Pipelines

The pipeline special variable $in

Best practices for pipeline commands

Interaction with Unix pipes

Handling stdout and stderr

You can handle stderr in multiple ways:

1. Do nothing, stderr will be printed directly
2. Pipe stderr to the next command, using e>| or e+o>|
3. Redirect stderr to a file, using e> file_path, or e+o> file_path
4. Use do -i { cmd } | complete to capture both stdout and stderr as structured data

For the next examples, let's assume this file:

# demo.nu
print "foo"
print -e "barbar"

It prints foo to stdout and barbar to stderr. The following table illustrates the differences between the different redirection styles:

Redirection to a pipeline:

type	command	$result contents	printed to terminal
|	let result = nu demo.nu | str upcase	"FOO"	"barbar"
e>|	let result = nu demo.nu e>| str upcase	"BARBAR"	"foo"
o+e>|	let result = nu demo.nu e+o>| str upcase	"FOO\nBARBAR"	nothing

Redirection to a file:

type	command	file.txt contents	printed to terminal
o> file_path	nu demo.nu o> file.txt	"foo"	"barbar"
e> file_path	nu demo.nu e> file.txt	"barbar"	"foo"
o+e> file_path	nu demo.nu o+e> file.txt	"foo/nbarbar"	nothing

complete command:

type	command	$result contents
use complete	let result = do { nu demo.nu } | complete	record containing both stdout and stderr

Note that e>| and o+e>| only work with external command, if you pipe internal commands' output through e>| and o+e>|, you will get an error:

❯ ls e>| str length
Error:   × `e>|` only works with external streams
   ╭─[entry #1:1:1]
 1 │ ls e>| str length
   ·    ─┬─
   ·     ╰── `e>|` only works on external streams
   ╰────

❯ ls e+o>| str length
Error:   × `o+e>|` only works with external streams
   ╭─[entry #2:1:1]
 1 │ ls e+o>| str length
   ·    ──┬──
   ·      ╰── `o+e>|` only works on external streams
   ╰────

You can also redirect stdout to a file, and pipe stderr to next command:

nu demo.nu o> file.txt e>| str upcase
nu demo.nu e> file.txt | str upcase

But you can't use redirection along with o+e>|, because it's ambiguous:

nu demo.nu o> file.txt o+e>| str upcase

Also note that complete is special, it doesn't work with e>|, o+e>|.