What does '-' (dash) mean after variable names?

Question

if [ -n "${BASH-}" -o -n "${ZSH_VERSION-}" ] ; then hash -r 2>/dev/null fi 

Where can I find a link to this? Thanks.

Answer 1

The variables inside ${...} are called "Parameter Extension".
Look for this term in the online manual or this manual (line 792).
The form ${var-} is similar in form to ${var:-} . The difference is explained just one line before the extension :- (line 810):

... bash checks for an unset or null parameter. Omitting the colon results in a check only for a parameter that is not set.

Thus, this form only checks when the variable is not set ( and not zero), and replaces the entire extension ${...} with the value after - , which in this case is zero .

Therefore, ${var-} becomes:

1. The value of var when var has a value (and not zero).
2. Also the value of var (colon: absent!), When var is null: '' , thus: also null.
3. The value after - (in this case, zero '' ) if the variable is not set.

All this is only in fact:

1. Expand to '' when var is either not installed or is zero.
2. Expand to the value of the var variable (if it matters).

Therefore, the extension does not change anything in the value of var or in the extension, it simply avoids a possible error if the nounset option is installed in the shell.

This code will stop in both cases of using $var :

 #!/bin/bash set -u unset var echo "variable $var" [[ $var ]] && echo "var set" 

However, this code will work without errors:

 #!/bin/bash set -u unset var echo "variable ${var-}" [[ ${var-} ]] && echo "var set" 

Answer 2

This is a bash parameter extension that is used to check if a variable is set.

Explanation

When you use ${ZSH_VERSION-WORD} instead of $ZSH_VERSION in a bash script, bash will execute additional logic

 if $ZSH_VERSION is set then simply use the value of $ZSH_VERSION as per normal elseif $ZSH_VERSION is NOT set then use value of WORD - which isnt provided in your case - so null 

is used

References

The extension of the base parameter is described in the man bash docs (line 939 of the man bash page).

see: POSIX

also see this SO answer

Answer 3

Under normal conditions, ${FOO-} behaves exactly like ${FOO} .

However, with set -u , unlocking undefined variables becomes a default error.

So ${FOO} may be a mistake, but ${FOO-} will never be.