08-14-2014
The current standard says:
Quote:
A subshell environment shall be created as a duplicate of the shell environment, except that signal traps that are not being ignored shall be set to the default action. Changes made to the subshell environment shall not affect the shell environment. Command substitution, commands that are grouped with parentheses, and asynchronous lists shall be executed in a subshell environment. Additionally, each command of a multi-command pipeline is in a subshell environment; as an extension, however, any or all commands in a pipeline may be executed in the current environment. All other commands shall be executed in the current shell environment.
So, variable assignments in a while loop in a pipeline are allowed, but not required, to be visible in the current shell execution environment after the pipeline terminates.
The last element of a pipeline is run in the current shell execution environment in
ksh.
The last element of a pipeline is run in a subshell environment in
bash.
On this point, both are OK according to the standard.
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read(1) General Commands Manual read(1)
NAME
read - read a line from standard input
SYNOPSIS
var ...
DESCRIPTION
reads a single line from standard input. The line is split into fields as when processed by the shell (refer to shells in the first field
is assigned to the first variable var, the second field to the second variable var, and so forth. If there are more fields than there are
specified var operands, the remaining fields and their intervening separators are assigned to the last var. If there are more vars than
fields, the remaining vars are set to empty strings.
The setting of variables specified by the var operands affect the current shell execution environment.
Standard input to can be redirected from a text file.
Since affects the current shell execution environment, it is usually provided as a normal shell special (built-in) command. Thus, if it is
called in a subshell or separate utility execution environment similar to the following, it does not affect the shell variables in the
caller's environment:
Options
recognizes the following options:
Do not treat a backslash character in any special way.
Consider each backslash to be part of the input line.
Opperands
recognizes the following operands:
var The name of an existing or nonexisting shell variable.
EXTERNAL INFLUENCES
Environment Variables
determines the internal field separators used to delimit fields.
RETURN VALUE
exits with one of the following values:
0 Successful completion.
>0 End-of-file was detected or an error occurred.
EXAMPLES
Print a file with the first field of each line moved to the end of the line.
while read -r xx yy
do
printf "%s %s
" "$yy" "$xx"
done < input_file
SEE ALSO
csh(1), ksh(1), sh(1), sh-posix(1).
STANDARDS CONFORMANCE
read(1)