z/OS V1R9.0 UNIX System Services Command ... - Christian Grothoff

2. sh performs parameter substitution, command substitution, or arithmetic
substitution, as appropriate, in the order that the words appear on the command
line, expanding each word to a field (see the appropriate topics).
3. sh scans each field produced in step 2 for unquoted characters from the IFS
environment variable and further subdivides this field into one or more new
fields.
4. sh expands any aliases to their definitions.
5. sh performs pathname expansion on each unquoted field from step 3.
6. sh removes all quote mechanisms (\, ’, and ") that were present in the original
word unless they have themselves been quoted.
The shell considers the first field of the expanded result to be a command.
The expanded simple command can contain variable assignments and redirections.
Variable assignments affect the current execution environment. After expansion, the
shell handles all redirection constructs, and the command, if one was found, it
performs the redirection in a child shell environment (see “Shell Execution
Environments” on page 567).
When a simple command contains a command name, variable assignments in the
command affect only the execution of that command.
After the shell has expanded all appropriate arguments in a simple command, but
before it performs filename generation, it examines the command name (if the
command has one). sh first checks the names against currently defined aliases
(see the alias command) and functions (see function under “Reserved-Word
Commands” on page 552), and finally against the set of built-in commands:
commands that the shell can run directly without searching for program files. Built-in
commands are described in “Built-in Commands” on page 568.
The autoload command, an alias of typeset –fu, identifies functions which are not
yet defined. The first time an undefined function is called within the shell, the shell
will search directories in the FPATH shell variable for a file with the same name as
the function. If a matching file is found, it is assumed to contain the function
definition of the same name. The file is read and executed in the current shell
environment, storing the function in the shell’s memory for subsequent execution.
(Multiple function definitions may be contained in the same file. When the file is
processed by the shell, all the functions will be defined. Every function definition in
the file should be a link name to the file.)
If the command is a built-in or function, the shell executes it.
If the command name is not a function or a built-in command, the z/OS shell looks
for a program file or script file that contains an executable version of that command.
The shell uses the following procedure to locate the program file:
v If the command name typed to the shell has slash (/) characters in its name, the
command is taken to be a full pathname (absolute or relative). The shell tries to
execute the contents of that file.
v Otherwise, the shell performs a path search. To do this, the shell obtains the
value of the PATH environment variable. The value should be a list of directory
names. sh searches under each directory for a file, the name of which matches
the command name. If the FPATH shell variable is set, the shell will search the
PATH and FPATH directories. If a file with a name matching the command name
is found in the same directory in both PATH and FPATH, or if a matching file is
sh
Chapter 2. Shell command descriptions 557