用树莓派3B+做了一些测试(我使用的是覆盖文件系统，需要定期同步)，我自己比较了diff -q和cmp -s;注意，这是一个来自/dev/shm内部的日志，所以磁盘访问速度不是问题:

[root@mypi shm]# dd if=/dev/urandom of=test.file bs=1M count=100 ; time diff -q test.file test.copy && echo diff true || echo diff false ; time cmp -s test.file test.copy && echo cmp true || echo cmp false ; cp -a test.file test.copy ; time diff -q test.file test.copy && echo diff true || echo diff false; time cmp -s test.file test.copy && echo cmp true || echo cmp false
100+0 records in
100+0 records out
104857600 bytes (105 MB) copied, 6.2564 s, 16.8 MB/s
Files test.file and test.copy differ

real    0m0.008s
user    0m0.008s
sys     0m0.000s
diff false

real    0m0.009s
user    0m0.007s
sys     0m0.001s
cmp false
cp: overwrite âtest.copyâ? y

real    0m0.966s
user    0m0.447s
sys     0m0.518s
diff true

real    0m0.785s
user    0m0.211s
sys     0m0.573s
cmp true
[root@mypi shm]# pico /root/rwbscripts/utils/squish.sh

我试了几次。cmp -s在我使用的测试箱上的时间始终略短。所以如果你想使用cmp -s在两个文件之间做事情....

identical (){
  echo "$1" and "$2" are the same.
  echo This is a function, you can put whatever you want in here.
}
different () {
  echo "$1" and "$2" are different.
  echo This is a function, you can put whatever you want in here, too.
}
cmp -s "$FILEA" "$FILEB" && identical "$FILEA" "$FILEB" || different "$FILEA" "$FILEB"

对于没有不同的文件，任何方法都需要完全读取两个文件，即使读取是在过去。

除此之外别无选择。因此，在某个时间点上创建哈希或校验和需要读取整个文件。大文件需要时间。

文件元数据检索比读取大文件快得多。

那么，您是否可以使用任何文件元数据来确定这些文件是不同的? 文件大小?或者甚至文件命令只读取文件的一小部分的结果?

文件大小示例代码片段:

  ls -l $1 $2 | 
  awk 'NR==1{a=$5} NR==2{b=$5} 
       END{val=(a==b)?0 :1; exit( val) }'

[ $? -eq 0 ] && echo 'same' || echo 'different'  

如果文件大小相同，则只能读取完整的文件。

还可以尝试使用cksum命令:

chk1=`cksum <file1> | awk -F" " '{print $1}'`
chk2=`cksum <file2> | awk -F" " '{print $1}'`

if [ $chk1 -eq $chk2 ]
then
  echo "File is identical"
else
  echo "File is not identical"
fi

cksum命令将输出文件的字节计数。参见“man cksum”。

用树莓派3B+做了一些测试(我使用的是覆盖文件系统，需要定期同步)，我自己比较了diff -q和cmp -s;注意，这是一个来自/dev/shm内部的日志，所以磁盘访问速度不是问题:

[root@mypi shm]# dd if=/dev/urandom of=test.file bs=1M count=100 ; time diff -q test.file test.copy && echo diff true || echo diff false ; time cmp -s test.file test.copy && echo cmp true || echo cmp false ; cp -a test.file test.copy ; time diff -q test.file test.copy && echo diff true || echo diff false; time cmp -s test.file test.copy && echo cmp true || echo cmp false
100+0 records in
100+0 records out
104857600 bytes (105 MB) copied, 6.2564 s, 16.8 MB/s
Files test.file and test.copy differ

real    0m0.008s
user    0m0.008s
sys     0m0.000s
diff false

real    0m0.009s
user    0m0.007s
sys     0m0.001s
cmp false
cp: overwrite âtest.copyâ? y

real    0m0.966s
user    0m0.447s
sys     0m0.518s
diff true

real    0m0.785s
user    0m0.211s
sys     0m0.573s
cmp true
[root@mypi shm]# pico /root/rwbscripts/utils/squish.sh

我试了几次。cmp -s在我使用的测试箱上的时间始终略短。所以如果你想使用cmp -s在两个文件之间做事情....

identical (){
  echo "$1" and "$2" are the same.
  echo This is a function, you can put whatever you want in here.
}
different () {
  echo "$1" and "$2" are different.
  echo This is a function, you can put whatever you want in here, too.
}
cmp -s "$FILEA" "$FILEB" && identical "$FILEA" "$FILEB" || different "$FILEA" "$FILEB"

因为我很糟糕，没有足够的声誉点，我不能把这个花絮作为评论。

但是，如果您打算使用cmp命令(并且不需要/不想太冗长)，您可以只获取退出状态。根据cmp手册页:

如果FILE为'-'或缺失，则读取标准输入。退出状态为0 如果输入相同，1如果不同，2如果麻烦。

所以，你可以这样做:

STATUS="$(cmp --silent $FILE1 $FILE2; echo $?)"  # "$?" gives exit status for each comparison

if [[ $STATUS -ne 0 ]]; then  # if status isn't equal to 0, then execute code
    DO A COMMAND ON $FILE1
else
    DO SOMETHING ELSE
fi

编辑:感谢大家的评论!我在这里更新了测试语法。但是，如果您正在寻找与这个答案在可读性、风格和语法方面类似的东西，我建议您使用Vasili的答案。

我相信cmp将在第一个字节差处停止:

cmp --silent $old $new || echo "files are different"