- in section 1 (User commands):
- in section 2 (System calls):
| ugetrlimit | umask | umount | umount2 |
| uname | unimplemented | unlink | unlinkat |
| unshare | uselib | userfaultfd | ustat |
| utime | utimensat | utimes |
- in section 3 (Library functions):
| ualarm | ulckpwdf | ulimit | undocumented |
| ungetc | ungetwc | unlocked_stdio | unlockpt |
| unsetenv | updwtmp | updwtmpx | uselocale |
| usleep | utmpname | utmpxname |
- in section 3pm (Perl modules):
| Unicode::GCString | Unicode::LineBreak | URI | URI::data |
| URI::Escape | URI::file | URI::geo | URI::Heuristic |
| URI::icap | URI::icaps | URI::ldap | URI::QueryParam |
| URI::Split | URI::URL | URI::WithBase | URI::_punycode |
| UUID |
| uid_t | uint16_t | uint32_t | uint64_t |
| uint8_t | uintmax_t | uintN_t | uintptr_t |
| useconds_t |
- in section 4 (Device files):
| urandom |
- in section 5 (File formats):
| ucf.conf | udev.conf | udisks2.conf | update-initramfs.conf |
| update-motd | user-dirs.conf | user-dirs.defaults | user-dirs.dirs |
| user-runtime-dir@.service | user.conf.d | user@.service | user_caps |
| utmp | utmpx |
- in section 7 (Miscellaneous):
| udev | udp | udplite | unicode |
| units | unix | UPower | uri |
| url | urn | user-keyring | user-session-keyring |
| user_namespaces | utf-8 | utf8 | uts_namespaces |
- in section 8 (System administration commands):
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