在 bash 中动态构建命令
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Dynamically building a command in bash
提问by Joel
I am construcing a command in bash dynamically. This works fine:
我正在 bash 中动态构建命令。这工作正常：
COMMAND="java myclass"
${COMMAND}
Now I want to dynamically construct a command that redirectes the output:
现在我想动态构建一个重定向输出的命令：
LOG=">> myfile.log 2>&1"
COMMAND="java myclass $LOG"
${COMMAND}
The command still invokes the java process, but the output is not redirected to myfile.log
该命令仍会调用 java 进程，但不会将输出重定向到 myfile.log
Additionally, if I do:
此外，如果我这样做：
BACKGROUND="&"
COMMAND="java myclass $BACKGROUND"
${COMMAND}
The command isn't run in the background.
该命令不在后台运行。
Any clues on how to get the log redirect, and background bits working? (bash -x shows the commands being constructed as expected)
关于如何获得日志重定向和背景位工作的任何线索？（bash -x 显示按预期构造的命令）
(In reality, unlike this example, the values of LOG and BACKGROUND are set dynamically)
（实际上，与本例不同，LOG和BACKGROUND的值是动态设置的）
采纳答案by Aaron Digulla
It doesn't work because quotes disable the special meaning of >and &. You must execute the commands which implement these features of the shell.
它不起作用，因为引号禁用了>and的特殊含义&。您必须执行实现 shell 的这些功能的命令。
To redirect, call exec >> myfile.log 2>&1before the command you want to log.
要重定向，请exec >> myfile.log 2>&1在要记录的命令之前调用。
To run a program in the background, use nohup(nohup cmd args...).
要在后台运行程序，请使用nohup( nohup cmd args...)。
回答by tangens
You could do it with the evalcommand:
您可以使用以下eval命令执行此操作：
eval ${COMMAND}
回答by ezpz
evaldoes what you want.
eval做你想做的。
#!/bin/bash
CMD="echo foo"
OUT="> foo.log"
eval ${CMD} ${OUT}
CMD="sleep 5"
BG="&"
eval ${CMD} ${BG}
