打击“不”：反转命令

Question

我知道我能做到这一点的退出状态...

if diff -q $f1 $f2 
then 
    echo "they're the same" 
else 
    echo "they're different" 
fi 

但是，如果我想否定我正在检查的条件是什么？即是这样的（这显然是行不通的）

if not diff -q $f1 $f2 
then 
    echo "they're different" 
else 
    echo "they're the same" 
fi 

我可以做这样的事情......

diff -q $f1 $f2 
if [[ $? > 0 ]] 
then 
    echo "they're different" 
else 
    echo "they're the same" 
fi 

当我检查前一个命令的退出状态是否大于0但这感觉有点尴尬。有没有更习惯的方式来做到这一点？

Answer 1

if ! diff -q $f1 $f2; then ... 

Answer 2

如果你想否定，你正在寻找!：

if ! diff -q $f1 $f2; then 
    echo "they're different" 
else 
    echo "they're the same" 
fi 

或（simplty逆转的if/else动作）：

if diff -q $f1 $f2; then 
    echo "they're the same" 
else 
    echo "they're different" 
fi 

也或者，尝试使用cmp这样做：

if cmp &>/dev/null $f1 $f2; then 
    echo "$f1 $f2 are the same" 
else 
    echo >&2 "$f1 $f2 are NOT the same" 
fi 

Answer 3

否定使用if ! diff -q $f1 $f2;。记录在man test：

! EXPRESSION 
     EXPRESSION is false 

不明白为什么你需要的否定，因为你处理这两种情况......如果你只需要处理，他们不匹配的情况：

diff -q $f1 $f2 || echo "they're different"