A ``tilde-prefix'' consists of an unquoted <tilde> character at the beginning of a word, followed by all of the characters preceding the first unquoted <slash> in the word, or all the characters in the word if there is no <slash>. In an assignment (see [xref to XBD 4.23]), multiple tilde-prefixes can be used: one at the beginning of the word (that is, following the <equals-sign> of the assignment), or one following any unquoted <colon>, or both. A tilde-prefix in an assignment is terminated by the first unquoted <colon> or <slash>, or the end of the assignment word.

If the tilde-prefix consists of only the <tilde> character, it shall be replaced by the value of the variable HOME. If HOME is unset, the results are unspecified.

Otherwise, the characters in the tilde-prefix following the <tilde> shall be treated as a possible login name from the user database. If these characters do not form a portable login name (see the description of the LOGNAME environment variable in [xref to XBD 8.3]), the results are unspecified.

Note: since the tilde-prefix is not subject to further word expansions after the <tilde> is removed to obtain the login name, none of the following has a portable login name following the <tilde>:

~"string"
~'string'
~$var
~\/bin

owing to the presence of '"', '\'', '$', '\\' and '/' characters in the login name.

If the characters in the tilde-prefix following the <tilde> form a portable login name, the tilde-prefix shall be replaced by a pathname of the initial working directory associated with the login name. The pathname shall be obtained as if by using the getpwnam() function as defined in the System Interfaces volume of POSIX.1-2017. If the system does not recognize the login name, the results are unspecified.

The pathname resulting from tilde expansion shall be treated as if quoted to prevent it being altered by field splitting and pathname expansion.