[BUG] `npm completion` errors when run on cygwin terminals

Is there an existing issue for this?

• I have searched the existing issues

This issue exists in the latest npm version

• I am using the latest npm

Current Behavior

In a cygwin terminal:

$ npm completion
npm error code ENOTSUP
npm error npm completion supported only in MINGW / Git bash on Windows
npm error Log files were not written due to an error writing to the directory: C:\Users\User\AppData\Local\npm-cache\_logs
npm error You can rerun the command with `--loglevel=verbose` to see the logs in your terminal

Expected Behavior

I would expect this behavior:

$ npm completion
###-begin-npm-completion-###
#
# npm command completion script
#
# Installation: npm completion >> ~/.bashrc  (or ~/.zshrc)
# Or, maybe: npm completion > /usr/local/etc/bash_completion.d/npm
#

if type complete &>/dev/null; then
  _npm_completion () {
    local words cword
    if type _get_comp_words_by_ref &>/dev/null; then
      _get_comp_words_by_ref -n = -n @ -n : -w words -i cword
    else
      cword="$COMP_CWORD"
      words=("${COMP_WORDS[@]}")
    fi

    local si="$IFS"
    if ! IFS=$'\n' COMPREPLY=($(COMP_CWORD="$cword" \
                           COMP_LINE="$COMP_LINE" \
                           COMP_POINT="$COMP_POINT" \
                           npm completion -- "${words[@]}" \
                           2>/dev/null)); then
      local ret=$?
      IFS="$si"
      return $ret
    fi
    IFS="$si"
    if type __ltrim_colon_completions &>/dev/null; then
      __ltrim_colon_completions "${words[cword]}"
    fi
  }
  complete -o default -F _npm_completion npm
elif type compdef &>/dev/null; then
  _npm_completion() {
    local si=$IFS
    compadd -- $(COMP_CWORD=$((CURRENT-1)) \
                 COMP_LINE=$BUFFER \
                 COMP_POINT=0 \
                 npm completion -- "${words[@]}" \
                 2>/dev/null)
    IFS=$si
  }
  compdef _npm_completion npm
elif type compctl &>/dev/null; then
  _npm_completion () {
    local cword line point words si
    read -Ac words
    read -cn cword
    let cword-=1
    read -l line
    read -ln point
    si="$IFS"
    if ! IFS=$'\n' reply=($(COMP_CWORD="$cword" \
                       COMP_LINE="$line" \
                       COMP_POINT="$point" \
                       npm completion -- "${words[@]}" \
                       2>/dev/null)); then

      local ret=$?
      IFS="$si"
      return $ret
    fi
    IFS="$si"
  }
  compctl -K _npm_completion npm
fi
###-end-npm-completion-###

The reason I expect this behavior is because the output works as expected when sourced. Here's how I tested that:

1. Locally, I commented out https://github.com/npm/cli/blob/846118686849f821b084775f7891038013f7ba97/lib/commands/completion.js#L74-L79
2. I ran source <(npm completion) on my cygwin terminal
3. I typed npm install --legacy<tab key> on the CLI and it expanded to npm install --legacy-bundling.

In addition:

1. Locally, I edited https://github.com/npm/cli/blob/846118686849f821b084775f7891038013f7ba97/lib/utils/is-windows.js like so:

   const isWindowsShell = (process.platform === 'win32') &&
     !/^MINGW(32|64)$/.test(process.env.MSYSTEM) && process.env.TERM !== 'cygwin'
   
   console.log(`process.platform`, process.platform);
   console.log(`process.env.MSYSTEM`, process.env.MSYSTEM);
   console.log(`!/^MINGW(32|64)$/.test(process.env.MSYSTEM)`, !/^MINGW(32|64)$/.test(process.env.MSYSTEM));
   console.log(`process.env.TERM`, process.env.TERM);
   console.log(`isWindowsShell`, isWindowsShell);
   
   exports.isWindowsShell = isWindowsShell
   

2. When I run npm completion in vscode's integrated terminal, it outputs:

   $ npm completion
   process.platform win32
   process.env.MSYSTEM undefined
   !/^MINGW(32|64)$/.test(process.env.MSYSTEM) true
   process.env.TERM xterm-256color # <-------
   isWindowsShell true
   npm error code ENOTSUP
   

3. When I run npm completion in a cygwin terminal, it outputs:

   $ npm completion
   process.platform win32
   process.env.MSYSTEM undefined
   !/^MINGW(32|64)$/.test(process.env.MSYSTEM) true
   process.env.TERM xterm # <-------
   isWindowsShell true
   npm error code ENOTSUP
   

4. When I run npm completion in a windows cmd terminal and embed cygwin via c:\cygwin64\bin\bash.exe --login -i, it outputs:

   $ npm completion
   process.platform win32
   process.env.MSYSTEM undefined
   !/^MINGW(32|64)$/.test(process.env.MSYSTEM) true
   process.env.TERM xterm-256color # <-------
   isWindowsShell true
   npm error code ENOTSUP
   

In other words, process.env.TERM !== 'cygwin' does not catch all cygwin environments.

Steps To Reproduce

1. In windows 10
2. In a cygwin terminal
3. Type npm completion

Environment

• npm: 11.1.0
• Node.js: v22.13.0
• OS Name: Windows 10
• System Model Name:
• npm config:

; "builtin" config from C:\Users\User\AppData\Roaming\npm\node_modules\npm\npmrc

prefix = "C:\\Users\\User\\AppData\\Roaming\\npm"

; "global" config from C:\Users\User\AppData\Roaming\npm\etc\npmrc

save-exact = true
save-prefix = ""

; "user" config from C:\cygwin64\home\User\.npmrc

//registry.npmjs.org/:_authToken = (protected)
registry = "https://registry.npmjs.org/"
script-shell = "C:\\cygwin64\\bin\\bash.exe"

; "project" config from C:\App\.npmrc

install-links = true

; node bin location = C:\Program Files\nodejs\node.exe
; node version = v22.13.0
; npm local prefix = C:\App
; npm version = 11.1.0
; cwd = C:\App
; HOME = C:\cygwin64\home\User
; Run `npm config ls -l` to show all defaults.

Start with lib/commands/completion.js and lib/utils/is-windows.js, especially the Windows-shell detection linked in the report. Reproduce npm completion in Cygwin and the listed Windows terminals, then verify that completion output is produced there without breaking the existing MINGW and Git Bash behavior.