为什么同时有/sbin/sh和/bin/sh

为什么同时有/sbin/sh和/bin/sh

Both /sbin/sh and /usr/bin/sh shells are Bourne shells.
The difference is that /sbin/sh is statically linked and /usr/bin/sh is
dynamically linked.  There is also /bin/sh, however /bin is just a symbolic
link to /usr/bin.

# ls -l /sbin/sh
-r-xr-xr-x   2 bin      root      251712 Jul 16  1997 /sbin/sh

# file /sbin/sh
/sbin/sh:       ELF 32-bit MSB executable SPARC Version 1, statically linked, stripped

# ldd /sbin/sh
ldd: /sbin/sh: file is not a dynamic executable or shared object

# ls -l /usr/bin/sh
-r-xr-xr-x   3 bin      root       88620 Jul 16  1997 /usr/bin/sh

# file /usr/bin/sh
/usr/bin/sh:    ELF 32-bit MSB executable SPARC Version 1, dynamically linked, stripped

# ldd /usr/bin/sh
       libc.so.1 =>;     /usr/lib/libc.so.1
       libdl.so.1 =>;    /usr/lib/libdl.so.1

Dynamically linked binaries use shared libraries during their execution.  This
makes them smaller in size and when libraries get updated, the binaries will
automatically use the updated libraries.

Statically linked binaries contain all the code within them.  They do not
use shared libraries at all.  This makes them larger in size.  The useful thing
about statically linked binaries is that they can run all by themselves as
they do not depend on shared libraries.  If the system is "broken" and libraries
were removed, damaged, or /usr is not mounted, /sbin/sh can still be executed.

This is why it is recommended to use /sbin/sh as the default root shell.

大体意思就是/sbin/sh 是静态链接的, /bin/sh是动态链接的。 静态链接的sh虽然比动态链接的size大,但它有个好处就是它的运行不依赖共享库。也就是说如果系统崩溃导致库文件丢失或损坏,它仍然能执行。
最后一句: 这就是我们推荐用/sbin/sh作为root的默认shell的原因。

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