bash assign default value

Question

${parameter:=word} Assign Default Values. If parameter is unset or null, the expansion of word is assigned to parameter. The value of parameter is then substituted. Positional parameters and special parameters may not be assigned to in this way.

I thought I could use this feature to write ${LONG_VARIABLE_NAME:=hello} instead of the longer LONG_VARIABLE_NAME=${LONG_VARIABLE_NAME:-hello}, but now bash also tries to execute 'hello' and that gives a command not found. Any way to avoid that? Or will I have to stick to the latter? Can someone give an example where the assign default is actually useful?

Answer 1

Please look at http://www.tldp.org/LDP/abs/html/parameter-substitution.html for examples

${parameter-default}, ${parameter:-default} 

If parameter not set, use default. After the call, parameter is still not set.
Both forms are almost equivalent. The extra : makes a difference only when parameter has been declared, but is null.

unset EGGS echo 1 ${EGGS-spam}   # 1 spam echo 2 ${EGGS:-spam}  # 2 spam  EGGS= echo 3 ${EGGS-spam}   # 3 echo 4 ${EGGS:-spam}  # 4 spam  EGGS=cheese echo 5 ${EGGS-spam}   # 5 cheese echo 6 ${EGGS:-spam}  # 6 cheese 

---

${parameter=default}, ${parameter:=default} 

If parameter not set, set parameter value to default.
Both forms nearly equivalent. The : makes a difference only when parameter has been declared and is null

# sets variable without needing to reassign # colons suppress attempting to run the string unset EGGS : ${EGGS=spam} echo 1 $EGGS     # 1 spam unset EGGS : ${EGGS:=spam} echo 2 $EGGS     # 2 spam  EGGS= : ${EGGS=spam} echo 3 $EGGS     # 3        (set, but blank -> leaves alone) EGGS= : ${EGGS:=spam} echo 4 $EGGS     # 4 spam  EGGS=cheese : ${EGGS:=spam} echo 5 $EGGS     # 5 cheese EGGS=cheese : ${EGGS=spam} echo 6 $EGGS     # 6 cheese 

---

${parameter+alt_value}, ${parameter:+alt_value} 

If parameter set, use alt_value, else use null string. After the call, parameter value not changed.
Both forms nearly equivalent. The : makes a difference only when parameter has been declared and is null

unset EGGS echo 1 ${EGGS+spam}  # 1 echo 2 ${EGGS:+spam} # 2  EGGS= echo 3 ${EGGS+spam}  # 3 spam echo 4 ${EGGS:+spam} # 4  EGGS=cheese echo 5 ${EGGS+spam}  # 5 spam echo 6 ${EGGS:+spam} # 6 spam 

Answer 2

Use a colon:

: ${A:=hello} 

The colon is a null command that does nothing and ignores its arguments. It is built into bash so a new process is not created.