Re: difference between the -o and || conditionals.
From: Stephane CHAZELAS (this.address_at_is.invalid)
Date: 12/01/04
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Date: Wed, 1 Dec 2004 17:29:46 +0000
2004-12-1, 08:42(-08), Dexter:
[...]
> Can someone please tell me the difference between the following constructs.
>
> if [ $? -ne 0 ] -o [ "`cat $tmp_file | grep -i error`" ] ; then
> print "error"
> fi
> ==OR===
>
> if [ $? -ne 0 ] || [ "`cat $tmp_file | grep -i error`" ] ; then
> print "error"
> fi
> ==OR===
>
> if [ $? -ne 0 -o "`cat $tmp_file | grep -i error`" ] ; then
> print "error"
> fi
[...]
None is correct.
If should be:
if [ "$?" -ne 0 ] || grep -li error < "$tmp_file" > /dev/null; then
"[" is an ordinary (even though generally built in the shell,
but that makes no difference from a functionnal point of view)
command.
In the first case, you are passing it those arguments:
0: "["
1 (or 0 or more): The expansion of $? possibly splitted as you
forgot to quote it and I don't know the current value of IFS.
2: "-ne"
3: "0"
4: "]"
5: "-o"
6: "["
7: the output of cat $tmp_file | grep -i error minus the
trailing newline characters as a single argument
8: "]"
Given all those arguments, "[" will have a hard time guessing
was kind of test you ask it to perform (and will probably fail
as the above syntax is not correct).
In the second case, you are running a first "[" command with
those arguments:
0: "["
1 (or 0 or more): The expansion of $? possibly splitted as you
forgot to quote it and I don't know the current value of IFS.
2: "-ne"
3: "0"
4: "]"
If $? expands to a single argument, then "[" will be able to
parse that command and will check if its first argument
is the string representation of an integer number that is not
equal to the number whose string representation is in its third
argument. If true, it will return with a 0 exit status and with
a non-0 exit status otherwise.
Then if (and only if) the exit status of that first command is
non-zero, then the shell will run the second "[" command with
those arguments:
0: "["
1: the output of cat $tmp_file | grep -i error minus the
trailing newline characters as a single argument
2: "]"
Some "[" implementations when given only one argument beside the
closing "]" will test wether that argument is empty or not (and
report the corresponding exit status). Some other "["
implementations will first check if that argument looks like a
test operator (such as "-f", "!"...), so it's wiser not to use
that at all and use the more obvious:
[ -n "..." ]
In the third case, one "[" command is run with those arguments
0: "["
1 (or 0 or more): The expansion of $? possibly splitted as you
forgot to quote it and I don't know the current value of IFS.
2: "-ne"
3: "0"
4: "-o"
5: the output of cat $tmp_file | grep -i error minus the
trailing newline characters as a single argument
6: "]"
Generally, when given more than 3 arguments (beside the "]"),
"[" implementations get very confused about what you are trying
to test, especially if operands look like test operators. That's
why the second case is prefered. In that particular case,
moreover, the cat and grep commands are run and its (possibly
huge) output is stored in memory and passed to the "[" command
even if $? -ne 0
Now, let's compare with my suggested solution.
if [ "$?" -ne 0 ] || grep -li error < "$tmp_file" > /dev/null; then
The first "[" tests wether the expansion of $? is a the string
representation of a number that is not-equal to 0.
Then, if that command returns false (a non-zero exit status),
then the shell runs the grep command, with those arguments:
0: "grep"
1: "-li"
2: "error"
(and that's all)
with its stdin connected to the file whose path is stored in
$tmp_file and its stdout connected to "/dev/null".
grep will then read one line at a time from its stdin, and as
soon as it finds a line that contains "error" (regardless of
case), it will output something like "(stdin)" to its stdout and
exit immediately with a 0 (true) exit status. On the other hand,
if it reaches the end of the file without finding "error", then
it will return with a non-zero exit status.
Compare that with
[ "`cat $tmp_file | grep -i error`" ]
which creates two pipes, forks at least two processs, runs two
commands, cat reading the whole file, then writing it without
even doing any modification on it, grep reading from that pipe,
outputing every line (til the end) containing "error", the shell
storing all that output in memory, and then, when grep is
finished, calls "[" with that string stored in memory as
argument and then the "[" command in turns tests wether its
argument is empty or not and return the corresponding exit
status.
-- Stephane
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