GNU Info
(autoconf.info)Limitations of Usual Tools
Limitations of Usual Tools
==========================
The small set of tools you can expect to find on any machine can
still include some limitations you should be aware of.
`awk'
Don't leave white spaces before the parentheses in user functions
calls, GNU awk will reject it:
$ gawk 'function die () { print "Aaaaarg!" }
BEGIN { die () }'
gawk: cmd. line:2: BEGIN { die () }
gawk: cmd. line:2: ^ parse error
$ gawk 'function die () { print "Aaaaarg!" }
BEGIN { die() }'
Aaaaarg!
If you want your program to be deterministic, don't depend on `for'
on arrays:
$ cat for.awk
END {
arr["foo"] = 1
arr["bar"] = 1
for (i in arr)
print i
}
$ gawk -f for.awk </dev/null
foo
bar
$ nawk -f for.awk </dev/null
bar
foo
Some AWK, such as HPUX 11.0's native one, have regex engines
fragile to inner anchors:
$ echo xfoo | $AWK '/foo|^bar/ { print }'
$ echo bar | $AWK '/foo|^bar/ { print }'
bar
$ echo xfoo | $AWK '/^bar|foo/ { print }'
xfoo
$ echo bar | $AWK '/^bar|foo/ { print }'
bar
Either do not depend on such patterns (i.e., use `/^(.*foo|bar)/',
or use a simple test to reject such AWK.
`cat'
Don't rely on any option. The option `-v', which displays
non-printing characters, _seems_ portable, though.
`cc'
When a compilation such as `cc foo.c -o foo' fails, some compilers
(such as CDS on Reliant UNIX) leave a `foo.o'.
HP-UX `cc' doesn't accept `.S' files to preprocess and assemble.
`cc -c foo.S' will appear to succeed, but in fact does nothing.
`cmp'
`cmp' performs a raw data comparison of two files, while `diff'
compares two text files. Therefore, if you might compare DOS
files, even if only checking whether two files are different, use
`diff' to avoid spurious differences due to differences of newline
encoding.
`cp'
SunOS `cp' does not support `-f', although its `mv' does. It's
possible to deduce why `mv' and `cp' are different with respect to
`-f'. `mv' prompts by default before overwriting a read-only
file. `cp' does not. Therefore, `mv' requires a `-f' option, but
`cp' does not. `mv' and `cp' behave differently with respect to
read-only files because the simplest form of `cp' cannot overwrite
a read-only file, but the simplest form of `mv' can. This is
because `cp' opens the target for write access, whereas `mv'
simply calls `link' (or, in newer systems, `rename').
`date'
Some versions of `date' do not recognize special % directives, and
unfortunately, instead of complaining, they just pass them through,
and exit with success:
$ uname -a
OSF1 medusa.sis.pasteur.fr V5.1 732 alpha
$ date "+%s"
%s
`diff'
Option `-u' is nonportable.
Some implementations, such as Tru64's, fail when comparing to
`/dev/null'. Use an empty file instead.
`dirname'
Not all hosts have a working `dirname', and you should instead use
`AS_DIRNAME' (Note: Programming in M4sh). For example:
dir=`dirname "$file"` # This is not portable.
dir=`AS_DIRNAME(["$file"])` # This is more portable.
This handles a few subtleties in the standard way required by
POSIX. For example, under UN*X, should `dirname //1' give `/'?
Paul Eggert answers:
No, under some older flavors of Unix, leading `//' is a
special path name: it refers to a "super-root" and is used to
access other machines' files. Leading `///', `////', etc.
are equivalent to `/'; but leading `//' is special. I think
this tradition started with Apollo Domain/OS, an OS that is
still in use on some older hosts.
POSIX allows but does not require the special treatment for
`//'. It says that the behavior of dirname on path names of
the form `//([^/]+/*)?' is implementation defined. In these
cases, GNU `dirname' returns `/', but it's more portable to
return `//' as this works even on those older flavors of Unix.
