Each time a command is executed, the substitutions described above are carried out in the following order:
quoting
parameter substitution
tilde substitution
aliasing
file name generation
I/O redirection
command substitution
If the command name matches the name of one of the built-in commands, it is executed within the current POSIX shell process. Next, the command name is checked to see if it matches one of the user-defined functions. If it does, the positional parameters are saved and then reset to the arguments of the function call. When the function completes or executes a built-in return command, the positional parameter list is restored and any trap set on EXIT within the function is executed. The exit status is that of the last command executed in the function. A function is also executed in the current POSIX shell process. If a command name is not a built-in command or a user-defined function, a process is spawned and an attempt is made to execute the command via the system call exec.
The shell variable PATH defines the search path for the directory containing the command. Alternative directory names are separated by a colon (:). The default path is /usr/bin: (specifying /usr/bin and the current directory in that order). The current directory can be specified by two or more adjacent colons, or by a colon at the beginning or end of the path list. If the command name contains a slash, the search path is not used. Otherwise, each directory in the path is searched for an executable file. If the file has execute permission but is not a directory or an a.out file, it is assumed to contain a shell script. A subshell is spawned to read it. Any non-exported aliases, functions and variables are removed in this case. A parenthesized command is executed in a sub-shell without removing non-exported quantities.