What are the POSIX "mandatory utilities"?

It always depends on how "compliant" a system is, but for the POSIX utilities, you can check the lists below. If it is marked as "DEVELOPMENT" then it is considered optional:

"Mandatory" utilities:

alias - define or display aliases ar - create and maintain library archives asa - interpret carriage-control characters at - execute commands at a later time awk - pattern scanning and processing language basename - return non-directory portion of a pathname batch - schedule commands to be executed in a batch queue bc - arbitrary-precision arithmetic language bg - run jobs in the background c99 - compile standard C programs cal - print a calendar cat - concatenate and print files cd - change the working directory chgrp - change the file group ownership chmod - change the file modes chown - change the file ownership cksum - write file checksums and sizes cmp - compare two files comm - select or reject lines common to two files command - execute a simple command compress - compress data cp - copy files crontab - schedule periodic background work csplit - split files based on context cut - cut out selected fields of each line of a file date - write the date and time dd - convert and copy a file df - report free disk space diff - compare two files dirname - return the directory portion of a pathname du - estimate file space usage echo - write arguments to standard output ed - edit text env - set the environment for command invocation ex - text editor expand - convert tabs to spaces expr - evaluate arguments as an expression false - return false value fc - process the command history list fg - run jobs in the foreground file - determine file type find - find files fold - filter for folding lines fort77 - FORTRAN compiler (FORTRAN) fuser - list process IDs of all processes that have one or more files open gencat - generate a formatted message catalog getconf - get configuration values getopts - parse utility options grep - search a file for a pattern hash - remember or report utility locations head - copy the first part of files iconv - codeset conversion id - return user identity ipcrm - remove an XSI message queue, semaphore set, or shared memory segment identifier ipcs - report XSI interprocess communication facilities status jobs - display status of jobs in the current session join - relational database operator kill - terminate or signal processes link - call link function ln - link files locale - get locale-specific information localedef - define locale environment logger - log messages logname - return the user's login name lp - send files to a printer ls - list directory contents m4 - macro processor mailx - process messages man - display system documentation mesg - permit or deny messages mkdir - make directories mkfifo - make FIFO special files more - display files on a page-by-page basis mv - move files newgrp - change to a new group nice - invoke a utility with an altered nice value nl - line numbering filter nohup - invoke a utility immune to hangups od - dump files in various formats paste - merge corresponding or subsequent lines of files patch - apply changes to files pathchk - check pathnames pax - portable archive interchange pr - print files printf - write formatted output ps - report process status pwd - return working directory name qalter - alter batch job qdel - delete batch jobs qhold - hold batch jobs qmove - move batch jobs qmsg - send message to batch jobs qrerun - rerun batch jobs qrls - release batch jobs qselect - select batch jobs qsig - signal batch jobs qstat - show status of batch jobs qsub - submit a script read - read from standard input into shell variables renice - set nice values of running processes rm - remove directory entries rmdir - remove directories sed - stream editor sh - shell, the standard command language interpreter sleep - suspend execution for an interval sort - sort, merge, or sequence check text files split - split a file into pieces strings - find printable strings in files stty - set the options for a terminal tabs - set terminal tabs tail - copy the last part of a file talk - talk to another user tee - duplicate standard input test - evaluate expression time - time a simple command touch - change file access and modification times tput - change terminal characteristics tr - translate characters true - return true value tsort - topological sort tty - return user's terminal name type - write a description of command type ulimit - set or report file size limit umask - get or set the file mode creation mask unalias - remove alias definitions uname - return system name uncompress - expand compressed data unexpand - convert spaces to tabs uniq - report or filter out repeated lines in a file unlink - call the unlink function uucp - system-to-system copy uudecode - decode a binary file uuencode - encode a binary file uustat - uucp status enquiry and job control uux - remote command execution vi - screen-oriented (visual) display editor wait - await process completion wc - word, line, and byte or character count who - display who is on the system write - write to another user xargs - construct argument lists and invoke utility zcat - expand and concatenate data 

DEVELOPMENT ("optional") utilities:

admin - create and administer SCCS files (DEVELOPMENT) cflow - generate a C-language flowgraph (DEVELOPMENT) ctags - create a tags file (DEVELOPMENT, FORTRAN) cxref - generate a C-language program cross-reference table (DEVELOPMENT) delta - make a delta (change) to an SCCS file (DEVELOPMENT) get - get a version of an SCCS file (DEVELOPMENT) lex - generate programs for lexical tasks (DEVELOPMENT) make - maintain, update, and regenerate groups of programs (DEVELOPMENT) nm - write the name list of an object file (DEVELOPMENT) prs - print an SCCS file (DEVELOPMENT) rmdel - remove a delta from an SCCS file (DEVELOPMENT) sact - print current SCCS file-editing activity (DEVELOPMENT) sccs - front end for the SCCS subsystem (DEVELOPMENT) strip - remove unnecessary information from strippable files (DEVELOPMENT) unget - undo a previous get of an SCCS file (DEVELOPMENT) val - validate SCCS files (DEVELOPMENT) what - identify SCCS files (DEVELOPMENT) yacc - yet another compiler compiler (DEVELOPMENT) 

