meaning of ?= in shell script

[clife]

What is the meaning of

    INSTALL_ROOT_DIR ?= ${INSTALL_DIR}/${TARGET_ROOT_DIR}

I googled for ?= in shell script , but none showed up.

Thanks

?= is a conditional assignment and will only assign the value if it not yet set. So in your example above, the line following the conditional you asked about will only be set to ${INSTALL_DIR/${TARGET_ROOT_DIR} if the INSTALL_ROOT_DIR is not already set.

If it is previously set, it will bypass this line in the script.

[clife]

Thanks a lot TKS.

You're welcome ;)