为什么在Ubuntu上“ let”不能与“ sh myscript”一起使用，而对“ ./myscript”却可以呢？

Question

By running the following script in sh and ./ mode get different result on Ubuntu

#!/bin/bash
x=80
z=90
let "a=$x+$z"
echo $a

Result:

sh mode gives me "blank" output, while ./ mode yields 170.

Any explanation?

Answer 1

Shell Selection via Invocation Method

./mode honors the shebang, which specifies that your script be invoked with bash . By contrast, sh mode explicitly uses sh instead, ignoring the shebang. ( . mode doesn't run a new shell at all, but executes the commands in the script inside the same shell you're already using interactively).

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Why Shell Selection Matters

let is a bash extension for compatibility with pre-POSIX shells.

On a platform (such as Ubuntu) where /bin/sh is implemented by an ash derivative, very little functionality not specified in the POSIX sh standard is available.

To perform arithmetic in a manner compatible with all POSIX-compliant shells (from the initial 1992 publication of the POSIX sh standard), use the following syntax:

a=$(( x + z ))