METHOD

# ksh93: Version AJM 93u+ 2012-08-01 ## lots of ugly, lazy hacks but ## it should be readable enough to give you the idea cd /tmp [ ! -s /tmp/posix.list ] && curl --location --silent http://pubs.opengroup.org/onlinepubs/9699919799/utilities/ | sed -e 's/<[^>]*>//g' | awk '/^[a-z].*.html/ { print $1 }' | tee /tmp/posix.list rm -f /tmp/posix.txt /tmp/all_tools.txt echo '#!/bin/sh' > /tmp/clean_all.sh cat /tmp/posix.list | while read P do echo p=$(basename "${P}" .html) echo "Getting ${p} details" [ ! -s "${p}.txt" ] && curl --location --silent "http://pubs.opengroup.org/onlinepubs/9699919799/utilities/${P}" | sed -e 's/<[^>]*>//g' > "${p}.txt" grep -A3 ^NAME "${p}.txt" | grep ^${p} | tee -a /tmp/posix.txt echo "${p}.txt" >> /tmp/all_tools.txt done sed 's|^|rm -f /tmp/|' < /tmp/all_tools.txt >> /tmp/clean_all.sh chmod +x /tmp/clean_all.sh echo 'rm -f /tmp/posix.list' >> /tmp/clean_all.sh echo 'rm -f /tmp/posix.txt /tmp/all_tools.txt' >> /tmp/clean_all.sh grep -v DEVELOPMENT /tmp/posix.txt grep DEVELOPMENT /tmp/posix.txt 

Answers of Kajukenbo and schily are incorrect

I have not enough "reputation" to "comment" on the answers, but I have to say that they are just wrong or, to be fair, misled by a POSIX specifications layout that was made for printers, not for browsers.

Misled by missing introduction / bad specs layout

The OP links correctly to the introduction of chapter 4: "Utilities" of POSIX' "Shell & Utilities" (= "XCU" part of POSIX) where a distinction between "mandatory" and "optional" utilities is given - see Gilles' explanation (as excellent as usual, but outdated for alias as example, see below). Unfortunately in the POSIX 2004 specifications version there are no links to the rest of this chapter, and esp. not to the utilities itself. So eg. Kajukenbo obviously searched elsewhere and found the "utilities" index of the POSIX 2017 specifications - but only the index (see the "idx" in the url) and nothing else from XCU's chapter 4 to explain how to read this list.

The attempt to interpret the pure list of utilities leads to a "mandatory" list that mixes mandatory and optional utilities and creating an own (non-POSIX conform) category for "optional".

Proper specs navigation

Now we have the 2018 edition of POSIX 2017 with some improvements for browsing, so if you consult the XCU 2018 edition, you have "Previous" and "Next" in header and footer to navigate and a (badly named) "Home" that leads to the chapter's TOC. Unfortunately chapter 4 gives no simple list of all utilities, you have to consult the index link found by Kajukenbo. From there or following the order of chapter 4 you get to descriptions for each utility considered by POSIX. In these descriptions the decisive element is not the presence or absence of the word "DEVELOPMENT" that makes admin non-mandatory (optional) and alias mandatory, but the presence or absence of a "margin code", given as a superscript link in square brackets in the upper left corner of the SYNOPSIS paragraph like "[XSI]" for admin. Note that esp. "XSI" is referred to in the POSIX specs under Conformance as "may support ... XSI", not "must".

The upper left corner Codes are introduced in the introduction of XCU's chapter 4 (see above) with a link to a code list that also has a section explaining this "Margin Code Notation". The "shading" of the SYNPOSIS paragraph mentioned there is usually rendered by some steel blue background, but this is by nature just a differentiating feature, not an identifying one, i.e. you may notice it only on switching between descriptions of mandatory and optional utilities, but not when directly accessing an optional one (and a blue link label on a steel blue background is not the best design idea).

POSIX versions

Note that since Gilles' answer alias has changed from optional in POSIX 2004 to mandatory in POSIX 2008. So you have to take also in account the POSIX version against which compliance is claimed or searched. This might be confusing due to "standard" vs. "revision" vs. "edition" vs. "version". Here you best use the IEEE standard number as given in the header of specifications on opengroup.com, eg. the current "IEEE Std 1003.1-2017". The IEEE versioning is well documented in Wikipedia's POSIX entry, while the division in parts like XCU is well covered in Wikipedia's SUS entry which, on the other hand, sticks to POSIX "editions" instead of IEEE numbers.

Painful as it is, you might benefit by delving deeper into some UNIX and / or engineer's way of thinking.

All utilities are mandatory except when they are marked special.

Look e.g. at the documentation for the get command that is maked with the "Development" tag.

Check e.g. Chapter 1.7.1 and look for the tag "optional".

I can't find any listing of such utilities. Is there one somewhere in the POSIX docs?

No. You have to check each utility individually, and, based on this, could create a list on your own. In reality, however, you would always rather need detailed information on one utility than superficial information on many.

Granted, the links given above point to older version of the docs

They do. But you may have also to work with "older" systems that don't know newer ones.

Maybe the nomenclature has changed since then

No. That's an industry standard. Industry standards would not survive if they change too much and / or too often. Don't confuse it with an end-user product of a single manufacturer who might be eager to change much and often so that customers are forced to buy anew.