Leave caches out of the walk by default (closes #181)

The default excludes now cover caches that programs and package
managers fill again on their own. On Linux: /var/cache and, for root
and each user under /home, .cache, .npm, .cargo/registry and
go/pkg/mod. On macOS: /private/var/folders, /Library/Caches and, for
each user, Library/Caches plus the same four. README and scan --help
list them; --no-default-excludes still turns them all off.

Model: opus-5-5

README.md

@@ -129,8 +129,8 @@ walk"). As the file `/etc/cron.d/sfdupes`:
     - one line naming the database;
     - progress lines, as described under "Progress";
     - a warning for each path it skips;
-    - a line for each system or temporary directory the walk leaves out (see
+    - a line for each system, temporary or cache directory the walk leaves out
-      "Rules for the walk");
+      (see "Rules for the walk");
     - a line for each path left out because its filesystem is one the kernel,
       the core of the operating system, makes up or keeps in memory, such as
       `tmpfs`;
@@ -302,8 +302,9 @@ The reference below is also the program's specification.
   directories on other filesystems, as the same flag does for `du` and `rsync`.
   By default the walk also enters directories on other filesystems.
 - `--no-default-excludes` (`scan`) lets the walk enter the default excludes.
-  Those are the system and temporary directories the walk otherwise leaves out,
+  Those are the system, temporary and cache directories the walk otherwise
-  such as `/var/tmp` on Linux (see "Rules for the walk").
+  leaves out, such as `/var/tmp` and `/var/cache` on Linux (see "Rules for the
+  walk").
 - `--exclude PATH` (`scan`) keeps the walk out of `PATH` and everything below
   it. `PATH` may be a directory or a file. The flag may be given more than once.
   The walk knows `PATH` by its device ID and inode number, as it knows the
@@ -570,13 +571,18 @@ walk practical on large or busy storage pools.
     it, as described under "What a scan does to the database".
     `--no-default-excludes` does not change this rule.
 
-- Below an operand, the walk does not enter a default exclude. On Linux, the
+- Below an operand, the walk does not enter a default exclude. The default
-  default excludes hold temporary files and the kernel's view of the running
+  excludes hold the kernel's view of the running system, temporary files, and
-  system. They depend on the system, and `sfdupes scan --help` lists those of
+  caches. A cache here is data that programs and package managers download or
-  the system it runs on. On Linux they are:
+  build again on their own. The default excludes depend on the system, and
+  `sfdupes scan --help` lists those of the system it runs on. In the lists, `*`
+  stands for any one directory name, so `/home/*/.cache` covers each user's
+  cache. On Linux they are:
 
     ```
-    /proc, /sys, /dev, /run, /tmp, /var/tmp
+    /proc, /sys, /dev, /run, /tmp, /var/tmp, /var/cache
+    /root/.cache, /root/.npm, /root/.cargo/registry, /root/go/pkg/mod
+    /home/*/.cache, /home/*/.npm, /home/*/.cargo/registry, /home/*/go/pkg/mod
     ```
 
     On macOS they are:
@@ -584,11 +590,15 @@ walk practical on large or busy storage pools.
     ```
     /dev, /private/tmp, /private/var/tmp, /private/var/vm
     /System/Volumes/VM, /System/Volumes/Data
+    /private/var/folders, /Library/Caches
+    /Users/*/Library/Caches, /Users/*/.cache, /Users/*/.npm
+    /Users/*/.cargo/registry, /Users/*/go/pkg/mod
     ```
 
-    A walk of `/` on macOS reaches everything in `/System/Volumes/Data` already,
+    `/private/var/folders` holds each user's temporary files and caches on
-    through firmlinks. Firmlinks are the links macOS uses to show the data
+    macOS. A walk of `/` on macOS reaches everything in `/System/Volumes/Data`
-    volume inside the system volume. Walking `/System/Volumes/Data` as well
+    already, through firmlinks. Firmlinks are the links macOS uses to show the
+    data volume inside the system volume. Walking `/System/Volumes/Data` as well
     would record each of those files twice. Any other system has no default
     excludes.
 

TODO.md

@@ -29,6 +29,12 @@
 Newest first. An entry ends with its date and its issue or branch, where those
 are known.
 
