打擊「不」：反轉命令

Question

我知道我能做到這一點的退出狀態...

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檢查的條件是什麼？即是這樣的（這顯然是行不通的）

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

我可以做這樣的事情......

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

當我檢查前一個命令的退出狀態是否大於0但這感覺有點尷尬。有沒有更習慣的方式來做到這一點？

Answer 1

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

Answer 2

如果你想否定，你正在尋找!：

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

或（simplty逆轉的if/else動作）：

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

也或者，嘗試使用cmp這樣做：

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;。記錄在man test：

! EXPRESSION 
     EXPRESSION is false 

不明白爲什麼你需要的否定，因爲你處理這兩種情況......如果你只需要處理，他們不匹配的情況：

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