which

Locate a program file in the user’s path.

For each of its arguments which prints to stdout the full path of the executable(s). It does this by searching the directories listed in the environment variable PATH.

Syntax
 which [options] [--] program_name [...]
Options
 --all, -a
 Print all matching executables in PATH, not just the first.
 --read-alias, -i
 Read aliases from stdin, reporting matching ones on stdout.
 This is useful in combination with using an alias for which itself.
 For example alias which='alias | which -i'.
 --skip-alias
 Ignore option --read-alias, if any. This is useful to explicity search
 for normal binaries, while using the --read-alias option in an alias for which.
 --skip-dot
 Skip directories in PATH that start with a dot.
 --skip-tilde
 Skip directories in PATH that start with a tilde and
 executables which reside in the HOME directory.
 --show-dot
 If a directory in PATH starts with a dot and a matching 
 executable was found for that path, then print
 "./program_name" rather than the full path.
 --show-tilde
 Output a tilde when a directory matches the HOME directory.
 This option is ignored when which is invoked as root.
 --tty-only
 Stop processing options on the right if not on tty.
 --version, -v, -V
 Print version information on standard output then exit successfully.
 --help
 Print usage information on standard output then exit successfully.
RETURN VALUE
 Which returns the number of failed arguments, or -1 when
 no programname was given.
EXAMPLE
 A useful way to use this utility is by adding an alias for
 which like the following:
 alias which='which --tty-only --show-tilde --show-dot'
 This will print the readable ~/ and ./ when starting which
 from your prompt, while still printing the full path when
 used from a script:
 > which q2
 ~/bin/q2
 > echo 'which q2'
 /home/carlo/bin/q2
 Aliases are also supported, through the use of an alias
 for which itself. An example alias for which that is
 using this feature is as follows:
 alias which='alias | which --tty-only --read-alias --show-tilde --show-dot'
 This will print the output of alias for each alias that
 matches one of the given arguments. For example, using
 this alias on itself in a tcsh:
 $ alias which alias \| /usr/bin/which -i !\*
 $ which which
 which (alias | ./which -i !*)
 /usr/bin/which
BUGS
 The HOME directory is determined by looking for the HOME environment variable,
 which exits when this variable doesn’t exist.
 Which will consider two equivalent directories to be different when one of
 them contains a path with a symbolic link.

Some shells may provide a builtin which command which is similar or identical to this utility.

"A man should never be ashamed to own he has been in the wrong, which is saying in other words, that he is wiser today than he was yesterday" ~ Alexander Pope (thoughts on various subjects)

Related Linux commands

pwd - Print Working Directory.
hash - Remember the full pathname of a name argument.
pathchk - Check file name portability.
whereis - Search the user’s $path, man pages and source files for a program.
Equivalent Windows commands: WHERE - Locate files /(get-command $file).Definition