Confounded by conditional expressions
in [Shell]
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From: boblatest on 14 Jul 2010 04:53 On Jul 12, 9:24 pm, Wayne <nos...(a)all.invalid> wrote: > On 7/12/2010 2:00 PM, Robert Latest wrote: > > > I've written a script that contains this line: > > > if [ ($last_seen -gt 0) -a ($last_seen -lt $toolate) ] ; then > > # whatever > > fi > > > It throws this syntax error: > > > test.sh: 23: Syntax error: word unexpected (expecting ")") > > > Why? > > > robert > > The round parenthesis need quoting: > > if [ \($last_seen -gt 0\) -a \($last_seen -lt $toolate\) ] ; then > > -- > Wayne Thanks! But how on God's Earth is one supposed to see that from the manpage excerpt I quoted? I suppose it becomes clear when you read the entire page and work out the sequence of substitution the shell makes, but what the heck, the manpage is supposed to be a quick reference! Sheesh. robert
From: Janis Papanagnou on 14 Jul 2010 04:54 Kenny McCormack schrieb: > In article <tk2ug7-jtg.ln1(a)leafnode-msgid.gclare.org.uk>, > Geoff Clare <geoff(a)clare.See-My-Signature.invalid> wrote: > ... >> SUSv3 was superseded by SUSv4 in 2008. Sven was referring to the >> current standard, not the out-of-date one you were looking at. > > Don't confuse the pointed out with facts. They only confuse and annoy him. > LOL. Nice pun. :-)
From: boblatest on 14 Jul 2010 04:54 On Jul 12, 8:33 pm, "Chris F.A. Johnson" <cfajohn...(a)gmail.com> wrote: > On 2010-07-12, Robert Latest wrote: > > > > > > > Hello all, > > quoting the "sh" manpage: > > > test expression > > > [ expression ] > > The test utility evaluates the expression and, if it evaluates to > > true, returns a zero (true) exit status; otherwise it returns 1 > > (false). If there is no expression, test also returns 1 (false). > > > All operators and flags are separate arguments to the test util??? > > ity. > > > The following primaries are used to construct expression: > > > [...] > > > expression1 -a expression2 > > True if both expression1 and expression2 are true. > > > expression1 -o expression2 > > True if either expression1 or expression2 are true. > > > (expression) True if expression is true. > > > I've written a script that contains this line: > > > if [ ($last_seen -gt 0) -a ($last_seen -lt $toolate) ] ; then > > # whatever > > fi > > Avoid -a and -o; use && and ||: > > if [ $last_seen -gt 0 ] && [ $last_seen -lt $toolate ] ; then > > -- > Chris F.A. Johnson, author <http://shell.cfajohnson.com/> > =================================================================== > Shell Scripting Recipes: A Problem-Solution Approach (2005, Apress) > Pro Bash Programming: Scripting the GNU/Linux Shell (2009, Apress) Thanks, that did it. Thanks to everybody for the good replies and the interesting discussion! robert
From: Thomas 'PointedEars' Lahn on 14 Jul 2010 12:26 boblatest wrote: > On Jul 12, 9:24 pm, Wayne <nos...(a)all.invalid> wrote: >> On 7/12/2010 2:00 PM, Robert Latest wrote: >> > I've written a script that contains this line: >> > >> > if [ ($last_seen -gt 0) -a ($last_seen -lt $toolate) ] ; then >> > # whatever >> > fi >> > >> > It throws this syntax error: >> > >> > test.sh: 23: Syntax error: word unexpected (expecting ")") >> > >> > Why? >> >> The round parenthesis need quoting: >> >> if [ \($last_seen -gt 0\) -a \($last_seen -lt $toolate\) ] ; then > > Thanks! But how on God's Earth is one supposed to see that from the > manpage excerpt I quoted? It is not clear from which "sh" manpage you have quoted. However, to answer your question, one would familiarize oneself with shell syntax before one attempted to use shell utilities like `test'. In your manpage, too, there is probably a warning just below the quoted section that the `test' grammar is inherently ambiguous, and that, when in doubt, the POSIX shell grammar would take precedence. The parentheses delimit a subshell expression in sh(1), which is obviously not allowed as positional argument to `[' (or `test'). So if they are to be interpreted by `test' instead of `sh', they need to be escaped (but can probably be safely omitted here instead). BTW, your complaints are misplaced here. This is _not_ the `sh' customer support. -- PointedEars
From: Wayne on 14 Jul 2010 12:40
On 7/14/2010 4:53 AM, boblatest wrote: > On Jul 12, 9:24 pm, Wayne <nos...(a)all.invalid> wrote: >> On 7/12/2010 2:00 PM, Robert Latest wrote: >> >>> I've written a script that contains this line: >> >>> if [ ($last_seen -gt 0) -a ($last_seen -lt $toolate) ] ; then >>> # whatever >>> fi >> >>> It throws this syntax error: >> >>> test.sh: 23: Syntax error: word unexpected (expecting ")") >> >>> Why? >> >>> robert >> >> The round parenthesis need quoting: >> >> if [ \($last_seen -gt 0\) -a \($last_seen -lt $toolate\) ] ; then >> >> -- >> Wayne > > Thanks! But how on God's Earth is one supposed to see that from the > manpage excerpt I quoted? I suppose it becomes clear when you read the > entire page and work out the sequence of substitution the shell makes, > but what the heck, the manpage is supposed to be a quick reference! > Sheesh. > > robert Shell scripting is too complex to have every facet described on every man page. It's elegant (in some ways), and even fun, but not simple. You might be better served taking a course in it, or reading a good book. To understand the more complex bits, there's always this newgroup! -- Wayne |