每两行显示一行（osX）zsh

Question

I'd to display every two line, a line from a file. I've seen the sed -n 'f~d' awk and per l method. But the sed one doesn't work on osX (As I understood) and the two others are are interpreted languages which i can't use.

Can you help me ?

Here's an exemple :

output before :

-rw-r--r--  1 mfassi-f  2013  22 Jul 17 12:36 test.sh
-rw-r--r--  1 mfassi-f  2013  29 Jul 17 12:30 test1.sh
-rw-r--r--  1 mfassi-f  2013  22 Jul 17 12:36 test2.sh
-rw-r--r--  1 mfassi-f  2013  29 Jul 17 12:30 test3.sh
-rw-r--r--  1 mfassi-f  2013  22 Jul 17 12:36 test4.sh
-rw-r--r--  1 mfassi-f  2013  29 Jul 17 12:30 test5.sh
-rw-r--r--  1 mfassi-f  2013  22 Jul 17 12:36 test6.sh

output after :

-rw-r--r--  1 mfassi-f  2013  29 Jul 17 12:30 test1.sh
-rw-r--r--  1 mfassi-f  2013  29 Jul 17 12:30 test3.sh
-rw-r--r--  1 mfassi-f  2013  29 Jul 17 12:30 test3.sh
-rw-r--r--  1 mfassi-f  2013  29 Jul 17 12:30 test5.sh

Answer 1

Here are two answers. One for a file, and one for command-line input.

['cause the question's changed ever so slightly, but these two seemed too similar to put as independent answers].

You can use zsh , ls , cut and paste to do this in a for loop. It's not the cleanest solution, but it does work (surprisingly).

for file in `ls -1 | paste - - | cut -f 1`
do
   ls -l -d $file
done

We take the output of ls -1 , then extract every second filename. (The way ls chooses to sort the files will have an impact here). Then, we do ls -l -d on each of these files. -d is necessary to stop ls from showing us the contents of $file , if $file is a directory. (Not sure if this is OS X specific, or if that's default POSIX ls behaviour).

---

Second answer: display every second line from a file.

If you're after a mostly zsh solution, you could do something like the following:

$ jot 8 0 7 >> sample.txt # Generate some numbers. 
$ count=0                 # Storage variable
$ for i in `cat sample.txt` 
do
if [ $(( $count % 2 )) -eq 0 ] ; then
echo $i
fi
count=`expr $count + 1`
done

This displays every second line.

Notes:
- This leaves a variable count in your session afterwards (it's not clean).
- This fails badly if sample.txt does not contain a single word per line.
- I'm almost sure that the modulus comparison I do isn't the most efficient: I grabbed it from here .
- I say it's mostly zsh because it does rely on cat , but I'm not sure how to avoid that.

Answer 2

The OS X version of sed is frustrating. Using sed -n '0~2p' <filename> doesn't work because, in the BSD sed, -n does something different:

-n
By default, each line of input is echoed to the standard output after all of the commands have been applied to it. The -n option suppresses this behavior.

I'd highly recommend installing GNU sed, which can be done using Homebrew :

brew install gnu-sed

And then you can use:

gsed -n '0~2p' filename # Display the 2nd, 4th etc
gsed -n '1~2p' filename # Display the 1st, 3rd etc.