Environment variables

Get with status

Get_environment_variable copies a variable such as APP_MODE into a string. Length reports the true size and status reports problems such as a missing name. Test status before trusting the value.

program env_get implicit none character(len=256) :: val integer :: L, stat call get_environment_variable("APP_MODE", val, length=L, status=stat) if (stat == 0) write(*, "(a)") val(1:L) end program env_get

Defaults

Most settings need a fallback when the variable is absent. Assign the default whenever status is nonzero, then continue as normal. The program then runs both on servers with full config and on plain laptops.

program env_default implicit none character(len=256) :: val integer :: L, stat call get_environment_variable("APP_MODE", val, length=L, status=stat) if (stat /= 0) val = "standard" write(*, "(a)") trim(val) end program env_default

Query length first

Fixed buffers can truncate long values, so query the length first. Call once with only length and status, allocate exactly that size, then call again to fetch the text. Deferred length strings fit any value with no waste.

program env_len implicit none character(len=:), allocatable :: val integer :: L, stat call get_environment_variable("APP_MODE", length=L, status=stat) if (stat == 0) then allocate(character(len=L) :: val) call get_environment_variable("APP_MODE", val) end if end program env_len

Env-driven config

Environment driven config reads numbers and flags at startup with zero files to ship. Parse each value with an internal read and keep a default for every failure path. Document each name in the usage text so operators know what to export.

program env_cfg implicit none character(len=256) :: buf integer :: nx = 100 integer :: L, stat, ios call get_environment_variable("NX", buf, length=L, status=stat) if (stat == 0) read(buf(1:L), *, iostat=ios) nx print *, nx end program env_cfg

Next: error stop

Article author: Arthur Isaev