summaryrefslogtreecommitdiff
path: root/doc
diff options
context:
space:
mode:
Diffstat (limited to 'doc')
-rw-r--r--doc/coreutils.texi32
1 files changed, 31 insertions, 1 deletions
diff --git a/doc/coreutils.texi b/doc/coreutils.texi
index 33be4d8ac..64419c769 100644
--- a/doc/coreutils.texi
+++ b/doc/coreutils.texi
@@ -12638,25 +12638,55 @@ test 0x100 -eq 1
@cindex logical connectives
@cindex connectives, logical
-The usual logical connectives.
+Note it's preferred to use shell logical primitives
+rather than these logical connectives internal to @command{test},
+because an expression may become ambiguous
+depending on the expansion of its parameters.
+
+For example, this becomes ambiguous when @samp{$1}
+is set to @samp{'!'} and @samp{$2} to the empty string @samp{''}:
+
+@example
+test "$1" -a "$2"
+@end example
+
+and should be written as:
+
+@example
+test "$1" && test "$2"
+@end example
+
+Note the shell logical primitives also benefit from
+short circuit operation, which can be significant
+for file attribute tests.
@table @samp
@item ! @var{expr}
@opindex !
True if @var{expr} is false.
+@samp{!} has lower precedence than all parts of @var{expr}.
+Note @samp{!} needs to be specified to the left
+of a binary expression, I.e., @samp{'!' 1 -gt 2}
+rather than @samp{1 '!' -gt 2}.
+Also @samp{!} is often a shell special character
+and is best used quoted.
+
@item @var{expr1} -a @var{expr2}
@opindex -a
@cindex logical and operator
@cindex and operator
True if both @var{expr1} and @var{expr2} are true.
+@samp{-a} is left associative,
+and has a higher precedence than @samp{-o}.
@item @var{expr1} -o @var{expr2}
@opindex -o
@cindex logical or operator
@cindex or operator
True if either @var{expr1} or @var{expr2} is true.
+@samp{-o} is left associative.
@end table