Diff for "BashFAQ/011"

Differences between revisions 26 and 27

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, 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:

   mapfile -ts $((n-1)) -n 1 < "$file"
   echo "${MAPFILE[0]}"

mapfile can also be used similarly to head while avoiding buffering issues in the event input is a pipe:

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

-  ⇤ ← Revision 26 as of 2012-02-27 01:42:40 → 
  Size: 1978
  Editor: ormaaj
  Comment: { mapfile -n $n; head -n 1; } <"$file"
+   ← Revision 27 as of 2012-02-27 02:27:07 → ⇥
  Size: 1538
  Editor: ormaaj
  Comment: Agree with Lhunath - rm useless note. Fix writing style.
-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:
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-but this reads the whole input file, even if you only wanted the third line.

This one avoids that problem:
+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 15:
+Line 13:
-At line $n the command "p" is run, printing it, with a "q" afterwards: quit the program.

Another way, more obvious to some, is to grab the last line from a listing of the first ''n'' lines:
+Another method is to grab the last line from a listing of the first {{{n}}} lines:
-Line 27:
+Line 23:
-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 36:
+Line 32:
-In Bash 4, this can also be done with mapfile builtin. By changing the value of argument for -n, you can get more than one line in the array MAPFILE:
+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 42:
+Line 38:
+{{{mapfile}}} can also be used similarly to {{{head}}} while avoiding buffering issues in the event input is a pipe:
-Line 43:
+Line 40:
-mapfile can also be used similarly to head while avoiding buffering issues in the event input is a pipe:
-Line 48:
+Line 44:
-=== 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]/}

   # POSIX
   n=$(printf "%s" "$n"|tr -cd '0-9')
}}}
 . One can argue that an `n` value of `Five brown horses with 3 feet and 9 little piggies.` should yield an error rather than silently behave as though it was really `39`.  If you want to code safe, quote your expansions and be done with it. --[[Lhunath]]
+=== See Also ===
 * [[BashFAQ/001]]
 * http://wiki.bash-hackers.org/commands/builtin/mapfile