Gnu/coreutils/Special-built 002din-utilities
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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.