Shell inference fails on commands (symlinks) invoked with trailing slash and paths with spaces

[andrewflbarnes]

When a shell is invoked with a trailing slash on symlinks, inference fails. Obviously this is a little weird, but possible - I had this recently with a typo when defining my IntelliJ shell as /opt/homebrew/bin/bash/ instead of /opt/homebrew/bin/bash.

This can also cause shell inference to be wrong e.g. you start a zsh shell from a bash shell.

Reproductions at bottom.

I believe this is caused by these lines which perform a split on / and takes the last entry regardless of whether it is empty or not.

Ultimately I get how this can be viewed as user error but it might be nice to either

• next_back to the last non blank entry
• emit a warning log/message that a command with a trailing slash was found and what that full command was

Might also be nice to have a debug log telling you what shell was inferred. RUST_LOG=debug fnm env will show commands which aren't supported shells ([2025-11-28T11:40:32Z DEBUG fnm::shell::infer] binary is not a supported shell: "") but not the command or shell which was successfully inferred:

# succesfully inferred as bash
$ RUST_LOG=debug fnm env >/dev/null 
[2025-11-28T11:41:51Z DEBUG fnm::shell::infer] binary is not a supported shell: "fnm"
[2025-11-28T11:41:51Z DEBUG fnm::shell::infer] binary is not a supported shell: ""

Shell inference fails in IntelliJ

$ ls -l /opt/homebrew/bin/bash
lrwxr-xr-x  1 xxxxxx  admin  29 16 Jul 16:11 /opt/homebrew/bin/bash -> ../Cellar/bash/5.3_1/bin/bash

$ ls -l /opt/homebrew/bin/bash/
-r-xr-xr-x  1 xxxxxx  xxxxxx  896992 16 Jul 16:11 /opt/homebrew/bin/bash/

$ pid=$$; while [ -n "$pid" ]; do read pid prog <<< "$(ps -o ppid,comm -p $pid | awk 'NR == 2 {print}')"; echo $pid $prog; done
3634 /opt/homebrew/bin/bash
1 /Users/xxxxxx/Applications/IntelliJ IDEA Ultimate.app/Contents/MacOS/idea
0 /sbin/launchd

$ fnm env
export PATH="/Users/xxxxxx/.local/state/fnm_multishells/20234_1764328797882/bin":"$PATH"
export FNM_MULTISHELL_PATH="/Users/xxxxxx/.local/state/fnm_multishells/20234_1764328797882"
export FNM_VERSION_FILE_STRATEGY="local"
export FNM_DIR="/Users/xxxxxx/.local/share/fnm"
export FNM_LOGLEVEL="info"
export FNM_NODE_DIST_MIRROR="https://artifactory.awstrp.net/artifactory/nodejs/"
export FNM_COREPACK_ENABLED="false"
export FNM_RESOLVE_ENGINES="true"
export FNM_ARCH="arm64"

# replace current process with shell started with trailing slash
# or just define the terminal shell with a trailing slash
$ exec /opt/homebrew/bin/bash/

# notice second DEBUG line
$ RUST_LOG=debug fnm env
[2025-11-28T11:24:07Z DEBUG fnm::shell::infer] binary is not a supported shell: "fnm"
[2025-11-28T11:24:07Z DEBUG fnm::shell::infer] binary is not a supported shell: ""
[2025-11-28T11:24:07Z DEBUG fnm::shell::infer] binary is not a supported shell: "IntelliJ"
[2025-11-28T11:24:07Z DEBUG fnm::shell::infer] binary is not a supported shell: "launchd"
[2025-11-28T11:24:07Z DEBUG fnm::shell::infer::unix] Can't read process info: entity not found
error: Can't infer shell!
fnm can't infer your shell based on the process tree.
Maybe it is unsupported? we support the following shells:
* bash
* zsh
* fish
* powershell                                                                                                                                                                                                                        

$ pid=$$; while [ -n "$pid" ]; do read pid prog <<< "$(ps -o ppid,comm -p $pid | awk 'NR == 2 {print}')"; echo $pid $prog; done
3634 /opt/homebrew/bin/bash/
1 /Users/xxxxxx/Applications/IntelliJ IDEA Ultimate.app/Contents/MacOS/idea
0 /sbin/launchd

Shell inference incorrect

# start bash shell then
$ ln -s /bin/zsh zsh

$ ./zsh/

xxxxxx@xxxxxx ~ % fnm completions | grep -q BASH && echo bash completions || echo zsh completions
bash completions

xxxxxx@xxxxxx ~ % fnm completions --shell=zsh | grep -q BASH && echo bash completions || echo zsh completions
zsh completions

[andrewflbarnes]

Note - this also highlights that paths with spaces aren't treated properly - in the above /Users/xxxxxx/Applications/IntelliJ IDEA Ultimate.app/Contents/MacOS/idea results in a check to IntelliJ rather than idea.

This could cause issues if say you were running a shell like /path with spaces/to/bash.