Diff for "BashFAQ/011"

Differences between revisions 1 and 28 (spanning 27 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 1 as of 2007-05-02 22:55:48 → 
  Size: 620
  Editor: redondos
  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.
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-[[Anchor(faq11)]]
+<<Anchor(faq11)>>
 Line 3:
-The dirty (but not quick) way would be {{{sed -n ${n}p "$file"}}} but this reads the whole input file, even if you only wanted the third line.

The following {{{sed}}} command line reads a file printing nothing (-n). At line $n the command "p" is run, printing it, with a "q" afterwards: quit the program.
+One dirty (but not quick) way is:
-Line 8:
+Line 6:
-    sed -n "$n{p;q;}" "$file"
+sed -n "${n}p" "$file"
-Line 11:
+Line 9:
-Another way, more obvious to some, is to grab the last line from a listing of the first n lines:
+But this reads the entire file even if only the third line is desired, which can be avoided by printing line `$n` using the `p` command, followed by a `q` to exit the script:
-Line 13:
+Line 12:
-   head -n $n $file | tail -n 1
+sed -n "$n{p;q;}" "$file"
-Line 16:
+Line 15:
-Using awk:
+Another method is to grab the last line from a listing of the first `n` lines:
 Line 18:
-   awk 'NR==n{print;exit}' file
+head -n "$n" "$file" | tail -n 1
 Line 20:
+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 [[ FileDescriptor|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 ===
 * [[BashFAQ/001]]
 * http://wiki.bash-hackers.org/commands/builtin/mapfile
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CategoryShell