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Written by Oleksandr Gavenko (AKA gavenkoa), compiled on 2017-01-30 from rev ccaa2f364422+.

POSIX

Download POSIX

http://www.opengroup.org/onlinepubs/009695399/download
download page for SYSV3
http://www.opengroup.org/onlinepubs/9699919799/download
download page for SYSV4

Shell command

command

Main semantic is to invoke command instead defined function with same name:

$ ls() { ls --color "$@"; }
$ ls                          # infinitely loop as func invoke itself
$ ls() { command ls --color "$@"; }
$ ls .                        # invoke /bin/ls with color output
$ unset ls                    # forget func definition
$ ls                          # invoke /bin/ls without color output

With '-p' arg Perform the command search using a default value for PATH that is guaranteed to find all of the standard utilities:

$ command -p getconf PATH

With '-v' arg work like 'which' command which not included in POSIX:

$ command -v ls
/usr/bin/ls
$ command -v echo             # for build-in command print command itself
echo                          # you can check for equality or for slash to
                              # distinguish from utilities