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BASIC's file I/O dates from the 1960s — surprisingly clean.

Sequential read

DIM line_buf AS STRING
OPEN "data.txt" FOR INPUT AS #1
DO WHILE NOT EOF(1)
    LINE INPUT #1, line_buf
    PRINT line_buf
LOOP
CLOSE #1

The #1 is a file number — like a file descriptor. Pick any number 1-255.

Sequential write

OPEN "output.txt" FOR OUTPUT AS #1     ' truncates existing
PRINT #1, "hello"
PRINT #1, "world"
CLOSE #1

OPEN "output.txt" FOR APPEND AS #1     ' append mode
PRINT #1, "appended"
CLOSE #1

Random / binary access

OPEN "data.bin" FOR BINARY AS #1
DIM buf AS STRING * 256
GET #1, , buf                          ' read 256 bytes
CLOSE #1

The STRING * 256 creates a 256-byte fixed-length string.

File modes summary

• INPUT — read text
• OUTPUT — write (truncate)
• APPEND — write at end
• BINARY — read+write raw bytes
• RANDOM — fixed-record-size files (legacy)

Error handling

DIM error_code AS INTEGER
OPEN "missing.txt" FOR INPUT AS #1
IF ERR <> 0 THEN
    PRINT "can't open: error "; ERR
END IF

The ERR function returns the last error code (0 = no error).

For cleaner handling:

IF (FREEFILE = 0) THEN
    PRINT "no free file numbers"
ELSE
    DIM f AS INTEGER = FREEFILE     ' get an unused number
    OPEN "data.txt" FOR INPUT AS #f
    ' ...
    CLOSE #f
END IF

FREEFILE returns the next available file number — avoids manually tracking.

stdin / stdout

For scripts:

• INPUT "", var reads a line and parses
• LINE INPUT "", var reads a raw line (no parsing)
• PRINT writes to stdout

EOF(0) does not work on piped input

EOF(0) asks the console device whether more input is waiting, and that is the right question only when a person is typing. When the grader pipes a file into your program, file number 0 is not the console and EOF(0) is true before you have read a single line - so a DO WHILE NOT EOF(0) loop simply never runs. Open the console device yourself and loop on its own file number:

DIM f AS INTEGER = FREEFILE
OPEN CONS FOR INPUT AS #f
DO WHILE NOT EOF(f)
    LINE INPUT #f, line_buf
    ' ... one line per pass ...
LOOP
CLOSE #f

Measured on the grader with three lines piped in: the EOF(0) loop counts 0, the EOF(f) loop counts 3.

OS commands

DIM exit_code AS INTEGER
exit_code = SHELL("ls -la")     ' run external command

FreeBASIC additions

FreeBASIC has more modern functions:

• FILEEXISTS(path) — does file exist?
• KILL path — delete file
• NAME old_path AS new_path — rename
• MKDIR path, RMDIR path
• CHDIR path, CURDIR()

Environment variables

DIM home AS STRING = ENVIRON("HOME")    ' or "USERPROFILE" on Windows
PRINT home

Why file I/O in BASIC

• BASIC was made for hobbyist programs that read/write text files
• The PRINT/INPUT pairing makes simple text I/O dead easy
• Game saves, configuration files, log files — straightforward

For structured I/O (XML, JSON), use FreeBASIC libraries or roll your own parser. The standard library doesn't have JSON support built-in.