#Builtins

Every builtin is a compile-time choice. The shell you ship contains the ones you asked for and nothing else — which is what makes both the container and the WebAssembly story work: the utilities a script needs are inside the binary, not on a PATH.

#The three sets

cmake/Builtins.cmake defines them.

Minimal — the shell language itself, plus what POSIX requires a shell to provide:

[ alias break cd command continue eval exec exit export expr getopts hash
history jobs kill local printf pwd read readonly return set shift source test
times trap type ulimit umask unset wait

Default — the minimal set plus echo, true, false, help and fdtable. This is what a plain cfg build gives you.

Extra — the reason a shish container image can be a single file:

awk basename cat chmod compress cp cut date digest dirname dirs dump env find
grep head hostname id link ln ls mkdir mktemp mv nl paste popd pushd readlink
realpath rm rmdir sed sleep sort split tail tee timeout touch tr uname
uncompress uniq unlink wc which xargs

src/builtin/builtins.map is the one list these come from (name, source file, tier); the lists above are generated from it. compress and uncompress are the whole family of compressors and decompressors under their usual names (gzip, bzip2, xz, zstd, zcat, gunzip, ...), see below.

cp and mv are one source file (builtin_cp.c, one function that looks at argv[0]); mv also needs rm's tree removal, so enabling mv compiles builtin_rm.c too.

These are off by default. Turn them on and a script stops needing coreutils (or grep/sed):

$ shish -c 'PATH=; mkdir -p a/b; echo hi > a/b/f; cat a/b/f; rm -r a'
hi
$ shish -c 'PATH=; printf "a\nb\nc\n" | grep b | sed s/b/B/'
B

src/builtin/core/ (coreutils), src/builtin/extra/ and src/builtin/filter/ are where these live in the tree (as opposed to src/builtin/ for the language's own required builtins); a builtin goes there when it stands in for an external program rather than implementing shell syntax. find, grep and sed are the larger ones and get their own subsystem under text/ (see "Regex backend" below) instead of living entirely in one builtin_*.c file.

#Choosing

cfg -DENABLE_ALL_BUILTINS=ON              # everything
cfg -DBUILTIN_CAT=ON -DBUILTIN_MKDIR=ON   # just these two on top of the default set
cfg -DBUILTIN_HISTORY=OFF                 # and this one off

The configure step writes <builddir>/src/builtin_config.h, which is what src/builtin/builtin_table.c is compiled against. Nothing you left out is linked in.

With autotools, the same choice is ./configure --enable-builtins="cat mkdir rm".

#Compression codecs

compress and uncompress sit on libarchive: a system copy when cmake finds one, otherwise libarchive and every codec library it needs are built from source. Each codec has its own switch, all ON:

A codec that is switched off is not built and its names are not in the table, which is what makes a small WebAssembly build possible (see WebAssembly):

cfg -DENABLE_ALL_BUILTINS=ON -DLIBARCHIVE_ZSTD=OFF -DLIBARCHIVE_LZ4=OFF -DLIBARCHIVE_LZO=OFF

#Regex backend: text/dfa or regex.h

expr and sed always match via text/dfa (an in-tree, from-scratch POSIX BRE/ERE engine, text/dfa.h) — no libc dependency, so both work the same way on a static/musl/dietlibc/WASM build as anywhere else. sed additionally needs s///'s substitution machinery (dfa_replace()), which has no <regex.h> equivalent, so it has no backend switch at all.

grep can go either way:

cfg -DGREP_USE_SYSTEM_REGEX=ON   # link the system's <regex.h> instead

Off (the default) is what makes grep work on a target with no libc regex implementation at all (WASI's libc has none); on is smaller where glibc/musl's regex.h is already being linked in for other reasons, and picks up the host's own conformance/locale behavior instead of text/dfa's. Either way grep's matching results should agree — see tests/dev/dfa-diff.c for the differential test between the two.

#What a builtin costs

Not much. A default dynamic build is 197 KB stripped (186 KB of text); the same build with -DENABLE_ALL_BUILTINS=ON is 392 KB (369 KB of text), about 195 KB for the whole extra set; compress/uncompress link libarchive. Static against glibc the default build is 1.2 MB, and with every builtin and the bundled libarchive (all codecs) 2.8 MB.

#Precedence

A builtin is found before PATH is searched, as POSIX requires for the special builtins and permits for the rest. To reach the real binary anyway, call it by path:

/bin/cat file        # the coreutils one
command -p cat file  # the standard utility, via the default PATH

type name says which one you would get.