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Full Discussion: tee vs output redirection confusion
Top Forums UNIX for Dummies Questions & Answers tee vs output redirection confusion Post 302299811 by c_d on Saturday 21st of March 2009 01:54:21 PM
Old 03-21-2009
Question tee vs output redirection confusion
ok,
suppose i have a file called f1
Code:
[c_d@localhost pipesnfilters]$ cat f1
this is file1
the quick brown fox jumped over the lazy dog
this is file1
who let the dogs out
this is unix
this is file1

and i have another file f2
Code:
[c_d@localhost pipesnfilters]$ cat f2
this is file2

the task is to eliminate the repeated lines in f1 and add the content of f2 into f1
so according to the task given my projected output is something like this
Code:
[c_d@localhost pipesnfilters]$ cat f1

the quick brown fox jumped over the lazy dog
this is file1
this is file2
this is unix
who let the dogs out

notice that the contents of f1 and f2 are merged into f1 and the repeated line "this is file1" has an entry only once.

Now, I have accomplished the task using the following piece of code
Code:
[c_d@localhost pipesnfilters]$ cat f1 f2 | sort -u | tee f1

the quick brown fox jumped over the lazy dog
this is file1
this is file2
this is unix
who let the dogs out

but suppose i use the following code to perform my task, i dont get the required output.
Code:
[c_d@localhost pipesnfilters]$ cat f1 f2 | sort -u >f1
[c_d@localhost pipesnfilters]$ cat f1
this is file2

All I am doing in the above is displaying contents of f1 and f2 and filtering out the repeated lines and redirecting this output to the file f1...so how come only content of file f2 is stored in f1?
i mean the output of
Code:
[c_d@localhost pipesnfilters]$ cat f1 f2 | sort -u

is
Code:
[c_d@localhost pipesnfilters]$ cat f1 f2 | sort -u


the quick brown fox jumped over the lazy dog
this is file1
this is file2
this is unix
who let the dogs out

isnt this supposed to be redirected into the file f1???
but why isnt this the case. why do i get my file f1 as
Code:
this is file2

and tee is
Code:
[c_d@localhost pipesnfilters]$ whatis tee
tee []               (1p)  - duplicate standard input
tee []               (1)  - read from standard input and write to standard output and files
tee []               (2)  - duplicating pipe content

so how come tee works but redirection does not work here?

I wish to know whats going on here.
what actually do the makers of unix mean when they say redirection and what they mean when they say duplication?

thanks in advance.
 

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