ok i need help understanding the following points in the command by radoulov
1.why was rm f1 issued and then && applied?
You need to remove the f1 file to protect its content from the following cat/sort pipeline redirection.
The shell will read a block of data, process the lines in that data, and the converted lines will be written as a block. When you remove the file its content remains available on the disk until the file is closed and you can happily attach the cat/sort pipeline standard output to the new file f1.
Consider the following:
Code:
% printf '%-100000s\n' f1_line{1..3}>f1
% { rm f1 && cat - |sort > f1;}< f1
% wc -l f1
3 f1
% { cat - |sort > f1;}< f1
% wc -l f1
1 f1
Quote:
2.which redirection has precedence over the other? < one first , or > one first? according to me output redirection(>) should run first because, its inside the { } ... but i dont think i am right...
A subshell is started with its standard input connected to f1.
The subshell runs the rm command (it happens to have its stdin connected
to f1, but this doesn't matter as rm ignores the input).
A new f1 is created and attached to the standard output of the pipeline, so then the pipeline has the
old f1 on its standard input, and the new one on its standard output. Notice that this:
Code:
% cat f1 && sort f1 > f1
a
b
c
... is different than this:
Code:
% cat f1|sort > f1
% wc -l f1
0 f1
Quote:
[...]
I am not really sure of that though...because replacing && with ; also works.
Yes, the ; works, but in some circumstances you can get an inconsistent result. Consider the following:
Code:
% head f[12]
==> 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
==> f2 <==
this is file2
% chmod u-w .
% { rm f1 && cat - f2 | sort -u > f1;} <f1
rm: cannot remove `f1': Permission denied
% wc -l f1
6 f1
% { rm f1; cat - f2 | sort -u > f1;} <f1
rm: cannot remove `f1': Permission denied
% wc -l f1
1 f1
Hope this helps.
Last edited by radoulov; 03-22-2009 at 09:03 AM..
Reason: example adjusted
i was think of this again and i thought of this one
Code:
[c_d@localhost pipesnfilters]$ cat f1 f2 | sort -u 1>f1
[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
[c_d@localhost pipesnfilters]$
any advise on this ? any cases where it could fail?
EDIT: now i see that in your above post you gave the same answer...why would it fail ? and in what cases?
1. Begin executing the cat command reading a chunk of data from its first argument or STDIN (a limited number of bytes that varies from system to system) and passing that data through the pipe to the sort command.
2. Begin executing the sort command: first set up the redirection (f1 gets truncated) then wait for the entire input to come because of the nature of the ordering.
These too commands run in parallel.
Once the cat command has read and passed the first chunk of data, it will try to read the next one and in the case of f1 it won't be available anymore because of the following redirect.
Consider the following:
Code:
% printf 'f1%1000s\n' {1..1000} > f1
% printf 'f2%1000s\n' {1..1000} > f2
% wc f[12]
1000 2000 1003000 f1
1000 2000 1003000 f2
2000 4000 2006000 total
% cat f[12] | sort -u | wc -l
2000
% cat f[12] | sort -u > f1
% wc -l f1
1000 f1
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03-22-2009
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