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The small set of tools you can expect to find on any machine can still include some limitations you should be aware of.
awk
$ 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 | $AWK '/^bar|foo/ { print }' bar |
Either do not depend on such patterns (i.e., use `/^(.*foo|bar)/', or use a simple test to reject such AWK.
cat
cc
cmp
cmp
performs a raw data comparison of two files, while
diff
compares two text files. Therefore, if you might compare
DOS files, even if only checking whether two files are different, use
diff
to avoid spurious differences due to differences of
newline encoding.
cp
cp
does not support `-f', although its
mv
does. It's possible to deduce why mv
and
cp
are different with respect to `-f'. mv
prompts by default before overwriting a read-only file. cp
does not. Therefore, mv
requires a `-f' option, but
cp
does not. mv
and cp
behave differently
with respect to read-only files because the simplest form of
cp
cannot overwrite a read-only file, but the simplest form of
mv
can. This is because cp
opens the target for
write access, whereas mv
simply calls link
(or, in
newer systems, rename
).
diff
Some implementations, such as Tru64's, fail when comparing to `/dev/null'. Use an empty file instead.
dirname
dirname
, but it is reasonably easy to
emulate, e.g.:
dir=`expr "x$file" : 'x\(.*\)/[^/]*' \| '.' : '.' |
But there are a few subtilities, e.g., under UN*X, should `//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.2 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.I have heard rumors that this special treatment of `//' may be dropped in future versions of POSIX, but for now it's still the standard.
egrep
> 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
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
(`|')
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
(`:')
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/^\(a\)$/\1/' |
will output `a' on most hosts, but `aa' on QNX 4.25. A
simple work around consists in testing expr
and use a variable
set to expr
or to false
according to the result.
find
find
commands do not understand it.
grep
grep
to `/dev/null'. Check the exit
status of grep
to determine whether it found a match.
Don't use multiple regexps with `-e', as some grep
will only
honor the last pattern (eg., IRIX 6.5 and Solaris 2.5.1). Anyway,
Stardent Vistra SVR4 grep
lacks `-e'... Instead, use
alternation and egrep
.
ln
ln
having a `-f' option. Symbolic links
are not available on old systems, use `ln' as a fall back.
For versions of the DJGPP before 2.04, ln
emulates soft links
for executables by generating a stub that in turn calls the real
program. This feature also works with nonexistent files like in the
Unix spec. So `ln -s file link' will generate `link.exe',
which will attempt to call `file.exe' if run. But this feature only
works for executables, so `cp -p' is used instead for these
systems. DJGPP versions 2.04 and later have full symlink support.
mv
Moving individual files between file systems is portable (it was in V6), but it is not always atomic: when doing `mv new existing', there's a critical section where neither the old nor the new version of `existing' actually exists.
Moving directories across mount points is not portable, use cp
and rm
.
sed
sed
will reject `s/[^/]*$//': 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 not portable. Anchors (`^' and `$') inside groups are not portable.
Nested groups are extremely portable, but there is at least one
sed
(System V/68 Base Operating System R3V7.1) that does not
support it.
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".
sed
(`t')
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
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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