[英]Substitution in shell script
I like to evaluate the following in linux shell 我喜欢在linux shell中评估以下内容
CMD_6='ls'
CMD_7='ls -l'
VER=6
CMD="CMD_"$VER
I'm expecting $CMD
to execute CMD_6 and list the directory content, but it is throwing an error: 我期望
$CMD
执行CMD_6并列出目录内容,但是会引发错误:
-bash: CMD_6: command not found
Can someone explain how to do this substitution? 有人可以解释如何进行替换吗?
Use functions. 使用功能。
cmd_6 () { ls; }
cmd_7 () { ls -l; }
ver=6
cmd="cmd_$ver"
"$cmd"
The value of cmd
shouldn't be any more complicated than a single function or command name: no arguments, no other shell syntax. cmd
的值不应比单个函数或命令名称更复杂:没有参数,没有其他shell语法。
... or use aliases: ...或使用别名:
$ alias cmd_6='ls'
$ ver=6
$ alias cmd="cmd_$ver"
$ alias
alias cmd='cmd_6'
alias cmd_6='ls'
The other answers explain how to to it with functions and aliases, but it can be done with variables: 其他答案解释了如何使用函数和别名来实现它,但是可以通过变量来实现:
foo="ls -l" bar=/bin/bash
$foo $bar
Output: 输出:
-rwxr-xr-x 1 root root 1099016 May 17 2017 /bin/bash
But this is a little different: 但这有点不同:
CMD="CMD_"$VER
To run it, do: 要运行它,请执行以下操作:
eval \$$CMD
Be careful with eval
, only feed it known or checked input. 小心
eval
,仅输入已知或检查的输入。
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