Linux 重定向 bash for 循环的输出
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Redirecting output of bash for loop
提问by Matthew Kirkley
I have a simple BASH command that looks like
我有一个简单的 BASH 命令，看起来像
for i in `seq 2`; do echo $i; done; > out.dat
When this runs the output of seq 2is output to the terminal and nothing is output to the data file (out.dat)
运行时，将输出seq 2输出到终端，并且没有任何内容输出到数据文件 (out.dat)
I am expecting standard out to be redirected to out.dat like it does simply running the command seq 2 > out.dat
我期待标准输出被重定向到 out.dat 就像它只是运行命令一样 seq 2 > out.dat
采纳答案by konsolebox
Remove your semicolon.
删除你的分号。
for i in `seq 2`; do echo "$i"; done > out.dat
SUGGESTIONS
建议
Also as suggested by Fredrik Pihl, try not to use external binaries when they are not needed, or at least when practically not:
同样按照 Fredrik Pihl 的建议，尽量不要在不需要时使用外部二进制文件，或者至少在实际上不需要时：
for i in {1..2}; do echo "$i"; done > out.dat
for ((i = 1; i <= 2; ++i )); do echo "$i"; done > out.dat
for i in 1 2; do echo "$i"; done > out.dat
Also, be careful of outputs in wordsthat may cause pathname expansion.
此外，请注意words可能导致路径名扩展的输出。
for A in $(echo '*'); do echo "$A"; done
Would show your files instead of just a literal *.
将显示您的文件，而不仅仅是文字*.
$()is also recommended as a clearer syntax for command substitution in Bash and POSIX shells than backticks (`), and it supports nesting.
$()在 Bash 和 POSIX shell 中，还推荐使用比反引号 ( `)更清晰的命令替换语法，并且它支持嵌套。
The cleaner solutions as well for reading output to variables are
将输出读取到变量的更干净的解决方案是
while read VAR; do
...
done < <(do something)
And
和
read ... < <(do something) ## Could be done on a loop or with readarray.
for A in "${ARRAY[@]}"; do
:
done
Using printf can also be an easier alternative with respect to the intended function:
就预期功能而言，使用 printf 也可以是一种更简单的替代方法：
printf '%s\n' {1..2} > out.dat
回答by svante
Try:
尝试：
(for i in `seq 2`; do echo $i; done;) > out.dat
回答by tobias_k
Another possibility, for the sake of completeness: You can move the output inside the loop, using >>to append to the file, if it exists.
另一种可能性，为了完整起见：您可以将输出移动到循环内，使用>>附加到文件（如果存在）。
for i in `seq 2`; do echo $i >> out.dat; done;
Which one is better certainly depends on the use case. Writing the file in one go is certainly better than appending to it a thousand times. Also, if the loop contains multiple echostatements, all of which shall go to the file, doing done > out.datis probably more readable and easier to maintain. The advantage of this solution, of course, is that it gives more flexibility.
哪个更好当然取决于用例。一次写入文件肯定比附加一千次要好。此外，如果循环包含多个echo语句，所有这些语句都将写入文件，这样做done > out.dat可能更具可读性且更易于维护。当然，这种解决方案的优势在于它提供了更大的灵活性。
