Difference between "Command substitution" and "Process substitution" Post: 302268243

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Top Forums Shell Programming and Scripting Difference between "Command substitution" and "Process substitution" Post 302268243 by royalibrahim on Monday 15th of December 2008 08:45:31 AM

12-15-2008

Registered User

Difference between "Command substitution" and "Process substitution"

Hi,

What is the actual difference between these two? Why the following code works for process substitution and fails for command substitution?

Code:

while IFS= read -r line; do  echo $line; done < <(cat file)

executes successfully and display the contents of the file

But,

Code:

while IFS='\n' read -r line; do  echo $line; done < $(cat file)

gives error as: bash: $(cat file): ambiguous redirect (same results for backticks syntax instead of "$()")

Also, please clarify to me whether 'IFS' is really required in this while loop as it seems to be redundant because by default read command will fetches the entire line if only one argument is supplied and no delimiter is needed here to print every line???
Is there any use case(s) available that the IFS presence is mandatory to print each and every line of a file?

Last edited by royalibrahim; 12-15-2008 at 10:00 AM..

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fc-pattern

FC-PATTERN(1)															     FC-PATTERN(1)

NAME

       fc-pattern - parse and show pattern

SYNOPSIS

       fc-pattern [ -cdVh ]  [ --config ]  [ --default ]  [  [ -f format ]  [ --format format ]  ]  [ --version ]  [ --help ]

	[ pattern  [ element... ]   ]

DESCRIPTION

       fc-pattern  parses  pattern (empty pattern by default) and shows the parsed result.  If --config is given, config substitution is performed
       on the pattern before being displayed.  If --default is given, default substitution is performed on the pattern before being displayed.

       If any elements are specified, only those are printed.

OPTIONS

       This program follows the usual GNU command line syntax, with long options starting with two dashes (`-'). A summary of options is  included
       below.

       -c     Perform config substitution on pattern.

       -d     Perform default substitution on pattern.

       -f     Format output according to the format specifier format.

       -V     Show version of the program and exit.

       -h     Show summary of options.

       pattern
	      Parses and displays pattern (uses empty pattern by default).

       element
	      If set, the element property is displayed for parsed pattern.

SEE ALSO

       FcNameParse(3)  FcConfigSubstitute(3)  FcDefaultSubstitute(3)  FcPatternPrint(3)  FcPatternFormat(3)  fc-cat(1)	fc-cache(1) fc-list(1) fc-
       match(1) fc-query(1) fc-scan(1)

       The fontconfig user's guide, in HTML format: /usr/share/doc/fontconfig/fontconfig-user.html.

AUTHOR

       This manual page was updated by Behdad Esfahbod <behdad@behdad.org>.

								   Apr 20, 2010 						     FC-PATTERN(1)