Diff for "BashFAQ/011"

Differences between revisions 1 and 24 (spanning 23 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

In Bash 4, this can also be done with mapfile builtin:

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

By changing the value of argument for -n, you can get more than one line in the array MAPFILE.

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 1 as of 2007-05-02 22:55:48 → 
  Size: 620
  Editor: redondos
  Comment:
+   ← Revision 24 as of 2011-03-01 15:42:15 → ⇥
  Size: 1808
  Editor: sn18
  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"
}}}
but this reads the whole input file, even if you only wanted the third line.

This one avoids that problem:
-Line 10:
+Line 15:
+At line $n the command "p" is run, printing it, with a "q" afterwards: quit the program.
-Line 11:
+Line 17:
-Another way, more obvious to some, is to grab the last line from a listing of the first n lines:
+Another way, more obvious to some, is to grab the last line from a listing of the first ''n'' lines:
-Line 13:
+Line 20:
    head -n $n $file | tail -n 1
-Line 15:
+Line 22:
+Another approach, using [[AWK]]:
-Line 16:
+Line 24:
-Using awk:
-Line 18:
+Line 25:
-   awk 'NR==n{print;exit}' file
+   awk "NR==$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
}}}
In Bash 4, this can also be done with mapfile builtin:

{{{
   mapfile -ts $((n-1)) -n 1
   echo "${MAPFILE[0]}"
}}}
By changing the value of argument for -n, you can get more than one line in the array MAPFILE.

=== 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]]

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CategoryShell