Disabling color through environment variables (#598)

* This patch adds support for disabling color through environment variables.
- NO_COLOR: https://no-color.org/
- CLICOLOR,CLICOLOR_FORCE: https://bixense.com/clicolors/ ( [logrus built-in](6699a89a23/text_formatter.go (L46)) )

* use viper/cobra and add documentation. (https://github.com/containrrr/watchtower/pull/598#pullrequestreview-463814669)

Co-authored-by: Simon Aronsson <simme@arcticbit.se>

cmd/root.go

@@ -63,6 +63,17 @@ func Execute() {
 func PreRun(cmd *cobra.Command, args []string) {
 	f := cmd.PersistentFlags()
 
+	if enabled, _ := f.GetBool("no-color"); enabled {
+		log.SetFormatter(&log.TextFormatter{
+			DisableColors: true,
+		})
+	} else {
+		// enable logrus built-in support for https://bixense.com/clicolors/
+		log.SetFormatter(&log.TextFormatter{
+			EnvironmentOverrideColors: true,
+		})
+	}
+
 	if enabled, _ := f.GetBool("debug"); enabled {
 		log.SetLevel(log.DebugLevel)
 	}

docs/arguments.md

@@ -88,6 +88,16 @@ Environment Variable: WATCHTOWER_TRACE
              Default: false
 ```
 
+## ANSI colors
+Disable ANSI color escape codes in log output.
+
+```
+            Argument: --no-color
+Environment Variable: NO_COLOR
+                Type: Boolean
+             Default: false
+```
+
 ## Docker host
 Docker daemon socket to connect to. Can be pointed at a remote Docker host by specifying a TCP endpoint as "tcp://hostname:port".
 
@@ -116,7 +126,7 @@ Will also include created and exited containers.
 Environment Variable: WATCHTOWER_INCLUDE_STOPPED
                 Type: Boolean
              Default: false
-```   
+```
 
 ## Revive stopped
 Start any stopped containers that have had their image updated. This argument is only usable with the `--include-stopped` argument.
@@ -126,7 +136,7 @@ Start any stopped containers that have had their image updated. This argument is
 Environment Variable: WATCHTOWER_REVIVE_STOPPED
                 Type: Boolean
              Default: false
-```   
+```
 
 ## Poll interval
 Poll interval (in seconds). This value controls how frequently watchtower will poll for new images. Either `--schedule` or a poll interval can be defined, but not both.
@@ -136,7 +146,7 @@ Poll interval (in seconds). This value controls how frequently watchtower will p
 Environment Variable: WATCHTOWER_POLL_INTERVAL
                 Type: Integer
              Default: 300
-```   
+```
 
 ## Filter by enable label
 Update containers that have a `com.centurylinklabs.watchtower.enable` label set to true.
@@ -146,13 +156,13 @@ Update containers that have a `com.centurylinklabs.watchtower.enable` label set
 Environment Variable: WATCHTOWER_LABEL_ENABLE
                 Type: Boolean
              Default: false
-```   
+```
 
 ## Filter by disable label
 **Do not** update containers that have `com.centurylinklabs.watchtower.enable` label set to false and no `--label-enable` argument is passed. Note that only one or the other (targeting by enable label) can be used at the same time to target containers.
 
 ## Without updating containers
-Will only monitor for new images, not update the containers. 
+Will only monitor for new images, not update the containers.
 
 > ### ⚠️ Please note
 >
@@ -163,7 +173,7 @@ Will only monitor for new images, not update the containers.
 Environment Variable: WATCHTOWER_MONITOR_ONLY
                 Type: Boolean
              Default: false
-``` 
+```
 
 ## Without restarting containers
 Do not restart containers after updating. This option can be useful when the start of the containers
@@ -186,10 +196,10 @@ them to a registry.
 Environment Variable: WATCHTOWER_NO_PULL
                 Type: Boolean
              Default: false
-``` 
+```
 
 ## Without sending a startup message
-Do not send a message after watchtower started. Otherwise there will be an info-level notification. 
+Do not send a message after watchtower started. Otherwise there will be an info-level notification.
 
 ```
             Argument: --no-startup-message
@@ -216,7 +226,7 @@ Runs Watchtower in HTTP API mode, only allowing image updates to be triggered by
 Environment Variable: WATCHTOWER_HTTP_API
                 Type: Boolean
              Default: false
-``` 
+```
 
 ## HTTP API Token
 Sets an authentication token to HTTP API requests.
@@ -226,7 +236,7 @@ Sets an authentication token to HTTP API requests.
 Environment Variable: WATCHTOWER_HTTP_API_TOKEN
                 Type: String
              Default: -
-``` 
+```
 
 ## Filter by scope
 Update containers that have a `com.centurylinklabs.watchtower.scope` label set with the same value as the given argument. This enables [running multiple instances](https://containrrr.github.io/watchtower/running-multiple-instances).
@@ -239,7 +249,7 @@ Environment Variable: WATCHTOWER_SCOPE
 ``` 
 
 ## Scheduling
-[Cron expression](https://pkg.go.dev/github.com/robfig/cron@v1.2.0?tab=doc#hdr-CRON_Expression_Format) in 6 fields (rather than the traditional 5) which defines when and how often to check for new images. Either `--interval` or the schedule expression 
+[Cron expression](https://pkg.go.dev/github.com/robfig/cron@v1.2.0?tab=doc#hdr-CRON_Expression_Format) in 6 fields (rather than the traditional 5) which defines when and how often to check for new images. Either `--interval` or the schedule expression
 can be defined, but not both. An example: `--schedule "0 0 4 * * *"`
 
 ```
@@ -268,7 +278,7 @@ Timeout before the container is forcefully stopped. When set, this option will c
 Environment Variable: WATCHTOWER_TIMEOUT
                 Type: Duration
              Default: 10s
-``` 
+```
 
 ## TLS Verification
 Use TLS when connecting to the Docker socket and verify the server's certificate. See below for options used to configure notifications.

internal/flags/flags.go

@@ -140,7 +140,12 @@ func RegisterSystemFlags(rootCmd *cobra.Command) {
 		"",
 		viper.GetString("WATCHTOWER_HTTP_API_TOKEN"),
 		"Sets an authentication token to HTTP API requests.")
-	
+	// https://no-color.org/
+	flags.BoolP(
+		"no-color",
+		"",
+		viper.IsSet("NO_COLOR"),
+		"Disable ANSI color escape codes in log output")
 	flags.StringP(
 		"scope",
 		"",