Diff for "BashFAQ/011"

Differences between revisions 14 and 28 (spanning 14 versions)

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

One dirty (but not quick) way is:

sed -n "${n}p" "$file"

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:

sed -n "$n{p;q;}" "$file"

Another method is to grab the last line from a listing of the first n lines:

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

Another approach, using AWK:

awk "NR==$n{print;exit}" "$file"

If more than one line is needed, it's 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, 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:

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

Or a counter with a simple read loop:

# 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:

# Bash4
mapfile -ts $((n-1)) -n 1 x <"$file"
printf '%s\n' "$x"

-  ⇤ ← Revision 14 as of 2010-04-06 15:36:18 → 
  Size: 1252
  Editor: 109-126-52-27
  Comment:
+   ← Revision 28 as of 2013-03-12 07:38:28 → ⇥
  Size: 1793
  Editor: ormaaj
  Comment: Quotes
-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 6:
-    sed -n ${n}p "$file"
+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 printing line `$n` using the `p` command, followed by a `q` to exit the script:
 Line 11:
-This one avoids that problem:
-Line 13:
+Line 12:
-    sed -n "$n{p;q;}" "$file"
+sed -n "$n{p;q;}" "$file"
-Line 16:
+Line 15:
-At line $n the command "p" is run, printing it, with a "q" afterwards: quit the program.
+Another method is to grab the last line from a listing of the first `n` lines:
-Line 18:
+Line 17:
-Another way, more obvious to some, is to grab the last line from a listing of the first ''n'' lines:
-Line 20:
+Line 18:
-   head -n $n $file | tail -n 1
+head -n "$n" "$file" | tail -n 1
-Line 24:
+Line 22:
-Line 25:
+Line 24:
-   awk "NR==$n{print;exit}" file
+awk "NR==$n{print;exit}" "$file"
-Line 28:
+Line 27:
-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 31:
+Line 30:
-   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 - $x + 1))   # Same
   awk "NR>=$x{print} NR==$y{exit}" "$file"        # Same
+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}" "$file"        # Same
 Line 37:
-=== Note ===
In most cases, you should sanitize your variable n to be sure, that it's not containing any of non-digits, before feeding it to sed or awk. You can do it with such simple code:
+In Bash 4, `mapfile` can be used similarly to `head` while avoiding buffering issues in the event input is a pipe, because it guarantees the [[ FileDescriptor|fd ]] will be seeked to where you left it:
 Line 40:
-   # Bash
   n=${n//[!0-9]/}
+# Bash4
{ mapfile -n "$n"; head -n 1; } <"$file"
}}}
-Line 43:
+Line 44:
-   # POSIX
   n=$(printf "%s" "$n"|tr -cd '0-9')
+Or a counter with a simple `read` loop:

{{{
# Bash/ksh
m=0
while ((m++ < n)) && read -r _; do
    :
done

head -n 1
-Line 46:
+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:

{{{
# 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