Multiple -a with greater than / less than break ba

问题:

I wrote a bash script that performs a curl call only during business hours. For some reason, the hourly comparison fails when I add an "-a" operator (and for some reason my bash does not recognize "&&").

Though the script is much larger, here is the relevant piece:

HOUR=`date +%k`

if [ $HOUR > 7 -a $HOUR < 17 ];
then
  //do sync
fi

The script gives me the error:

./tracksync: (last line): Cannot open (line number): No such file

However, this comparison does not fail:

if [ $DAY != "SUNDAY" -a $HOUR > 7 ];
then
  //do sync
fi

Is my syntax wrong or is this a problem with my bash?

回答1:

You cannot use < and > in bash scripts as such. Use -lt and -gt for that:

if [ $HOUR -gt 7 -a $HOUR -lt 17 ]

< and > are used by the shell to perform redirection of stdin or stdout.

The comparison that you say is working is actually creating a file named 7 in the current directory.

As for &&, that also has a special meaning for the shell and is used for creating an "AND list" of commands.

The best documentation for all these: man bash (and man test for details on comparison operators)

回答2:

There are a few answers here but none of them recommend actual numerical context.

Here is how to do it in bash:

if (( hour > 7 && hour < 17 )); then
   ...
fi

Note that "$" is not needed to expand variables in numerical context.

回答3:

I suggest you use quotes around variable references and "standard" operators:

if [ "$HOUR" -gt 7 -a "$HOUR" -lt 17 ]; ...; fi

回答4:

Try using [[ instead, because it is safer and has more features. Also use -gt and -lt for numeric comparison.

if [[ $HOUR -gt 7 && $HOUR -lt 17 ]]
then
    # do something
fi