+- The default excludes cover caches as well: `/var/cache`, and each user's
+  `.cache`, `.npm`, `.cargo/registry` and `go/pkg/mod`, root's included. On
+  macOS they add `/private/var/folders`, `/Library/Caches` and each user's
+  `Library/Caches` and the same four. `--no-default-excludes` still turns them
+  all off (2026-11-11, https://git.eeqj.de/sneak/sfdupes/issues/181)
+
 - New `scan` flag `--exclude PATH`, which may be repeated. The walk neither
   enters nor records an excluded path, and keeps the records below it. An
   operand at or below one is a fatal error (2026-11-11,

internal/sfdupes/exclude.go

@@ -72,14 +72,27 @@ type walkRules struct {
 func defaultExcludes(goos string) []string {
 	switch goos {
 	case goosLinux:
-		return []string{"/proc", "/sys", "/dev", "/run", "/tmp", "/var/tmp"}
+		// The kernel's view of the running system, temporary files, and
+		// caches that programs and package managers fill again on their
+		// own: the system's, root's and each user's.
+		return []string{
+			"/proc", "/sys", "/dev", "/run", "/tmp", "/var/tmp", "/var/cache",
+			"/root/.cache", "/root/.npm", "/root/.cargo/registry",
+			"/root/go/pkg/mod",
+			"/home/*/.cache", "/home/*/.npm", "/home/*/.cargo/registry",
+			"/home/*/go/pkg/mod",
+		}
 	case goosDarwin:
 		// A walk of / reaches everything in /System/Volumes/Data through
 		// the firmlinks, so walking it as well would record each of
-		// those files twice.
+		// those files twice. /private/var/folders holds each user's
+		// temporary files and caches.
 		return []string{
 			"/dev", "/private/tmp", "/private/var/tmp", "/private/var/vm",
 			"/System/Volumes/VM", "/System/Volumes/Data",
+			"/private/var/folders", "/Library/Caches",
+			"/Users/*/Library/Caches", "/Users/*/.cache", "/Users/*/.npm",
+			"/Users/*/.cargo/registry", "/Users/*/go/pkg/mod",
 		}
 	case goosWindows:
 		// Relative to the root of the drive an operand lies on. The

internal/sfdupes/exclude_test.go

@@ -24,10 +24,19 @@ func TestDefaultExcludes(t *testing.T) {
 	// on any other system. The Windows list exists in the code only: the
 	// binary does not build for Windows, so the README leaves it out.
 	cases := map[string][]string{
-		goosLinux: {"/proc", "/sys", "/dev", "/run", "/tmp", "/var/tmp"},
+		goosLinux: {
+			"/proc", "/sys", "/dev", "/run", "/tmp", "/var/tmp", "/var/cache",
+			"/root/.cache", "/root/.npm", "/root/.cargo/registry",
+			"/root/go/pkg/mod",
+			"/home/*/.cache", "/home/*/.npm", "/home/*/.cargo/registry",
+			"/home/*/go/pkg/mod",
+		},
 		goosDarwin: {
 			"/dev", "/private/tmp", "/private/var/tmp", "/private/var/vm",
 			"/System/Volumes/VM", "/System/Volumes/Data",
+			"/private/var/folders", "/Library/Caches",
+			"/Users/*/Library/Caches", "/Users/*/.cache", "/Users/*/.npm",
+			"/Users/*/.cargo/registry", "/Users/*/go/pkg/mod",
 		},
 		goosWindows: {
 			"pagefile.sys", "swapfile.sys", "hiberfil.sys",
@@ -63,6 +72,28 @@ func TestIsDefaultExclude(t *testing.T) {
 			t.Errorf("isDefaultExclude(%q) = %v, want %v", path, got, want)
 		}
 	}
+
+	// The Linux list matches each user's cache directory itself, and no
+	// .cache directory elsewhere.
+	linux := map[string]bool{
+		"/home/u/.cache":           true,
+		"/home/u/go/pkg/mod":       true,
+		"/root/.cache":             true,
+		"/var/cache":               true,
+		"/home/u/.cache/pip":       false,
+		"/home/u/src/.cache":       false,
+		"/srv/home/u/.cache":       false,
+		"/home/u/.cargo/bin":       false,
+		"/home/u/go/pkg/mod/cache": false,
+	}
+
+	for path, want := range linux {
+		got := isDefaultExclude(path, defaultExcludes(goosLinux))
+		if got != want {
+			t.Errorf("isDefaultExclude(%q, Linux list) = %v, want %v", path,
+				got, want)
+		}
+	}
 }
 
 func TestReadMountInfo(t *testing.T) {