getusershell, setusershell, endusershell - get permitted user shells
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
getusershell(), setusershell(), endusershell():_BSD_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE < 500)
The getusershell() function returns the next line from the file /etc/shells, opening the file if necessary. The line should contain the pathname of a valid user shell. If /etc/shells does not exist or is unreadable, getusershell() behaves as if /bin/sh and /bin/csh were listed in the file.
The setusershell() function rewinds /etc/shells.
The endusershell() function closes /etc/shells.
The getusershell() function returns a NULL pointer on end-of-file.
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