From 8e18ab0cd7cd8471a84d8d337bedeaefadd79a1e Mon Sep 17 00:00:00 2001
From: procr1337 <193802945+procr1337@users.noreply.github.com>
Date: Wed, 5 Aug 2026 10:16:06 +0200
Subject: [PATCH] util: Fix stdin redirect for ShellBuilder (#62078)
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# Objective

This is a follow-up to my own PR #59270 which fixed the hang it targeted
(#55042) only partially and broke `ShellBuilder::build` for fish.

**The hang is only half fixed.** The agent's terminal tool wraps
commands as `sh -i -c 'exec </dev/null; <command>'`. The redirect sits
on the same line as the command, but POSIX shells parse an entire line
before executing any of it. So when `<command>` has a syntax error, the
whole line is discarded — including the `exec` that was supposed to
close stdin. Interactive `dash` then recovers from the error, finds
stdin still attached to the PTY, and sits at a `$` prompt until the user
presses Ctrl+D.

That means #59270 only helped when the redirect had already run, i.e.
multi-line commands whose error is on a later line. A single-line
command still hangs, including `cat <(echo hi)`, the example from that
PR's own description. Probably prompts or tool definitions changed since
that PR landed which now make it more likely again to run into this
issue, at least with Anthropic models.

**fish runs no command at all.** #59270 also applied the `exec
</dev/null; …` form to fish, replacing its `begin; …; end </dev/null`
wrapper. But fish's `exec` requires a command to replace the shell with,
so a bare `exec </dev/null` is a usage error: fish prints `exec`'s help
text, skips the rest of the command, and exits 0.

## Solution

For POSIX shells, put the redirect on its own line:

```sh
sh -i -c 'exec </dev/null
<command>'
```

That makes it a separate parse-and-execute unit, so it runs before the
command line is parsed. Any syntax error is then reported by a shell
that already has `/dev/null` on stdin, and it exits instead of
prompting.

For fish, go back to `begin; <command>; end </dev/null`. fish exits on a
parse error even when interactive, so it never needed the `exec` form to
begin with.

Only the `Posix` and `Fish` arms change; the other shells' redirects are
untouched.

Note this is still a targeted fix for `dash`. The underlying reason a
syntax error becomes an interactive prompt is that agent terminals are
built with `-i` at all — non-interactive `dash` exits cleanly even with
no redirect. Dropping `-i` would remove the whole class of hang, but it
would also stop sourcing users' interactive shell config in agent
terminals.

## Testing

`cargo test -p util --lib shell_builder` covers the changed command
assembly.

Reviewers can check the shell behavior directly on a PTY, without
building Zed:

```sh
# hangs at a `$` prompt until Ctrl+D
script -qec "sh -i -c 'exec </dev/null; echo hi ;;'" /dev/null

# exits immediately with just the syntax error
script -qec "sh -i -c 'exec </dev/null
echo hi ;;'" /dev/null
```

```sh
fish -i -c 'exec </dev/null; echo test'        # prints exec's help, no "test", exit 0
fish -i -c 'begin; echo test; end </dev/null'  # prints "test"
fish -i -c 'if true'                           # exit 127, no hang
```

macOS is unaffected in practice because `/bin/sh` there is bash, which
exits on a syntax error even with `-i`; Windows goes through the
`Cmd`/`PowerShell` arms, which this doesn't touch. A `zsh`-as-`/bin/sh`
setup is likewise unaffected.

## Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content adheres to Zed's UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable


---

Release Notes:

- Fixed agent terminal commands hanging at a shell prompt after a syntax
error
---
 crates/util/src/shell_builder.rs | 25 +++++++++++++++++++------
 1 file changed, 19 insertions(+), 6 deletions(-)

diff --git a/crates/util/src/shell_builder.rs b/crates/util/src/shell_builder.rs
index 57ce0319a4..76ff3923e6 100644
--- a/crates/util/src/shell_builder.rs
+++ b/crates/util/src/shell_builder.rs
@@ -99,8 +99,17 @@ impl ShellBuilder {
             });
             if self.redirect_stdin {
                 match self.kind {
-                    ShellKind::Fish | ShellKind::Posix => {
-                        combined_command.insert_str(0, "exec </dev/null; ");
+                    ShellKind::Posix => {
+                        // Perform the STDIN redirection prior to the actual
+                        // command on a separate line, so that it is already
+                        // active if the command contains a syntax error.
+                        // Otherwise, with -i, dash will fall back to an
+                        // interactive shell in this case.
+                        combined_command.insert_str(0, "exec </dev/null\n");
+                    }
+                    ShellKind::Fish => {
+                        combined_command.insert_str(0, "begin; ");
+                        combined_command.push_str("; end </dev/null");
                     }
                     ShellKind::Nushell
                     | ShellKind::Csh
@@ -143,8 +152,12 @@ impl ShellBuilder {
             });
             if self.redirect_stdin {
                 match self.kind {
-                    ShellKind::Fish | ShellKind::Posix => {
-                        combined_command.insert_str(0, "exec </dev/null; ");
+                    ShellKind::Posix => {
+                        combined_command.insert_str(0, "exec </dev/null\n");
+                    }
+                    ShellKind::Fish => {
+                        combined_command.insert_str(0, "begin; ");
+                        combined_command.push_str("; end </dev/null");
                     }
                     ShellKind::Nushell
                     | ShellKind::Csh
@@ -282,7 +295,7 @@ mod test {
             .build(Some("echo".into()), &["test".to_string()]);
 
         assert_eq!(program, "fish");
-        assert_eq!(args, vec!["-i", "-c", "exec </dev/null; echo test"]);
+        assert_eq!(args, vec!["-i", "-c", "begin; echo test; end </dev/null"]);
     }
 
     #[test]
@@ -298,7 +311,7 @@ mod test {
         assert_eq!(program, "sh");
         assert_eq!(
             args,
-            vec!["-i", "-c", "exec </dev/null; cat <<EOF\nhello\nEOF"]
+            vec!["-i", "-c", "exec </dev/null\ncat <<EOF\nhello\nEOF"]
         );
     }