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−-*- mode: outline -*-
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−
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−* Download POSIX.
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−
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−  http://www.opengroup.org/onlinepubs/009695399/download
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−                download page for SYSV3
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−
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−  http://www.opengroup.org/onlinepubs/9699919799/download
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−                download page for SYSV4
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−
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−* Shell command.
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−
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−** command.
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−
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−Main semantic is to invoke command instead defined function with same name:
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−
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−  $ ls() { ls --color "$@"; }
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−  $ ls                          # infinitely loop as func invoke itself
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−  $ ls() { command ls --color "$@"; }
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−  $ ls .                        # invoke /bin/ls with color output
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−  $ unset ls                    # forget func definition
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−  $ ls                          # invoke /bin/ls without color output
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−
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−With '-p' arg Perform the command search using a default value for PATH that
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−is guaranteed to find all of the standard utilities:
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−
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−  $ command -p getconf PATH
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−
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−
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−With '-v' arg work like 'which' command which not included in POSIX:
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−
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−  $ command -v ls
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−/usr/bin/ls
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−  $ command -v echo             # for build-in command print command itself
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−echo                            # you can check for equality or for slash to
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−                                # distinguish from utilities