yocto-public/2-usage/syntax.md

Configuration file syntax

# VAR value (the brackets are mandatory)
${VAR}

# Set value if not defined (ie set default value)
VAR ?= "defvalue"

# Set value at the end of parsing if the value is not defined
VAR ??= "defvalue"

# Set VAR to "value" immediately
VAR := "value"


# Append "value" to VAR
# Warning: inserts a space between current value and appended value
VAR += "value"
# Same with prepend:
VAR =+ "value"

# Same to append/prepend but without space
VAR .= "value"
VAR =. "value"

# Same to append/prepend
#   but without space and deferred at the end of parsing
VAR_append = "value"
VAR_prepend = "value"

# Remove elements (words) in a space-separated value list
VAR_remove = "value"


# Like bash functions
my_function() {
	echo "Hello world"
}
# Note that _append suffix also works with functions


# Function written in python
python my_function2() {
	print "Hello world"
}

- `${VAR}` is a Yocto variable. - `$VAR` is a shell variable. - They are very different !