Diff for "BashFAQ/011"

Differences between revisions 8 and 19 (spanning 11 versions)

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

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.

This one avoids that problem:

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

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:

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

Another approach, using AWK:

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

If you want more than one line, it's pretty 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

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

CategoryShell

-  ⇤ ← Revision 8 as of 2007-10-16 13:51:05 → 
  Size: 1057
  Editor: p54BFE162
  Comment: make regex work (10 would match 102 as well). although only sed really makes sense here, using nl is still way less pointless than pr!
+   ← Revision 19 as of 2011-02-13 08:10:08 → ⇥
  Size: 1592
  Editor: Lhunath
  Comment:
-Deletions are marked like this.
+Additions are marked like this.
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-[[Anchor(faq11)]]
+<<Anchor(faq11)>>
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+Line 23:
-Another approach, using ["AWK"]:
+Another approach, using [[AWK]]:
-Line 30:
+Line 31:
-   x=3 y=4;
+   x=3 y=4
-Line 32:
+Line 33:
-   head -n $y "$file" | tail -n $(($y - $x + 1))   # Same
+   head -n $y "$file" | tail -n $((y - x + 1))     # Same
   head -n $y "$file" | tail -n +$x                # If your tail supports it
-Line 36:
+Line 38:
-Using nl(1) (and grep(1) (and cut(1))):
+=== Note ===
In most cases, you should sanitize your variable n to be sure, that it's not containing any of non-digits, before feeding it to sed or awk. You can do it with such simple code:
{{{
   # Bash
   n=${n//[!0-9]/}
-Line 38:
+Line 44:
-{{{
   nl -b a file | grep '^  *<number of line>\W' | cut -f 2
+   # POSIX
   n=$(printf "%s" "$n"|tr -cd '0-9')
-Line 41:
+Line 47:
+   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]]

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CategoryShell