Re: Some question regarding tail and option "-c" (original) (raw)

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From: Jim Meyering
Subject: Re: Some question regarding tail and option "-c"
Date: Wed, 03 Oct 2012 20:09:26 +0200

Robert Milasan wrote:

On Wed, 3 Oct 2012 11🔞59 -0600 "Bob Proulx" <address@hidden> wrote:

> What version of tail are you using? > > tail --version > > I think the easiest way to see what is happening at run time is to use > strace. Install it if you don't have it. Then run: > > cat /etc/passwd | strace -v -o tail.strace tail -c 10 > > That should produce about a 14K output file with all of the system > calls logged. >

# tail --version tail (GNU coreutils) 8.16

# cat /etc/passwd | strace -v -o tail.strace tail -c 10 tail: cannot open ‘10’ for reading: No such file or directory

I can reproduce that by specifying an older version of POSIX:

$ : | _POSIX2_VERSION=0 tail -c 10
tail: cannot open '10' for reading: No such file or directory

Depending on how your version of tail was compiled, that may be the default behavior, or it may be due to however your _POSIX2_VERSION envvar is set.

There is some documentation on the effects of that variable in "info coreutils":

2.13 Standards conformance
==========================

In a few cases, the GNU utilities' default behavior is incompatible
with the POSIX standard.  To suppress these incompatibilities, define
the `POSIXLY_CORRECT' environment variable.  Unless you are checking
for POSIX conformance, you probably do not need to define
`POSIXLY_CORRECT'.

   Newer versions of POSIX are occasionally incompatible with older
versions.  For example, older versions of POSIX required the command
`sort +1' to sort based on the second and succeeding fields in each
input line, but starting with POSIX 1003.1-2001 the same command is
required to sort the file named `+1', and you must instead use the
command `sort -k 2' to get the field-based sort.

   The GNU utilities normally conform to the version of POSIX that is
standard for your system.  To cause them to conform to a different
version of POSIX, define the `_POSIX2_VERSION' environment variable to
a value of the form YYYYMM specifying the year and month the standard
was adopted.  Three values are currently supported for
`_POSIX2_VERSION': `199209' stands for POSIX 1003.2-1992, `200112'
stands for POSIX 1003.1-2001, and `200809' stands for POSIX 1003.1-2008.
For example, if you have a newer system but are running software that
assumes an older version of POSIX and uses `sort +1' or `tail +10', you
can work around any compatibility problems by setting
`_POSIX2_VERSION=199209' in your environment.