Hi all,
I'm trying to detect whether a string holds a dash but nothing seems to work for me (I'm new to shell).
if [ "$m" -eq "-" ]
then
echo "has dash"
else
echo "has no dash"
fi
views:
60answers:
3
+4
A:
-eq is used for testing equality of integers. To test for string equality, use = instead:
if [ "$m" = - ]
See the man page for test for further details.
Phil Ross
2009-11-28 22:21:22
Important note: if `$m` is the empty string, this will evaluate to a syntax error. To prevent that, a common idiom is to test `if [ "x$m" = x- ]`.
Adam Rosenfield
2009-11-28 22:23:19
Actually, with the `"$m"` quotes around `$m`, there will be an empty string on `[`'s `args` list, and therefore no syntax error. The reason for quoting stuff beginning with `x` is to prevent the problems when `$m` starts with a `-` and `[` interprets it as a command line switch.
ndim
2009-11-28 22:25:41
@Adam I don't get a syntax error in bash (version 3.1). The empty double quotes are treated as the first argument.
Phil Ross
2009-11-28 22:28:46
@ndim I also don't see any problems when `$m` starts with a `-`.
Phil Ross
2009-11-28 22:31:21
I personally have not observed problems either, but then I have not tested a similar number of Unix shells the Autoconf people need their shell code run on. In http://www.gnu.org/software/hello/manual/autoconf/Limitations-of-Builtins.html, they write that Posix says `[ "..." = "..." ]` should work for every possible string, but in reality, you do run into issues on some systems if the string starts with `-`.
ndim
2009-11-28 22:48:17
+2
A:
if [ "x$m" = "x-" ]; then
echo "is a dash"
else
echo "is not a dash"
fi
Uses string comparison, quotes everything, and avoids possible [ command line switch confusion (on some not-quite-Posix shells) if $m starts with a -.
ndim
2009-11-28 22:27:19
+1
A:
The '-eq' operator performs an arithmetic comparison. You need to use the '=' operator instead. ie:
if test "$m" = '-'; then echo "is a dash"; else echo "has no dash"; fi
Howard Hong
2009-11-28 23:30:46