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nix: allow users to set shell startup hooks (#93)
Add a `shell.init_hook` field to devbox.json which can contain arbitrary commands to run at shell startup. { "packages": ["go"], "shell": { "init_hook": "echo hello from a hook!" } } This change also merges the `shell` package into `nix`, making it clearer that it handles Nix shells specifically. The `nix.Shell` function is replaced by the `nix.Shell` struct, which does everything that the `shell.Shell` struct did. The new API looks something like: sh, err := nix.DetectShell() sh.UserInitHook = cfg.Shell.InitHook sh.Run(nixPath) Finally, we now build the devbox shellrc with a template since it was getting a bit complex.
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‎config.go‎

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@@ -16,6 +16,12 @@ type Config struct {
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// Packages is the slice of Nix packages that devbox makes available in
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// its environment.
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Packages []string `cue:"[...string]" json:"packages"`
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// Shell configures the devbox shell environment.
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Shell struct {
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// InitHook contains commands that will run at shell startup.
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InitHook string `json:"init_hook,omitempty"`
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} `json:"shell,omitempty"`
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}
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// ReadConfig reads a devbox config file.

‎devbox.go‎

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@@ -131,7 +131,13 @@ func (d *Devbox) Shell() error {
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return errors.WithStack(err)
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}
133133
nixDir := filepath.Join(d.srcDir, ".devbox/gen/shell.nix")
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return nix.Shell(nixDir)
134+
sh, err := nix.DetectShell()
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if err != nil {
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// Fall back to using a plain Nix shell.
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sh = &nix.Shell{}
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}
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sh.UserInitHook = d.cfg.Shell.InitHook
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return sh.Run(nixDir)
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}
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// saveCfg writes the config file to the devbox directory.

‎nix/nix.go‎

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@@ -7,94 +7,9 @@ import (
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"bytes"
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"encoding/json"
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"fmt"
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"os"
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"os/exec"
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"strings"
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"github.com/pkg/errors"
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"go.jetpack.io/devbox/debug"
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"go.jetpack.io/devbox/shell"
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)
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func Shell(path string) error {
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// nix-shell only runs bash, which isn't great if the user has a
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// different default shell. Here we try to detect what their current
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// shell is, and then `exec` it to replace the bash process inside
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// nix-shell.
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sh, err := shell.Detect()
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if err != nil {
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// Fall back to running the vanilla Nix bash shell.
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return runFallbackShell(path)
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}
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// Naively running the user's shell has two problems:
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//
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// 1. The shell will source the user's rc file and potentially reorder
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// the PATH. This is especially a problem with some shims that prepend
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// their own directories to the front of the PATH, replacing the
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// Nix-installed packages.
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// 2. If their shell is bash, we end up double-sourcing their ~/.bashrc.
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// Once when nix-shell launches bash, and again when we exec it.
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//
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// To workaround this, first we store the current (outside of devbox)
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// PATH in ORIGINAL_PATH. Then we run a "pure" nix-shell to prevent it
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// from sourcing their ~/.bashrc. From inside the nix-shell (but before
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// launching the user's preferred shell) we store the PATH again in
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// PURE_NIX_PATH. When we're finally in the user's preferred shell, we
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// can use these env vars to set the PATH so that Nix packages are up
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// front, and all of the other programs come after.
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//
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// ORIGINAL_PATH is set by sh.StartCommand.
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// PURE_NIX_PATH is set by the shell hook in shell.nix.tmpl.
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sh.PreInitHook = `
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# Update the $PATH so that the user's init script has access to all of their
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# non-devbox programs.
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export PATH="$PURE_NIX_PATH:$ORIGINAL_PATH"
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`
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sh.PostInitHook = `
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# Update the $PATH again so that the Nix packages take priority over the
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# programs outside of devbox.
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export PATH="$PURE_NIX_PATH:$ORIGINAL_PATH"
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# Prepend to the prompt to make it clear we're in a devbox shell.
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export PS1="(devbox) $PS1"
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`
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if debug.IsEnabled() {
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sh.PostInitHook += `echo "POST-INIT PATH=$PATH"
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`
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}
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cmd := exec.Command("nix-shell", path)
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cmd.Args = append(cmd.Args, "--pure", "--command", sh.ExecCommand())
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cmd.Stdin = os.Stdin
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cmd.Stdout = os.Stdout
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cmd.Stderr = os.Stderr
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debug.Log("Executing nix-shell command: %v", cmd.Args)
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return errors.WithStack(cmd.Run())
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}
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func runFallbackShell(path string) error {
79-
cmd := exec.Command("nix-shell", path)
80-
cmd.Stdin = os.Stdin
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cmd.Stdout = os.Stdout
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cmd.Stderr = os.Stderr
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debug.Log("Unrecognized user shell, falling back to: %v", cmd.Args)
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return errors.WithStack(cmd.Run())
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}
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func Exec(path string, command []string) error {
89-
runCmd := strings.Join(command, " ")
90-
cmd := exec.Command("nix-shell", "--run", runCmd)
91-
cmd.Stdin = os.Stdin
92-
cmd.Stdout = os.Stdout
93-
cmd.Stderr = os.Stderr
94-
cmd.Dir = path
95-
return errors.WithStack(cmd.Run())
96-
}
97-
9813
func PkgExists(pkg string) bool {
9914
_, found := PkgInfo(pkg)
10015
return found

