Redirect output and error to a file

New to Julia
3

As you know I think there are two things to distinguish:

  • Output generated by your code
  • Output generated by the REPL functions

The code

dtf = DataFrame(a=1:5)
dtf

generates no output. But if you type it in the REPL, your are executing another piece of code: a REPL function that evaluates your code and prints the result.

If you want to redirect output from your code, you can use Suppressor.jl, or simply something like this:

julia> open("stdout.txt", "w") do io
           redirect_stdout(io) do
               println("Hello")
           end
       end

But I don’t know if it’s possible to redirect output generated by the REPL. For that I would pipe the script to a new Julia process, so I can redirect the output from the shell.

For example, say I have a file a.jl containing the following:

using DataFrames

dtf = DataFrame([8.04   9.14  7.46   6.58
                 6.95   8.14  6.77   5.76
                 8.33   9.26  7.81   8.47], :auto)

x = 3

If I run julia a.jl in the terminal, I get no output because the code runs without a REPL and generates no output.

However on Linux for example I can run it by calling julia <a.jl >stdout.txt : this feeds the code through standard input. Apparently this is enough to make Julia start a REPL, so it prints results as you want. And the shell redirects the results to stdout.txt:

cat stdout.txt 
3×4 DataFrame
 Row │ x1       x2       x3       x4
     │ Float64  Float64  Float64  Float64
─────┼────────────────────────────────────
   1 │    8.04     9.14     7.46     6.58
   2 │    6.95     8.14     6.77     5.76
   3 │    8.33     9.26     7.81     8.47
3

To redirect both stdout and stderr, you could use julia >output.txt 2>&1, or with a recent Bash version: julia &>output.txt. Example:

$ echo 'x = y' | julia &>output.txt

$ cat output.txt 
ERROR: UndefVarError: y not defined
Stacktrace:
 [1] top-level scope
   @ none:1
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