to adapt any of the previous methods:
-Line 30:
+Line 35:
-{{{
   x=3 y=4
   sed -n "$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
+{{{#!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 38:
+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:
{{{
   # Bash
   n=${n//[!0-9]/}
+Or a counter with a simple `read` loop:
-Line 44:
+Line 45:
-   # POSIX
   n=$(printf "%s" "$n"|tr -cd '0-9')
+{{{#!highlight bash
# Bash/ksh
m=0
while ((m++ < n)) && read -r _; do
    :
done

head -n 1
-Line 47:
+Line 54:
+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