pipes

[azertyazerty]

Hi guys

I couldn't find anything on the forum related to this:

One of the questions on an exam Informatics at Ghent University a few years ago consisted of writing one command (pipes, I/O-redirection allowed) that we have to put in a script named pipe4. As I have my exam monday, I'm preparing myself by making the old ones, but got stuck with this question.

$ echo studenten.txt 2 | pipe4 | xargs echo "Tweede regel:" 
   Tweede regel: 00600248 Desmet Guillaume

studenten.txt is of the form:

$ cat studenten.txt
...
20053544 De Wolf Kristof
20053680 Lens Yoeri
20043436 Ronsyn Tim
20045050 Tchebakov Andreï
20045090 El-Cherkaoui Rajaa
...

The argument 2 means that the second line (in dutch: Tweede regel) need to be written to stdout and does not change.

Any suggestions?
Thanks!

[Cabhan]

So your pipe4 script receives as input a file and a number, and must output that number line of the file?

What are you confused about? What isn't making sense to you? There are a number of ways to accomplish this: what are you doing so far?

[azertyazerty]

The script recieves a filename and always the number 2 (in dutch, "Second line" means "Tweede regel") as input.

As a matter of fact, there are many solutions to achieve this, and I found a solution, but it's not a "one-line-command". So now I have

pipe4:

#! /bin/bash
sed 's/\([^ ]*\) 2/cat \1 | sed -n 2p/'

which gives me

cat studenten.txt | sed -n 2p

Now I want to execute this command, that's my problem.
I know there's no need to put it in a script, but this was the assignment.

[miafl]

Maybe, this would serve your needs:

[1607]user@host:/tmp$ cat studenten.txt 
20053544 De Wolf Kristof
20053680 Lens Yoeri
20043436 Ronsyn Tim
20045050 Tchebakov Andreï
20045090 El-Cherkaoui Rajaa
[1608]user@host:/tmp$ cat pipe4 
read FILE RECORD && sed -n "${RECORD}p" "$FILE"
[1609]user@host:/tmp$ echo studenten.txt 2 | ./pipe4 | xargs echo "Tweede regel:"
Tweede regel: 20053680 Lens Yoeri
[1610]user@host:/tmp$ ^2^3
echo studenten.txt 3 | ./pipe4 | xargs echo "Tweede regel:"
Tweede regel: 20043436 Ronsyn Tim
[1611]user@host:/tmp$ exit

[azertyazerty]

Maybe, this would serve your needs:

[1607]user@host:/tmp$ cat studenten.txt 
20053544 De Wolf Kristof
20053680 Lens Yoeri
20043436 Ronsyn Tim
20045050 Tchebakov Andreï
20045090 El-Cherkaoui Rajaa
[1608]user@host:/tmp$ cat pipe4 
read FILE RECORD && sed -n "${RECORD}p" "$FILE"
[1609]user@host:/tmp$ echo studenten.txt 2 | ./pipe4 | xargs echo "Tweede regel:"
Tweede regel: 20053680 Lens Yoeri
[1610]user@host:/tmp$ ^2^3
echo studenten.txt 3 | ./pipe4 | xargs echo "Tweede regel:"
Tweede regel: 20043436 Ronsyn Tim
[1611]user@host:/tmp$ exit

Thanks! You really helped me out.
After reading you're solution I've found the exact one:

sed "s/^\(.*\) \([^ ]*\)/\sed -n \2p \1/" | bash -

[miafl]

Perhaps, I do not fully understand what exactly it is you're trying to achieve. But if executing a command from a string is what you need, you might also want to consider Bash's built-in eval function.

[azertyazerty]

Perhaps, I do not fully understand what exactly it is you're trying to achieve. But if executing a command from a string is what you need, you might also want to consider Bash's built-in eval function.

eval makes things even simpeler, thanks again!