Diff for "BashFAQ/011"

Differences between revisions 1 and 17 (spanning 16 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')

CategoryShell

-  ⇤ ← Revision 1 as of 2007-05-02 22:55:48 → 
  Size: 620
  Editor: redondos
  Comment:
+   ← Revision 17 as of 2011-02-13 08:03:13 → ⇥
  Size: 1329
  Editor: Lhunath
  Comment:
-Deletions are marked like this.
+Additions are marked like this.
 Line 1:
-[[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 dirty (but not quick) way would be:
 Line 5:
-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.
+{{{
    sed -n ${n}p "$file"
}}}
-Line 7:
+Line 9:
+but this reads the whole input file, even if you only wanted the third line.

This one avoids that problem:
-Line 11:
+Line 16:
-Another way, more obvious to some, is to grab the last line from a listing of the first n lines:
+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:
-Line 16:
+Line 23:
-Using awk:
+Another approach, using [[AWK]]:
-Line 18:
+Line 25:
-   awk 'NR==n{print;exit}' file
+   awk "NR==$n{print;exit}" file
-Line 20:
+Line 27:
+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')
}}}
----
CategoryShell