`egrep'
The empty alternative is not portable, use `?' instead. For
instance with Digital Unix v5.0:
> printf "foo\n|foo\n" | egrep '^(|foo|bar)$'
|foo
> printf "bar\nbar|\n" | egrep '^(foo|bar|)$'
bar|
> printf "foo\nfoo|\n|bar\nbar\n" | egrep '^(foo||bar)$'
foo
|bar
`egrep' also suffers the limitations of `grep'.
`expr'
No `expr' keyword starts with `x', so use `expr x"WORD" :
'xREGEX'' to keep `expr' from misinterpreting WORD.
Don't use `length', `substr', `match' and `index'.
`expr' (`|')
You can use `|'. Although POSIX does require that `expr '''
return the empty string, it does not specify the result when you
`|' together the empty string (or zero) with the empty string. For
example:
expr '' \| ''
GNU/Linux and POSIX.2-1992 return the empty string for this case,
but traditional Unix returns `0' (Solaris is one such example).
In the latest POSIX draft, the specification has been changed to
match traditional Unix's behavior (which is bizarre, but it's too
late to fix this). Please note that the same problem does arise
when the empty string results from a computation, as in:
expr bar : foo \| foo : bar
Avoid this portability problem by avoiding the empty string.
`expr' (`:')
Don't use `\?', `\+' and `\|' in patterns, they are not supported
on Solaris.
The POSIX.2-1992 standard is ambiguous as to whether `expr a : b'
(and `expr 'a' : '\(b\)'') output `0' or the empty string. In
practice, it outputs the empty string on most platforms, but
portable scripts should not assume this. For instance, the QNX
4.25 native `expr' returns `0'.
You may believe that one means to get a uniform behavior would be
to use the empty string as a default value:
expr a : b \| ''
unfortunately this behaves exactly as the original expression, see
the ``expr' (`:')' entry for more information.
Older `expr' implementations (e.g. SunOS 4 `expr' and Solaris 8
`/usr/ucb/expr') have a silly length limit that causes `expr' to
fail if the matched substring is longer than 120 bytes. In this
case, you might want to fall back on `echo|sed' if `expr' fails.
Don't leave, there is some more!
The QNX 4.25 `expr', in addition of preferring `0' to the empty
string, has a funny behavior in its exit status: it's always 1
when parentheses are used!
$ val=`expr 'a' : 'a'`; echo "$?: $val"
0: 1
$ val=`expr 'a' : 'b'`; echo "$?: $val"
1: 0
$ val=`expr 'a' : '\(a\)'`; echo "?: $val"
1: a
$ val=`expr 'a' : '\(b\)'`; echo "?: $val"
1: 0
In practice this can be a big problem if you are ready to catch
failures of `expr' programs with some other method (such as using
`sed'), since you may get twice the result. For instance
$ expr 'a' : '\(a\)' || echo 'a' | sed 's/^\(a\)$/\1/'
will output `a' on most hosts, but `aa' on QNX 4.25. A simple
work around consists in testing `expr' and use a variable set to
`expr' or to `false' according to the result.
`find'
The option `-maxdepth' seems to be GNU specific. Tru64 v5.1,
NetBSD 1.5 and Solaris 2.5 `find' commands do not understand it.
The replacement of `{}' is guaranteed only if the argument is
exactly _{}_, not if it's only a part of an argument. For
instance on DU, and HP-UX 10.20 and HP-UX 11:
$ touch foo
$ find . -name foo -exec echo "{}-{}" \;
{}-{}
while GNU `find' reports `./foo-./foo'.
`grep'
Don't use `grep -s' to suppress output, because `grep -s' on
System V does not suppress output, only error messages. Instead,
redirect the standard output and standard error (in case the file
doesn't exist) of `grep' to `/dev/null'. Check the exit status of
`grep' to determine whether it found a match.
Don't use multiple regexps with `-e', as some `grep' will only
honor the last pattern (eg., IRIX 6.5 and Solaris 2.5.1). Anyway,
Stardent Vistra SVR4 `grep' lacks `-e'... Instead, use
alternation and `egrep'.
`ln'
Don't rely on `ln' having a `-f' option. Symbolic links are not
available on old systems, use `ln' as a fall back.
