壳牌可变容量

Question

假设我有一个shell脚本,其中有一个语句,如:

a=$(find / -type f)

这表示存在一定的文件列表,其文件路径将存储在变量"a"中.

它可以存储的最大限制或行数是多少.我怎么找到它？

Answer 1

IIRC,bash并没有对变量可以存储多少数据施加限制.但是受到bash执行环境的限制.有关更全面的解释,请参阅此答案.

Answer 2

作为数据点，我在 OS X 10.10.5 中尝试了以下脚本，在配备 2.8 GHz Intel Core i7 的 Macbook Pro Retina 上使用内置 bash：

#!/bin/bash

humombo="X"

while true; do
    humombo="$humombo$humombo"
    echo "Time $(date "+%H:%M:%S"), chars $(echo "$humombo" | wc -c)"
done

结果：大小一次又一次地愉快地翻倍（请注意，大小包括单行结尾的额外字节）。当humombo超过 4MB时，事情开始变慢；从 256MB 翻倍到 512MB 需要 48 秒，然后脚本爆炸：

mbpe:~ griscom$ ./delme.sh 
Time 16:00:04, chars        3
Time 16:00:04, chars        5
Time 16:00:04, chars        9
Time 16:00:04, chars       17
Time 16:00:04, chars       33
Time 16:00:04, chars       65
Time 16:00:04, chars      129
Time 16:00:04, chars      257
Time 16:00:04, chars      513
Time 16:00:04, chars     1025
Time 16:00:04, chars     2049
Time 16:00:04, chars     4097
Time 16:00:04, chars     8193
Time 16:00:04, chars    16385
Time 16:00:04, chars    32769
Time 16:00:04, chars    65537
Time 16:00:04, chars   131073
Time 16:00:04, chars   262145
Time 16:00:04, chars   524289
Time 16:00:04, chars  1048577
Time 16:00:04, chars  2097153
Time 16:00:05, chars  4194305
Time 16:00:05, chars  8388609
Time 16:00:07, chars  16777217
Time 16:00:09, chars  33554433
Time 16:00:15, chars  67108865
Time 16:00:27, chars  134217729
Time 16:00:51, chars  268435457
Time 16:01:39, chars  536870913
bash(80722,0x7fff77bff300) malloc: *** mach_vm_map(size=18446744071562072064) failed (error code=3)
*** error: can't allocate region
*** set a breakpoint in malloc_error_break to debug
./delme.sh: xrealloc: cannot allocate 18446744071562068096 bytes
mbpe:~ griscom$ 

两个注意事项：

• 我怀疑崩溃更多是整个过程占用了太多内存，而不是我达到了单个变量容量的限制。

• 在玩这个时，我以交互方式运行了相同的命令，当循环退出时 bash 被破坏了；我必须打开一个新的终端窗口才能做任何事情。因此，过多的内存分配会以未知的方式破坏 bash；我的猜测是在脚本中执行此操作会在退出时进行清理。

编辑：我刚刚在高性能 Ubuntu 18 系统上尝试了相同的代码：

Time 18:03:02, chars 3
Time 18:03:02, chars 5
Time 18:03:02, chars 9
Time 18:03:02, chars 17
Time 18:03:02, chars 33
Time 18:03:02, chars 65
Time 18:03:02, chars 129
Time 18:03:02, chars 257
Time 18:03:02, chars 513
Time 18:03:02, chars 1025
Time 18:03:02, chars 2049
Time 18:03:02, chars 4097
Time 18:03:02, chars 8193
Time 18:03:02, chars 16385
Time 18:03:02, chars 32769
Time 18:03:02, chars 65537
Time 18:03:02, chars 131073
Time 18:03:02, chars 262145
Time 18:03:02, chars 524289
Time 18:03:02, chars 1048577
Time 18:03:02, chars 2097153
Time 18:03:02, chars 4194305
Time 18:03:02, chars 8388609
Time 18:03:03, chars 16777217
Time 18:03:04, chars 33554433
Time 18:03:07, chars 67108865
Time 18:03:12, chars 134217729
Time 18:03:23, chars 268435457
Time 18:03:43, chars 536870913
./delme.sh: xrealloc: cannot allocate 18446744071562068096 bytes

花了不到一半的时间，死得更干净一点，但字符大小相同。（顺便说一句，错误消息中的数字，十进制 18446744071562068096，是 0xffff ffff 8000 0080，所以很明显我们在这里遇到了一些数字容量限制。）