How can I retrieve environment variables in COBOL?

What are environment variables?

An environment variable is a value stored by the operating system and accessible by programs.

They are used to define an application's execution context:

• server name
• path to a database
• operating mode (DEV / PROD)
• etc...

In COBOL, you can access them using the ACCEPT instruction.

Reading an environment variable in COBOL

Here's a simple example to retrieve the environment variable APP_MODE:

       IDENTIFICATION DIVISION.
       PROGRAM-ID. EnvExample.

       DATA DIVISION.
       WORKING-STORAGE SECTION.
       01  APP-MODE       PIC X(10).

       PROCEDURE DIVISION.
           ACCEPT APP-MODE FROM ENVIRONMENT "APP_MODE"
           DISPLAY "Mode d'exécution : " APP-MODE
           STOP RUN.

• ACCEPT allows you to read an external input.
• By specifying FROM ENVIRONMENT "APP_MODE", we ask COBOL to read the corresponding environment variable.
• The value is stored in the APP-MODE variable.

Practical example

Let's imagine you want to execute certain instructions depending on the environment:

       IF APP-MODE = "DEV"
           DISPLAY "Connexion à la base de test..."
       ELSE
           DISPLAY "Connexion à la base de production..."
       END-IF.

But before running your program, you can set the variable according to the context:

Mode d'exécution : DEV
Connexion à la base de test...

Handling special cases

Numeric values

One thing to remember is that an environment variable is always passed to a program as a string. It's possible to convert it automatically in COBOL, by declaring it as a numeric value, like this with port which would equal 3000:

01 PORT PIC 9(4).
ACCEPT PORT FROM ENVIRONMENT "PORT"

But this isn't recommended.

If your variable mistakenly contains a value that can't be parsed, then there's a good chance your program will crash (depending on your environment).

It's better to read an alphanumeric value, then check and convert it, like this:

01 PORT-STR PIC X(10).
01 PORT     PIC 9(4).

ACCEPT PORT-STR FROM ENVIRONMENT "PORT"

IF PORT-STR IS NUMERIC
    MOVE PORT-STR TO PORT
ELSE
    DISPLAY "PORT invalide"
END-IF

Empty values

If an environment variable is read, but it's empty (or simply not defined), then your program will only receive a set of spaces equivalent to the length of your variable.

To check whether your variable is defined or not, and assign it a default value, all you need to do is this:

ACCEPT APP-MODE FROM ENVIRONMENT "APP_MODE"
IF APP-MODE = SPACES
    MOVE "DEV" TO APP-MODE
DISPLAY "Mode d'exécution : " APP-MODE
STOP RUN.

The SPACES constant is a "figurative" constant, which adapts to the context, and corresponds to the number of spaces needed to fill your variable.