Shell Programming and Scripting

Hi All,

i m facing very strange problem . like suppose i am in one directory and then i type ls
it will display the content of directory then i redirect the output as

Code:

sneha-pardus $   ls > test
sneha-pardus $   cat test
a.sh b.sh c.sh
sneha-pardus $ awk ' { print $1 } ' test > test
sneha-pardus $ cat test
sneha-pardus $

it shows nothing why can't we use same file in awk and then for redirection

please help me regarding this

Hi.

You're trying to write to the same file you are reading.

Given that redirection happens first, and that:

Code:

> temp awk ......

is much the same as

Code:

awk .... > temp

you can see the dilemma.

If you only want the first file from an ls, why not just get the first file into the output file in one step?

i.e.

Code:

ls | sed 1q > test

hi scottn

can u please explain me the above code which u wrote and one more thing i am trying to redirect the output so first awk will do it process then whatever the result be it would be redirected to the same file

please correct if i am wrong

Hi.

If awk had finished reading the file completely before you tried to write to it, you'd be quids in!

Given that the redirection happens before the command you are running (regardless of where you place it), i.e.

Code:

> test awk ...

is the same as

Code:

awk ... > test

you have destroyed the input file before you even had a chance to read it.

As for the sed command, it prints the first line (file, in this case - as you ran an ls), then exits.