‎nix/shell.go‎

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@@ -0,0 +1,184 @@
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// Copyright 2022 Jetpack Technologies Inc and contributors. All rights reserved.
2+
// Use of this source code is governed by the license in the LICENSE file.
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4+
package nix
5+
6+
import (
7+
"bytes"
8+
_ "embed"
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"fmt"
10+
"os"
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"os/exec"
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"path/filepath"
13+
"strings"
14+
"text/template"
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16+
"github.com/pkg/errors"
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"go.jetpack.io/devbox/debug"
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)
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20+
//go:embed shellrc.tmpl
21+
var shellrcText string
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var shellrcTmpl = template.Must(template.New("shellrc").Parse(shellrcText))
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24+
type name string
25+
26+
const (
27+
shUnknown name = ""
28+
shBash name = "bash"
29+
shZsh name = "zsh"
30+
shKsh name = "ksh"
31+
shPosix name = "posix"
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)
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34+
// Shell configures a user's shell to run in Devbox. Its zero value is a
35+
// fallback shell that launches a regular Nix shell.
36+
type Shell struct {
37+
name name
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binPath string
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userShellrcPath string
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// UserInitHook contains commands that will run at shell startup.
42+
UserInitHook string
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}
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// DetectShell attempts to determine the user's default shell.
46+
func DetectShell() (*Shell, error) {
47+
path := os.Getenv("SHELL")
48+
if path == "" {
49+
return nil, errors.New("unable to detect the current shell")
50+
}
51+
52+
sh := &Shell{binPath: filepath.Clean(path)}
53+
base := filepath.Base(path)
54+
// Login shell
55+
if base[0] == '-' {
56+
base = base[1:]
57+
}
58+
switch base {
59+
case "bash":
60+
sh.name = shBash
61+
sh.userShellrcPath = rcfilePath(".bashrc")
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case "zsh":
63+
sh.name = shZsh
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sh.userShellrcPath = rcfilePath(".zshrc")
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case "ksh":
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sh.name = shKsh
67+
sh.userShellrcPath = rcfilePath(".kshrc")
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case "dash", "ash", "sh":
69+
sh.name = shPosix
70+
sh.userShellrcPath = os.Getenv("ENV")
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72+
// Just make up a name if there isn't already an init file set
73+
// so we have somewhere to put a new one.
74+
if sh.userShellrcPath == "" {
75+
sh.userShellrcPath = ".shinit"
76+
}
77+
default:
78+
sh.name = shUnknown
79+
}
80+
debug.Log("Detected shell: %s", sh.binPath)
81+
debug.Log("Recognized shell as: %s", sh.binPath)
82+
debug.Log("Looking for user's shell init file at: %s", sh.userShellrcPath)
83+
return sh, nil
84+
}
85+
86+
// rcfilePath returns the absolute path for an rcfile, which is usually in the
87+
// user's home directory. It doesn't guarantee that the file exists.
88+
func rcfilePath(basename string) string {
89+
home, err := os.UserHomeDir()
90+
if err != nil {
91+
return ""
92+
}
93+
return filepath.Join(home, basename)
94+
}
95+
96+
func (s *Shell) Run(nixPath string) error {
97+
// Launch a fallback shell if we couldn't find the path to the user's
98+
// default shell.
99+
if s.binPath == "" {
100+
cmd := exec.Command("nix-shell", nixPath)
101+
cmd.Stdin = os.Stdin
102+
cmd.Stdout = os.Stdout
103+
cmd.Stderr = os.Stderr
104+
105+
debug.Log("Unrecognized user shell, falling back to: %v", cmd.Args)
106+
return errors.WithStack(cmd.Run())
107+
}
108+
109+
cmd := exec.Command("nix-shell", nixPath)
110+
cmd.Args = append(cmd.Args, "--pure", "--command", s.execCommand())
111+
cmd.Stdin = os.Stdin
112+
cmd.Stdout = os.Stdout
113+
cmd.Stderr = os.Stderr
114+
115+
debug.Log("Executing nix-shell command: %v", cmd.Args)
116+
return errors.WithStack(cmd.Run())
117+
}
118+
119+
// execCommand is a command that replaces the current shell with s.
120+
func (s *Shell) execCommand() string {
121+
shellrc, err := writeDevboxShellrc(s.userShellrcPath, s.UserInitHook)
122+
if err != nil {
123+
debug.Log("Failed to write devbox shellrc: %v", err)
124+
return "exec " + s.binPath
125+
}
126+
127+
switch s.name {
128+
case shBash:
129+
return fmt.Sprintf(`exec /usr/bin/env ORIGINAL_PATH="%s" %s --rcfile "%s"`,
130+
os.Getenv("PATH"), s.binPath, shellrc)
131+
case shZsh:
132+
return fmt.Sprintf(`exec /usr/bin/env ORIGINAL_PATH="%s" ZDOTDIR="%s" %s`,
133+
os.Getenv("PATH"), filepath.Dir(shellrc), s.binPath)
134+
case shKsh, shPosix:
135+
return fmt.Sprintf(`exec /usr/bin/env ORIGINAL_PATH="%s" ENV="%s" %s `,
136+
os.Getenv("PATH"), shellrc, s.binPath)
137+
default:
138+
return "exec " + s.binPath
139+
}
140+
}
141+
142+
func writeDevboxShellrc(userShellrcPath string, userHook string) (path string, err error) {
143+
// We need a temp dir (as opposed to a temp file) because zsh uses
144+
// ZDOTDIR to point to a new directory containing the .zshrc.
145+
tmp, err := os.MkdirTemp("", "devbox")
146+
if err != nil {
147+
return "", fmt.Errorf("create temp dir for shell init file: %v", err)
148+
}
149+
150+
// This is a best-effort to include the user's existing shellrc. If we
151+
// can't read it, then just omit it from the devbox shellrc.
152+
userShellrc, err := os.ReadFile(userShellrcPath)
153+
if err != nil {
154+
userShellrc = []byte{}
155+
}
156+
157+
path = filepath.Join(tmp, filepath.Base(userShellrcPath))
158+
shellrcf, err := os.Create(path)
159+
if err != nil {
160+
return "", fmt.Errorf("write to shell init file: %v", err)
161+
}
162+
defer func() {
163+
cerr := shellrcf.Close()
164+
if err == nil {
165+
err = cerr
166+
}
167+
}()
168+
169+
err = shellrcTmpl.Execute(shellrcf, struct {
170+
OriginalInit string
171+
OriginalInitPath string
172+
UserHook string
173+
}{
174+
OriginalInit: string(bytes.TrimSpace(userShellrc)),
175+
OriginalInitPath: filepath.Clean(userShellrcPath),
176+
UserHook: strings.TrimSpace(userHook),
177+
})
178+
if err != nil {
179+
return "", fmt.Errorf("execute shellrc template: %v", err)
180+
}
181+
182+
debug.Log("Wrote devbox shellrc to: %s", path)
183+
return path, nil
184+
}

