There are numerous Functions built into REXX. The following reference section lists the built-in Functions you'll find in most interpreters. This reference lists the Functions by type, for example, all of the Functions that operate upon files or directories are listed on one page named "Files and Directories".

For each Function, a short description is given of its purpose.

Below this, its template is shown. The template lists the arguments passed to the Function, and gives a description of the purpose of each argument. Arguments that you must specify are listed in blue and arguments that you may specify or omit are listed in red. You can replace those arguments in the template with whatever items you wish to pass, be it the value of some variable, a literal string, the return value of another Function, etc.

Below the template, the return value of the Function is described.

Below this are any notes about the Function.

Finally, there is a chart showing several examples of calling the Function with various example arguments (literal strings in these examples), and the return value for each example is shown on the right-hand side of the chart.

Operations upon words

Operate upon a string that contains one or more words. Each word (within the string) is separated from the other words by one or more space characters (or any other character regarded as a space such as a TAB character, newline character, form feed, etc).
Operations upon strings
Operate upon strings (ie, the values of variables, return strings from other functions, literal strings, etc) which may contain any sort of data.
Math operations
Operate upon numeric values (numeric strings and numeric constants) to perform mathematical operations.
Conversions
Operate upon strings, converting them from one "type" to another, for example, converting a string containing a hexadecimal value into a string containing a decimal numeric string for use in math expressions. Or, you can convert a numeric string to an ASCII character, and vice versa.
Files and Directories
Create/read/write a file on a permanent media such as a hard drive or to read/write data to devices that don't permanently store their data such as the console (window) or serial port or some pipes. Also, create/delete directories, get information about drives/directories/files (such as listing the contents of a directory). Also, filename manipulation such as dealing with wildcards in filenames, or making a temporary filename. Also, finding the location of special directories such as the place where program files reside.
Error/Debug
Deal with error conditions (such as SYNTAX, HALT, NOVALUE, ERROR, FAILURE, and NOTREADY), or get error messages from an error number, or debug scripts.
System Information
Return information from the operating system, such as the current time and date, or the value of some environment variable.
Args/Variables
Deal with arguments/variables, such as retrieving arguments passed to your script, or checking whether a variable has a numeric value or has been assigned a value.
Add-on (External) Functions
Deal with Add-on Functions (ie, functions written in other languages which can be made available to be called from your script). Such functions are contained in Dynamic Link Libraries (ie, DLLs).
Miscellaneous
A variety of "odds and ends".