Setting the environment variable
POSIXLY_CORRECT turns off the
following warning and error messages, for strict compliance with
POSIX.2. Otherwise, the following diagnostics are issued:
trby default prints a usage message and exits, because set2 would not be used. The POSIX specification says that set2 must be ignored in this case. Silently ignoring arguments is a bad idea.
tr does not provide complete BSD or System V compatibility.
For example, it is impossible to disable interpretation of the POSIX
constructs `[:alpha:]', `[=c=]', and `[c*10]'. Also, GNU
tr does not delete zero bytes automatically, unlike traditional
Unix versions, which provide no way to preserve zero bytes.
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