Question

I am trying to run multiple similar commands using a for loop and string formatting.

I am able to create and then print the strings of the commands I would like to run:

$foo=( "bar" "baz" )
$for ((j=0;j<2;++j)); do printf "touch %s_file.txt; " "${foo[j]}"; done
touch bar_file.txt; touch baz_file.txt;

But I want to enter these strings as a command. Based on other questions I was thinking that eval would do what I need:

$for ((j=0;j<2;++j)); do eval "touch %s_file.txt; " "${foo[j]}"; done
-bash: bar: command not found
-bash: baz: command not found

I was expecting(or hoping) the output would be equivalent to the output of:

$touch bar_file.txt; touch baz_file.txt;

The touch here is just an example. I'm more interested in how to format a string and then run it as a command. Thanks.

Answer 1

首先，这里不需要使用字符串格式来生成代码，或者通常 - BashFAQ #50描述用例和更好的实践替代方案。这可以简单如下：

for j in in "${foo[@]}"; do touch "${j}_file.txt"; done

其次，如果你必须，那就这样做：

printf -v cmd 'touch %q_file.txt; ' "${foo[@]}"
eval "$cmd"

这将设置cmd='touch bar_file.txt; touch baz_file.txt '，然后执行它。

当shell稍后解析内容时使用%q（与eval一样）确保您的数组元素的格式化能够在shell解析后生存 - 所以如果你有foo=( "name with spaces" 'name with $(rm -rf $HOME) dangerous content' )，您将正确触摸'name with spaces_file.txt'和'name with $(rm -rf $HOME) dangerous content_file.txt'，而不是执行危险内容并根据空格触摸多个文件。

Answer 2

Instead of trying to put the whole command in a string, why not make a function to do the thing you want like:

update_file() {
    touch "$1"
}

foo=( "bar" "baz" )
for ((j=0;j<2;++j)); do
    update_file "$(printf "%s_file.txt" "${foo[j]}")"
done

then you can make that function do arbitrarily complex things with the files

As a general rule, variables hold data, not code. You can sometimes make it work putting code into data, but that way lies dragons. See here for some good reading on that

Bash run string formatted command in a for loop

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