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Homework and Emergencies Homework & Coursework Questions String editing using sed? awk? Post 302370938 by bakunin on Friday 13th of November 2009 01:36:45 AM
Old 11-13-2009
Carefully analyze the difference between what you have done before and what you are doing now and you will understand whats wrong yourself:

Code:
cat test1 | while read line

versus

Code:
cat test1 | read line

"cat test1" is a process, yes? It has some output, which you can observe by issuing this command, it will print the content of the file, line by line.

Now you pipe the output of this process into another process, which is set up by "while ... do .... done". To the outside this counts as one process, even if it contains several other processes inside.

In the second case you pipe the output into a completely different process, a command called "read", which tries to fill a variable with content. In the first case "read" is only invoked by the while-loop as a sub-process and is presented one line after the other which it dutifully assigns to the variable $line. In the second case it is fed the whole output of the first process at once. What is poor "read" supposed to do, hm?

bakunin
 

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CAT(1)							      General Commands Manual							    CAT(1)

NAME
cat, read, nobs - catenate files SYNOPSIS
cat [ file ... ] read [ -m ] [ -n nline ] [ file ... ] nobs [ file ... ] DESCRIPTION
Cat reads each file in sequence and writes it on the standard output. Thus cat file prints a file and cat file1 file2 >file3 concatenates the first two files and places the result on the third. If no file is given, cat reads from the standard input. Output is buffered in blocks matching the input. Read copies to standard output exactly one line from the named file, default standard input. It is useful in interactive rc(1) scripts. The -m flag causes it to continue reading and writing multiple lines until end of file; -n causes it to read no more than nline lines. Read always executes a single write for each line of input, which can be helpful when preparing input to programs that expect line-at-a- time data. It never reads any more data from the input than it prints to the output. Nobs copies the named files to standard output except that it removes all backspace characters and the characters that precede them. It is useful to use as $PAGER with the Unix version of man(1) when run inside a win (see acme(1)) window. SOURCE
/src/cmd/cat.c /src/cmd/read.c /bin/nobs SEE ALSO
cp(1) DIAGNOSTICS
Read exits with status eof on end of file or, in the -n case, if it doesn't read nlines lines. BUGS
Beware of and which destroy input files before reading them. CAT(1)
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