bash中变量的while循环 [英] While-loop over lines from variable in bash

问题描述

假设一个文件 file 有多行.

Assume a file file with multiple lines.

$ cat file
foo
bar
baz

进一步假设我希望用 while 循环遍历每一行.

Assume further that I wish to loop through each line with a while-loop.

$ while IFS= read -r line; do
$   echo $line
$   # do stuff
$ done < file
foo
bar
baz

最后，请假设我希望传递存储在变量中的行而不是存储在文件中的行.如何遍历保存为变量的行而不收到以下错误?

Finally, please assume that I wish to pass lines stored in a variable rather than lines stored in a file. How can I loop through lines that are saved as a variable without receiving the below error?

$ MY_VAR=$(cat file)
$ while IFS= read -r line; do
$   echo $line
$   # do stuff
$ done < $(echo "$MY_VAR")
bash: $(echo "$MY_VAR"): ambiguous redirect

推荐答案

您有几个选择:

• 此处字符串(请注意，这是一个非 POSIX 扩展):done <<<"$MY_VAR"

• 一个 heredoc(符合 POSIX，将与 /bin/sh 一起使用):

done <<EOF
$MY_VAR
EOF

一个进程替换(也是一个非 POSIX 扩展，但使用 printf 而不是 echo 使它在支持它的 shell 中更具可预测性；请参阅 APPLICATIONecho 的 POSIX 规范): done <<(printf '%s\n' "$MY_VAR")

A process substitution (also a non-POSIX extension, but using printf rather than echo makes it more predictable across shells that support it; see the APPLICATION USAGE note in the POSIX spec for echo): done < <(printf '%s\n' "$MY_VAR")

请注意，前两个选项(在 bash 中)将在磁盘上创建一个包含变量内容的临时文件，而最后一个使用 FIFO.

Note that the first two options will (in bash) create a temporary file on disk with the variable's contents, whereas the last one uses a FIFO.

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