Add support for specifying --host via environment variable

This commit adds support for specifying the `--host` option via the `RESTIC_HOST` environment variable. This is done by extending option processing in `cmd_backup.go` and for `restic.SnapshotFilter` in `find.go`.

changelog/unreleased/issue-4733

@@ -0,0 +1,9 @@
+Enhancement: Allow specifying `--host` via environment variable
+
+Restic commands that operate on snapshots, such as `restic backup` and
+`restic snapshots`, support the `--host` flag to specify the hostname for
+grouoping snapshots. They now permit selecting the hostname via the
+environment variable `RESTIC_HOST`. `--host` still takes precedence over the
+environment variable.
+
+https://github.com/restic/restic/issues/4733

cmd/restic/cmd_backup.go

@@ -114,7 +114,7 @@ func init() {
 	f.BoolVar(&backupOptions.StdinCommand, "stdin-from-command", false, "interpret arguments as command to execute and store its stdout")
 	f.Var(&backupOptions.Tags, "tag", "add `tags` for the new snapshot in the format `tag[,tag,...]` (can be specified multiple times)")
 	f.UintVar(&backupOptions.ReadConcurrency, "read-concurrency", 0, "read `n` files concurrently (default: $RESTIC_READ_CONCURRENCY or 2)")
-	f.StringVarP(&backupOptions.Host, "host", "H", "", "set the `hostname` for the snapshot manually. To prevent an expensive rescan use the \"parent\" flag")
+	f.StringVarP(&backupOptions.Host, "host", "H", "", "set the `hostname` for the snapshot manually (default: $RESTIC_HOST). To prevent an expensive rescan use the \"parent\" flag")
 	f.StringVar(&backupOptions.Host, "hostname", "", "set the `hostname` for the snapshot manually")
 	err := f.MarkDeprecated("hostname", "use --host")
 	if err != nil {
@@ -137,6 +137,11 @@ func init() {
 	// parse read concurrency from env, on error the default value will be used
 	readConcurrency, _ := strconv.ParseUint(os.Getenv("RESTIC_READ_CONCURRENCY"), 10, 32)
 	backupOptions.ReadConcurrency = uint(readConcurrency)
+
+	// parse host from env, if not exists or empty the default value will be used
+	if host := os.Getenv("RESTIC_HOST"); host != "" {
+		backupOptions.Host = host
+	}
 }
 
 // filterExisting returns a slice of all existing items, or an error if no

cmd/restic/find.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"context"
+	"os"
 
 	"github.com/restic/restic/internal/restic"
 	"github.com/spf13/pflag"
@@ -14,17 +15,31 @@ func initMultiSnapshotFilter(flags *pflag.FlagSet, filt *restic.SnapshotFilter,
 	if !addHostShorthand {
 		hostShorthand = ""
 	}
-	flags.StringArrayVarP(&filt.Hosts, "host", hostShorthand, nil, "only consider snapshots for this `host` (can be specified multiple times)")
+	flags.StringArrayVarP(&filt.Hosts, "host", hostShorthand, nil, "only consider snapshots for this `host` (can be specified multiple times) (default: $RESTIC_HOST)")
 	flags.Var(&filt.Tags, "tag", "only consider snapshots including `tag[,tag,...]` (can be specified multiple times)")
 	flags.StringArrayVar(&filt.Paths, "path", nil, "only consider snapshots including this (absolute) `path` (can be specified multiple times)")
+
+	if len(filt.Hosts) == 0 {
+		// parse host from env, if not exists or empty the default value will be used
+		if host := os.Getenv("RESTIC_HOST"); host != "" {
+			filt.Hosts = []string{host}
+		}
+	}
 }
 
 // initSingleSnapshotFilter is used for commands that work on a single snapshot
 // MUST be combined with restic.FindFilteredSnapshot
 func initSingleSnapshotFilter(flags *pflag.FlagSet, filt *restic.SnapshotFilter) {
-	flags.StringArrayVarP(&filt.Hosts, "host", "H", nil, "only consider snapshots for this `host`, when snapshot ID \"latest\" is given (can be specified multiple times)")
+	flags.StringArrayVarP(&filt.Hosts, "host", "H", nil, "only consider snapshots for this `host`, when snapshot ID \"latest\" is given (can be specified multiple times) (default: $RESTIC_HOST)")
 	flags.Var(&filt.Tags, "tag", "only consider snapshots including `tag[,tag,...]`, when snapshot ID \"latest\" is given (can be specified multiple times)")
 	flags.StringArrayVar(&filt.Paths, "path", nil, "only consider snapshots including this (absolute) `path`, when snapshot ID \"latest\" is given (can be specified multiple times)")
+
+	if len(filt.Hosts) == 0 {
+		// parse host from env, if not exists or empty the default value will be used
+		if host := os.Getenv("RESTIC_HOST"); host != "" {
+			filt.Hosts = []string{host}
+		}
+	}
 }
 
 // FindFilteredSnapshots yields Snapshots, either given explicitly by `snapshotIDs` or filtered from the list of all snapshots.