fix: relative symlink resolution in install script

[V-Subhankar-infy]

Summary

Fixes relative symlink resolution in the install script for split local layouts (e.g. a launcher in ~/.local/bin pointing to a payload in ~/.local/lib/devcontainers). The wrapper now resolves its own location with portable POSIX shell built-ins instead of GNU-specific readlink -f, so devcontainer works regardless of the caller's working directory and shell environment (Linux, macOS, BSD).

Fixes #1232

Root Cause

A single readlink call plus cd "$(dirname ...)" && pwd resolved relative symlink targets against the caller's CWD rather than the link's own directory, and the readlink -f fallback isn't portable to macOS/BSD.

Reproduction of error

./install.sh --prefix "$HOME/.local/lib/devcontainers"
ln -s ../lib/devcontainers/bin/devcontainer "$HOME/.local/bin/devcontainer"
export PATH="$HOME/.local/bin:$PATH"
cd "$HOME" && devcontainer --version   # cd: ../lib/devcontainers/bin: No such file or directory

Fix

The script-location resolver now walks the symlink chain one hop at a time, cd-ing into each link's directory before reading the next target, then canonicalizes the final directory. This replaces the previous single readlink resolution.

Why It Works

• Correct relative resolution: enters each link's directory before reading the next target, so relative symlinks resolve against the link, not the CWD.
• CWD-independent: finalized with cd -P … && pwd for an absolute, canonical path.
• Portable: POSIX-only (readlink, ${var%/*}, cd -P, pwd) — no readlink -f/realpath.
• Safe: runs in a subshell with CDPATH= cleared; no leaked cd or stray output.

Validation

Regression test confirms a relative symlink launched from another directory exits 0 and reports the installed CLI version.

Scope & Risk

One resolution block; no install/download changes. Direct execution and absolute symlinks are unchanged. Low risk, plus improved cross-platform portability.

Credits

Thanks to @klmr for the portable POSIX symlink-resolution approach.

[klmr]

Beware that readlink -f support has only been added to macOS fairly recently. A portable alternative is much more convoluted, alas:

--- a/devcontainer      2026-06-19 15:12:58
+++ b/devcontainer      2026-06-19 15:13:11
@@ -6,15 +6,22 @@

 # Resolve the installation directory
 # Handle both direct execution and symlinked scenarios
-if [ -L "$0" ]; then
-    # Follow symlink
-    SCRIPT_PATH="$(readlink "$0" 2>/dev/null || readlink -f "$0" 2>/dev/null || echo "$0")"
-else
-    SCRIPT_PATH="$0"
-fi
-
-# Get absolute path to script directory
-SCRIPT_DIR="$(cd "$(dirname "$SCRIPT_PATH")" && pwd)"
+SCRIPT_DIR="$(
+    CDPATH=
+    self=$0
+
+    while [ -L "$self" ]; do
+        cd -- "${self%/*}" >/dev/null || :
+        self=$(readlink "${self##*/}")
+    done
+
+    case "$self" in
+        (*/*) ;;
+        (*) self=./$self ;;
+    esac
+
+    cd -P -- "${self%/*}" >/dev/null && pwd
+)"
 INSTALL_DIR="$(dirname "$SCRIPT_DIR")"

 # Paths to bundled components

(Reference/explanation: https://stackoverflow.com/a/246128/1968)