Special built-in utilities (GNU Coreutils 9.0)
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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 history
, and suspend
, and with Bash the command ‘nice suspend
’ generates an error message instead of suspending.