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Full Discussion: Identifying the commands creating subshells
Top Forums UNIX for Dummies Questions & Answers Identifying the commands creating subshells Post 302594131 by Corona688 on Monday 30th of January 2012 02:08:30 PM
Old 01-30-2012
Not a all processes are shells, and none of those create subshells.

The cat | wc -l creates a useless cat.

If you pipe into shell commands, that creates a subshell, i.e.

Code:
cat file | while read LINE
do
...
done

better written as
Code:
while read LINE
do
...
done < file

The general idea is to use as few pipes and use as few external programs as possible. Someone might write a slow-performing loop like this:

Code:
while read LINE
do
        TOKEN1=`echo "$LINE" | awk '{ print $1 }'`
        TOKEN2=`echo "$LINE" | awk '{ print $2 }'`
        TOKEN3=`echo "$LINE" | awk '{ print $3 }'`
        TOKEN4=`echo "$LINE" | awk '{ print $4 }'`
done < file

which creates four processes per loop just to do something read can do as a builtin:

Code:
while read TOKEN1 TOKEN2 TOKEN3 TOKEN4
do
...
done < file

In summary, pipes create extra processes or subshells, external programs create extra processes, and grouping ( ) create subshells, i.e.
( echo foo ; echo bar ) | myprogram
This User Gave Thanks to Corona688 For This Post:
pandeesh
 

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LEARN ABOUT FREEBSD

echo

ECHO(1) 						    BSD General Commands Manual 						   ECHO(1)

NAME

     echo -- write arguments to the standard output

SYNOPSIS

     echo [-n] [string ...]

DESCRIPTION

     The echo utility writes any specified operands, separated by single blank (' ') characters and followed by a newline ('
') character, to the
     standard output.

     The following option is available:

     -n    Do not print the trailing newline character.

     The end-of-options marker -- is not recognized and written literally.

     The newline may also be suppressed by appending 'c' to the end of the string, as is done by iBCS2 compatible systems.  Note that the -n
     option as well as the effect of 'c' are implementation-defined in IEEE Std 1003.1-2001 (``POSIX.1'') as amended by Cor. 1-2002.  For porta-
     bility, echo should only be used if the first argument does not start with a hyphen ('-') and does not contain any backslashes ('').  If
     this is not sufficient, printf(1) should be used.

     Most shells provide a builtin echo command which tends to differ from this utility in the treatment of options and backslashes.  Consult the
     builtin(1) manual page.

EXIT STATUS

     The echo utility exits 0 on success, and >0 if an error occurs.

SEE ALSO

     builtin(1), csh(1), printf(1), sh(1)

STANDARDS

     The echo utility conforms to IEEE Std 1003.1-2001 (``POSIX.1'') as amended by Cor. 1-2002.

BSD
								 November 12, 2010							       BSD
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