Diff for "BashFAQ/011"

Differences between revisions 27 and 30 (spanning 3 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 the last line from a listing of the first n lines:

   1 head -n "$n" "$file" | tail -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

In Bash 4, mapfile can be used similarly to head while avoiding buffering issues in the event input is a pipe, because it guarantees the fd will be seeked to where you left it:

   1 # Bash4
   2 { mapfile -n "$n"; head -n 1; } <"$file"

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 27 as of 2012-02-27 02:27:07 → 
  Size: 1538
  Editor: ormaaj
  Comment: Agree with Lhunath - rm useless note. Fix writing style.
+   ← Revision 30 as of 2016-06-16 01:20:29 → ⇥
  Size: 1932
  Editor: tor-exit
  Comment: using ; as a command delimiter within the { command is specified as an optional extension by posix, and is not required.
-Deletions are marked like this.
+Additions are marked like this.
 Line 5:
-{{{
    sed -n ${n}p "$file"
+{{{#!highlight bash
sed -n "${n}p" "$file"
 Line 8:
-But this reads the entire file even if only the third line is desired, which can be avoided by printing line {{{$n}}} using the "p" command, followed by a "q" to exit the script:
-Line 10:
+Line 9:
-{{{
    sed -n "$n{p;q;}" "$file"
+But this reads the entire file even if only the third line is desired, which can be avoided by using the `q` command to quit on line `$n`, and deleting all other lines with the `d` command:

{{{#!highlight bash
sed "${n}q;d" "$file"
-Line 13:
+Line 14:
-Another method is to grab the last line from a listing of the first {{{n}}} lines:
 Line 15:
-{{{
   head -n $n $file | tail -n 1
+Another method is to grab the last line from a listing of the first `n` lines:

{{{#!highlight bash
head -n "$n" "$file" | tail -n 1
-Line 18:
+Line 20:
-Line 20:
+Line 23:
-{{{
   awk "NR==$n{print;exit}" file
+{{{#!highlight bash
awk "NR==$n{print;exit}" "$file"
-Line 23:
+Line 26:
-Line 25:
+Line 29:
-{{{
   x=3 y=4
   sed -n "$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 32:
+Line 36:
-In Bash 4, a pure-bash solution can be achieved succinctly using the {{{mapfile}}} builtin. More than one line can be read into the array {{{MAPFILE}}} by adjusting the argument to mapfile's -n option:
-Line 34:
+Line 37:
-{{{
   mapfile -ts $((n-1)) -n 1 < "$file"
   echo "${MAPFILE[0]}"
+In Bash 4, `mapfile` can be used similarly to `head` while avoiding buffering issues in the event input is a pipe, because it guarantees the [[ FileDescriptor|fd ]] will be seeked to where you left it:

{{{#!highlight bash
# Bash4
{ mapfile -n "$n"; head -n 1; } <"$file"
-Line 38:
+Line 43:
-{{{mapfile}}} can also be used similarly to {{{head}}} while avoiding buffering issues in the event input is a pipe:
-Line 40:
+Line 44:
-{{{
   { mapfile -n $n; head -n 1; } <"$file"
+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"

Diff for "BashFAQ/011"

How can I print the n'th line of a file?

See Also