Add info about echo, print and log commands to the book (#870)

* Add info about echo, print and log commands to the book

* Minor changes

---------

Co-authored-by: Mate Farkas <Mate.Farkas@oneidentity.com>

Author: presidento

assets/images/0_79_std_log.png

@@ -54,6 +54,24 @@ If we try to run the external `cat` on a file that doesn't exist, we can see wha
 ╰───────────┴─────────────────────────────────────────────╯
 ```
 
+## `echo`, `print`, and `log` commands
+
+The [`echo`](/commands/docs/echo.md) command is mainly for _pipes_. It returns its arguments, ignoring the piped-in value. There is usually little reason to use this over just writing the values as-is.
+
+In contrast, the [`print`](/commands/docs/print.md) command prints the given values to stdout as plain text. It can be used to write to standard error output, as well. Unlike [`echo`](/commands/docs/echo.md), this command does not return any value (`print | describe` will return "nothing"). Since this command has no output, there is no point in piping it with other commands.
+
+The [standard library](/book/standard_library.md) has commands to write out messages in different logging levels. For example:
+
+@[code](@snippets/book/std_log.nu)
+
+![Log message examples](../assets/images/0_79_std_log.png)
+
+The log level for output can be set with the `NU_LOG_LEVEL` environment variable:
+
+```
+NU_LOG_LEVEL=DEBUG nu std_log.nu
+```
+
 ## Using `out>`, `err>` to redirect stdout and stderr to files
 
 If you want to redirect output to file, you can just type something like this:

book/stdout_stderr_exit_codes.md

@@ -0,0 +1,13 @@
+use std 'log debug'
+use std 'log info'
+use std 'log warning'
+use std 'log error'
+use std 'log critical'
+
+def main [] {
+    log debug "Debug message"
+    log info "Info message"
+    log warning "Warning message"
+    log error "Error message"
+    log critical "Critical message"
+}