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"

See Also

CategoryShell