0001051: stderr redirection of the "time" utility

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Category

Severity

Type

Date Submitted

Last Update

0001051

[1003.1(2013)/Issue7+TC1] Shell and Utilities

Editorial

Omission

2016-05-04 14:30

2017-06-08 15:08

Reporter

stephane

View Status

public

Assigned To

Priority

normal

Resolution

Duplicate

Status

Closed

Name

Stephane Chazelas

Organization

User Reference

Section

"time" utility

Page Number

3259

Line Number

109327

Interp Status

---

Final Accepted Text

Summary

0001051: stderr redirection of the "time" utility

Description

ksh, pdksh/mksh, bash and zsh implement "time" as a keyword, where the syntax is:

time <pipeline>

Where pipeline is as defined in the shell grammar. Any redirection in that <pipeline> is considered to be part of the <pipeline> to be timed (with the exception of time (cmd) > redir in bash which seems to be an accident of implementation). The timing statistics are issued on the shell's stderr regardless of the stderr redirection made in the <pipeline> (with the exception of that bash corner case).

That "time" keyword is recognised as such only when it's the first word of a pipeline or pipesequence or follows "!". bash has a further restriction that it cannot be used inside a pipeline.

For instance in these cases, time is not recognised as a keyword

   E=x time ls
   <&3 time ls # except in zsh

and in bash

   cmd1 | time cmd2 | cmd3 # other shells time "cmd2 | cmd3"

When "time" is not considered as a keyword, then generally the "time" command from the filesystem is invoked ("a=1 time uname" causes a syntax error in zsh).

There has been an attempt to cover for that case in the spec but it is incomplete.

   > When time is used as part of a pipeline, the times reported
   > are unspecified, except when it is the sole command within a
   > grouping command (see Section 2.9.4.1, on page 2344) in that
   > pipeline.

It doesn't clarify what is meant by "pipeline". If it's by the grammar definition or 2.9.2, then that's every command. "time" can't be but to be used as part of such a pipeline.

Even if it means a pipe lines with at least 2 commands, we'd still need to clarify what should happen for stderr redirections in:

   { time cmd 2> file; } | ...

Note that all implementations except zsh seem to treat

2> file time cmd

and even

2> file time cmd | cmd2

or

cmd1 | 2>&1 time cmd | cmd3

the same (and time cmd only by invoking the filesytem's "cmd").

ksh93 also stops "time" being considered as a keyword when called with "-p".

Desired Action

Change the wording so "pipeline" becomes a "pipeline as per 2.9.2 with more than one command part".

Mention that redirection of stderr may not affect the timing statistics output if the redirection operator appears on the right of the "time" word in the time command.

It may be worth mentioning.

<any-redirection> time cmd

as a portable (ignoring zsh's time for now which is not POSIX compliant anyway) way to force "time" to behave (be parsed) as a normal utility.

Issue History
Date Modified	Username	Field	Change
2016-05-04 14:30	stephane	New Issue
2016-05-04 14:30	stephane	Name	=> Stephane Chazelas
2016-05-04 14:30	stephane	Section	=> "time" utility
2016-05-04 14:30	stephane	Page Number	=> 3259
2016-05-04 14:30	stephane	Line Number	=> 109327
2016-05-04 15:10	geoffclare	Relationship added	related to 0000267
2017-06-08 15:08	geoffclare	Interp Status	=> ---
2017-06-08 15:08	geoffclare	Status	New => Closed
2017-06-08 15:08	geoffclare	Resolution	Open => Duplicate
2017-06-08 15:08	geoffclare	Relationship replaced	duplicate of 0000267