#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 eval exec exit export expr getopts hash
history jobs kill local printf pwd read readonly return set shift
source test times trap type umask unset wait

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

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

basename cat chmod dirname hostname ln mkdir mktemp rm rmdir uname which

These are off by default. Turn them on and a script stops needing coreutils:

$ shish -c 'PATH=; mkdir -p a/b; echo hi > a/b/f; cat a/b/f; rm -r a'
hi

#Choosing

cfg -DENABLE_ALL_BUILTINS=ON              # everything
cfg -DENABLE_CAT=ON -DENABLE_MKDIR=ON     # just these two on top of the default set
cfg -DENABLE_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".

#What a builtin costs

Not much. A default dynamic build is 139 KB stripped (132 KB of text); the same build with -DENABLE_ALL_BUILTINS=ON is 164 KB (154 KB of text) — 25 KB for the whole extra set. Static against glibc, with everything on, it is 1.05 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.