open establishes access to a file, command, or other stream and returns an IO object. Use that object with functions such as print, println, readuntil, or join; close it when the operation is complete. section 5.0 section 5.5
The same function supports several practical I/O patterns:
- Call open(path) to open a file for reading; pass "w" to open it for writing. section 17.4.2
- Pass a do-end function so Julia receives the IO object and closes it automatically afterward. section 17.4.2
- Call open on a command object to execute a command and receive a Process connection whose output can be read like a file. section 17.2
Save structured rows to a file
function save_engines(path::AbstractString, engines) open(path, "w") do io println(io, "name,company,mass,thrust,Isp") for (name, egn) in engines row = [name, "", egn.mass, egn.thrust, egn.Isp] join(io, row, ',') println(io) end end end The IO argument directs println and join to the file instead of standard output, while the do-end form handles closing. section 17.4.2
shell> grep "Rocket Lab" rocket-engines.csv Curie,Rocket Lab,0.008,0.12,317 Rutherford,Rocket Lab,0.035,25,311 julia> io = open(`grep "Rocket Lab" rocket-engines.csv`); julia> readuntil(io, ',') "Curie" julia> readuntil(io, ',') "Rocket Lab" julia> readuntil(io, ',') "0.008" julia> position(io) ERROR: MethodError: no method matching position(::Base.Process) julia> close(io)
Inspect an interactive example where open runs grep, returns a Process connection, and lets readuntil consume the command output. section 17.2
