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Full Discussion: Peculiar behavior due to IFS
Operating Systems Linux Peculiar behavior due to IFS Post 302775491 by RudiC on Tuesday 5th of March 2013 01:36:02 AM
Old 03-05-2013
Quote:
Originally Posted by alister
Quote:
Originally Posted by RudiC Image
Does it? I don't think so. The | char opens a pipe and makes the shell wait for the next command in the pipe.
The shell command parsing step which looks for pipes has already concluded by the time parameter expansion occurs. There's nothing special about a pipe character in a variable. You'd have to use eval to make it so.
The command that was given in post#1 was
Code:
aa=|
echo $aa

Executing this on the two systems I have at hand (Linux, FreeBSD) issues the secondary prompt (asking for the next command in pipe, I assume). The variable aa is not assigned to, it is not even defined afterwards. So I think my statement holds.
Of course, assigning an escaped | is totally different and absolutely OK.
 

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pipe(2) 							System Calls Manual							   pipe(2)

NAME
pipe() - create an interprocess channel SYNOPSIS
DESCRIPTION
creates an I/O mechanism called a pipe and returns two file descriptors, fildes[0] and fildes[1]. fildes[0] is opened for reading and fildes[1] is opened for writing. A read-only file descriptor fildes[0] accesses the data written to fildes[1] on a first-in-first-out (FIFO) basis. For details of the I/O behavior of pipes see read(2) and write(2). By default, HP-UX pipes are not STREAMS-based. It is possible to generate the kernel so that all pipes created on a system are STREAMS- based. This can only be done for HP-UX releases 10.0 and later. STREAMS-based FIFOs (created by or are not supported on HP-UX. To generate a kernel that supports STREAMS-based pipes: o STREAMS/UX must be installed. o The module and the driver must be included in the file. (When STREAMS/UX is installed, and are automatically added to the system file.) o The tunable parameter (see streampipes(5)) must be set to 1 in the file. (This is not automatically done when STREAMS/UX is installed.) o The kernel must be generated and the system rebooted. Once this is done, all pipes created by will be STREAMS-based. For more information, see EXAMPLES
The following example uses to implement the command string RETURN VALUE
returns one of the following values: Successful completion. Failure. is set to indicate the error. ERRORS
sets to one of the following error values if the corresponding condition is true. or more file descriptors are currently open. The system file table is full. The file system lacks sufficient space to create the pipe. Could not allocate resources for both Stream heads (STREAMS-based pipes only). SEE ALSO
sh(1), read(2), write(2), popen(3S), privileges(5), streampipes(5), streamio(7). STANDARDS CONFORMANCE
pipe(2)
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