how to redirect stdout to a file in NuShell?

how to redirect some command output to a file in NuShell?

In zsh/bash we can redirect output of ls command to a file as below

ls > fileList.txt

how can we do the same in NuShell?

To redirect/save into a file, use the save command (see the manual).

However, save takes strings as input, while ls provides records. You can convert those records into strings by defining how.

One way, if your primary interest lies in saving the looks, is to explicitly render the table as shown in your shell using the table command (see the manual which even lists ls | table as one of the examples):

ls | table | save fileList.csv

But if you are more interested in the data conveyed, re-format it into a datatype of your choice using to (see a list of formats). For example, to re-format it as CSV use to csv, for HTML use to html, for JSON use to json etc. There is also to text which simply lists all records line by line with their key names prepended. Its manual page even lists ls | to text as one of the examples.

ls | to csv | save fileList.csv
# or
ls | to html | save fileList.html
# or
ls | to json | save fileList.json
# or
ls | to text | save fileList.txt
# etc.

Below command saved the ls output in json format and retrieved in exact format.

ls | to json | save -f fileList.json
open fileList.json

use --force option to overwrite file.

command help save is your friend on terminal:

> help save                                                                                                                                                     06/06/2023 10:50:48 PM
Save a file.

Search terms: write, write_file, append, redirection, file, io, >, >>

Usage:
  > save {flags} <filename>

    ...more info here...
    ...more info here...

Examples:
  Save a string to foo.txt in the current directory
  > 'save me' | save foo.txt

  Append a string to the end of foo.txt
  > 'append me' | save --append foo.txt

  Save a record to foo.json in the current directory
  > { a: 1, b: 2 } | save foo.json

  Save a running program's stderr to foo.txt
  > do -i {} | save foo.txt --stderr foo.txt

  Save a running program's stderr to separate file
  > do -i {} | save foo.txt --stderr bar.txt

To update this answer based on more recent Nu (v 0.84 as I write this):
Nushell now has redirection operators: out> out+err>, similar to sh-shells 1> 2>&1 etc. Can be used as follows:

> 10 + 20 >out t.t
> cat t.t
30

Redirection operators compared to save.

• redirection out+err> (abbreviated o+e>) can capture both streams to a single file. save f.f --stderr f.f refuses to save both streams to same file.
• redirection operators do not require the preceeding | save. They are, er, postfix operators in an expression.
• redirection operators always overwrite the file. They do not have an append option.
• like save, the input for redirection operators must be string or converted to string via to json as suggested above.