For versions of the DJGPP before 2.04, `ln' emulates soft links
for executables by generating a stub that in turn calls the real
program. This feature also works with nonexistent files like in
the Unix spec. So `ln -s file link' will generate `link.exe',
which will attempt to call `file.exe' if run. But this feature only
works for executables, so `cp -p' is used instead for these
systems. DJGPP versions 2.04 and later have full symlink support.
`mv'
The only portable options are `-f' and `-i'.
Moving individual files between file systems is portable (it was
in V6), but it is not always atomic: when doing `mv new existing',
there's a critical section where neither the old nor the new
version of `existing' actually exists.
Moving directories across mount points is not portable, use `cp'
and `rm'.
Moving/Deleting open files isn't portable. The following can't be
done on DOS/WIN32:
exec > foo
mv foo bar
nor can
exec > foo
rm -f foo
`sed'
Patterns should not include the separator (unless escaped), even
as part of a character class. In conformance with POSIX, the Cray
`sed' will reject `s/[^/]*$//': use `s,[^/]*$,,'.
Sed scripts should not use branch labels longer than 8 characters
and should not contain comments.
Don't include extra `;', as some `sed', such as NetBSD 1.4.2's,
try to interpret the second as a command:
$ echo a | sed 's/x/x/;;s/x/x/'
sed: 1: "s/x/x/;;s/x/x/": invalid command code ;
Input should have reasonably long lines, since some `sed' have an
input buffer limited to 4000 bytes.
Alternation, `\|', is common but POSIX.2 does not require its
support, so it should be avoided in portable scripts. Solaris 8
`sed' does not support alternation; e.g. `sed '/a\|b/d'' deletes
only lines that contain the literal string `a|b'.
Anchors (`^' and `$') inside groups are not portable.
Nested parenthesization in patterns (e.g., `\(\(a*\)b*)\)') is
quite portable to modern hosts, but is not supported by some older
`sed' implementations like SVR3.
Of course the option `-e' is portable, but it is not needed. No
valid Sed program can start with a dash, so it does not help
disambiguating. Its sole usefulness is helping enforcing
indenting as in:
sed -e INSTRUCTION-1 \
-e INSTRUCTION-2
as opposed to
sed INSTRUCTION-1;INSTRUCTION-2
Contrary to yet another urban legend, you may portably use `&' in
the replacement part of the `s' command to mean "what was
matched". All descendents of Bell Lab's V7 `sed' (at least; we
don't have first hand experience with older `sed's) have supported
it.
`sed' (`t')
Some old systems have `sed' that "forget" to reset their `t' flag
when starting a new cycle. For instance on MIPS RISC/OS, and on
IRIX 5.3, if you run the following `sed' script (the line numbers
are not actual part of the texts):
s/keep me/kept/g # a
t end # b
s/.*/deleted/g # c
: end # d
on
delete me # 1
delete me # 2
keep me # 3
delete me # 4
you get
deleted
delete me
kept
deleted
instead of
deleted
deleted
kept
deleted
Why? When processing 1, a matches, therefore sets the t flag, b
jumps to d, and the output is produced. When processing line 2,
the t flag is still set (this is the bug). Line a fails to match,
but `sed' is not supposed to clear the t flag when a substitution
fails. Line b sees that the flag is set, therefore it clears it,
and jumps to d, hence you get `delete me' instead of `deleted'.
When processing 3 t is clear, a matches, so the flag is set, hence
b clears the flags and jumps. Finally, since the flag is clear, 4
is processed properly.
There are two things one should remind about `t' in `sed'.
Firstly, always remember that `t' jumps if _some_ substitution
succeeded, not only the immediately preceding substitution,
therefore, always use a fake `t clear; : clear' to reset the t
flag where indeed.
Secondly, you cannot rely on `sed' to clear the flag at each new
cycle.
One portable implementation of the script above is:
t clear
: clear
s/keep me/kept/g
t end
s/.*/deleted/g
: end
`touch'
On some old BSD systems, `touch' or any command that results in an
empty file does not update the timestamps, so use a command like
`echo' as a workaround.
GNU `touch' 3.16r (and presumably all before that) fails to work
on SunOS 4.1.3 when the empty file is on an NFS-mounted 4.2 volume.
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