coreutils: Special built-in utilities

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1 2.12 Special built-in utilities
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1 Some programs like ‘nice’ can invoke other programs; for example, the
1 command ‘nice cat file’ invokes the program ‘cat’ by executing the
1 command ‘cat file’.  However, “special built-in utilities” like ‘exit’
1 cannot be invoked this way.  For example, the command ‘nice exit’ does
1 not have a well-defined behavior: it may generate an error message
1 instead of exiting.
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1    Here is a list of the special built-in utilities that are
1 standardized by POSIX 1003.1-2004.
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1      . : break continue eval exec exit export readonly return set shift
1      times trap unset
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1    For example, because ‘.’, ‘:’, and ‘exec’ are special, the commands
1 ‘nice . foo.sh’, ‘nice :’, and ‘nice exec pwd’ do not work as you might
1 expect.
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1    Many shells extend this list.  For example, Bash has several extra
1 special built-in utilities like ‘history’, and ‘suspend’, and with Bash
1 the command ‘nice suspend’ generates an error message instead of
1 suspending.
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