Variable substitution in the shell

${STR-NEWSTR} is only replaced when STR is NULL

${STR:-NEWSTR} replace when STR is null or if STR is an empty string

All in all,-number, when STR has no content, then replace,: The number just increases the case of an empty string. Note that the result is the value of the expression and does not change the original STR

${STR+NEWSTR} only replace when STR has content

${STR:+NEWSTR} replace when STR is not NULL

To sum up, the + number, when STR has content, is replaced by: The number just increases the case of an empty string.

${STR=NEWSTR}

${STR:=NEWSTR}

The = sign has the same function as the-number, except that he will give the value of the replaced expression to the original Str

${STR?NEWSTR}

${STR:?NEWSTR}

？ This means that if STR does not have a value, the NEWSTR is output to a standard error: The number only increases in the case of an empty string.

Variable substitution in the shell