‎nix/shellrc.tmpl‎

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{{- /*
2+
3+
// This template defines the shellrc file that the devbox shell will run at
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// startup.
5+
//
6+
// It includes the user's original shellrc, which varies depending on their
7+
// shell. It will either be ~/.bashrc, ~/.zshrc, a path set in ENV, or something
8+
// else. It also appends any user-defined shell hooks from devbox.json.
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//
10+
// Devbox also needs to ensure that the shell's PATH, prompt, and a few other
11+
// things are set correctly at startup. To do this, it must run some commands
12+
// before and after the user's shellrc. These commands are in the
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// "Devbox Pre/Post-init Hook" sections.
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//
15+
// The devbox pre/post-init hooks assume two environment variables are already
16+
// set:
17+
//
18+
// - ORIGINAL_PATH - embedded into the command built by Shell.execCommand. It
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// preserves the PATH at the time `devbox shell` is invoked.
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// - PURE_NIX_PATH - set by the shell hook in shell.nix.tmpl. It preserves the
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// PATH set by Nix's "pure" shell mode.
22+
23+
*/ -}}
24+
25+
# Begin Devbox Pre-init Hook
26+
27+
# Update the $PATH so that the user's init script has access to all of their
28+
# non-devbox programs.
29+
export PATH="$PURE_NIX_PATH:$ORIGINAL_PATH"
30+
31+
# End Devbox Pre-init Hook
32+
33+
{{- if .OriginalInit }}
34+
35+
# Begin {{ .OriginalInitPath }}
36+
37+
{{ .OriginalInit }}
38+
39+
# End {{ .OriginalInitPath }}
40+
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{{- end }}
42+
43+
# Begin Devbox Post-init Hook
44+
45+
# Update the $PATH again so that the Nix packages take priority over the
46+
# programs outside of devbox.
47+
export PATH="$PURE_NIX_PATH:$ORIGINAL_PATH"
48+
49+
# Prepend to the prompt to make it clear we're in a devbox shell.
50+
export PS1="(devbox) $PS1"
51+
52+
# End Devbox Post-init Hook
53+
54+
{{- if .UserHook }}
55+
56+
# Begin Devbox User Hook
57+
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{{ .UserHook }}
59+
60+
# End Devbox User Hook
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{{- end }}

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