info coreutils 'true invocation'
"16.2 `true': Do nothing, successfully
=====================================
`true' does nothing except return an exit status of 0, meaning
"success". It can be used as a place holder in shell scripts where a
successful command is needed, although the shell built-in command `:'
(colon) may do the same thing faster. In most modern shells, `true' is
a built-in command, so when you use `true' in a script, you're probably
using the built-in command, not the one documented here.
`true' honors the `--help' and `--version' options.
Note, however, that it is possible to cause `true' to exit with
nonzero status: with the `--help' or `--version' option, and with
standard output already closed or redirected to a file that evokes an
I/O error. For example, using a Bourne-compatible shell:
$ ./true --version >&-
./true: write error: Bad file number
$ ./true --version >&-
./true: write error: Bad file number
$ ./true --version > /dev/full
./true: write error: No space left on device
This version of `true' is implemented as a C program, and is thus
more secure and faster than a shell script implementation, and may
safely be used as a dummy shell for the purpose of disabling accounts."
Presumably you wouldn't actually use --help or --version when using true as a
place-holder.
However, that "secure" thing means "less likely that someone can bust into your
system if they manage to cause an IO error when a script uses true
--version/--help".
I like the last line. Would an execution of IEFBR14 (followed by nothing else)
at log-on be a way of disabling an account?
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