Special built-in utilities (GNU Coreutils 9.0)
2.12 Special built-in utilities
Some programs like
nice can invoke other programs; for example, the command ‘
nice cat file’ invokes the program
cat by executing the command ‘
cat file’. However, special built-in utilities like
exit cannot be invoked this way. For example, the command ‘
nice exit’ does not have a well-defined behavior: it may generate an error message instead of exiting.
Here is a list of the special built-in utilities that are standardized by POSIX 1003.1-2004.
. : break continue eval exec exit export readonly return set shift times trap unset
For example, because ‘
:’, and ‘
exec’ are special, the commands ‘
nice . foo.sh’, ‘
nice :’, and ‘
nice exec pwd’ do not work as you might expect.
Many shells extend this list. For example, Bash has several extra special built-in utilities like
suspend, and with Bash the command ‘
nice suspend’ generates an error message instead of suspending.