IEEE Standards Interpretations for IEEE Std 1003.2™-1992 IEEE Standard for Information Technology--Portable Operating System Interfaces (POSIX®)--Part 2: Shell and Utilities
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Interpretation Request #99
Topic: set -x Relevant Clauses: 3.14.11
POSIX.2 requires that "set -x" will write a trace for each command after the shell expands the command and before it executes it. The following script indicates that execution occurs before the trace is written, as the call to "set +x" is not recorded: set -x VAR1=1 VAR2=2 set +x VAR3=3 This behaviour is different to present Korn Shell, which has historically treated set +x specially; not tracing this. Several tools uses this to do selective tracing. We see that the standard doesn't specify a special case for this, but we know that it will break existing practice if we change this behavior. All other commands are traced before execution. Did the standard really intend to mandate the Bourne Shell behavior over Korn Shell, or was it just an oversight that they did not single out set +x as a special case?
The standard clearly states the behavior of set +x, and conforming implementations must conform to this. However, concerns have been raised about this which are being referred to the sponsor.
Rationale for Interpretation