bash 如何在shell中为for循环添加前导零？
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原文地址: http://stackoverflow.com/questions/18460123/
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How to add leading zeros for for-loop in shell?
提问by Bruce Blacklaws
I have a basic number for loop which increments the variable num by 1 over each iteration...
我有一个基本数字 for 循环，它在每次迭代中将变量 num 增加 1 ...
for (( num=1; num<=5; num++ ))
do
echo $num
done
Which outputs:
哪些输出：
1
2
3
4
5
I'm trying to make it produce the output (add leading zero before $num):
我试图让它产生输出（在 $num 之前添加前导零）：
01
02
03
04
05
Without doing:
不做：
echo 0$num
回答by Adrian Frühwirth
Use the following syntax:
使用以下语法：
$ for i in {01..05}; do echo "$i"; done
01
02
03
04
05
If you want to use printf, nothing prevents you from putting its result in a variable for further use:
如果您想使用printf，没有什么可以阻止您将其结果放入变量中以供进一步使用：
$ foo=$(printf "%02d" 5)
$ echo "${foo}"
05
Disclaimer: This only works in >=bash-4.
免责声明：这仅适用于>=bash-4.
回答by piojo
seq -wwill detect width and normalize it.
seq -w将检测宽度并将其标准化。
for num in $(seq -w 01 05); do
...
done
Apparently this doesn't work on the newest versions of iOS, so you can either install macports and use its version of seq, or you can set the format explicitly:
显然这不适用于最新版本的 iOS，因此您可以安装 macports 并使用其版本的seq，或者您可以明确设置格式：
seq -f '%02g' 1 3
01
02
03
But given the ugliness of format specifications for such a simple problem, I prefer the solution Henk and Adrian gave, which just uses Bash. Apple can't screw this up since there's no generic "unix" version of Bash:
但是考虑到这样一个简单问题的格式规范的丑陋，我更喜欢 Henk 和 Adrian 给出的解决方案，它只使用 Bash。Apple 不能搞砸，因为没有通用的“unix”版本的 Bash：
echo {01..05}
Or:
或者：
for number in {01..05}; do ...; done
回答by anubhava
Use printfcommand to have 0padding:
使用printf命令来0填充：
printf "%02d\n" $num
Your for loop will be like this:
您的 for 循环将是这样的：
for (( num=1; num<=5; num++ )); do printf "%02d\n" $num; done
01
02
03
04
05
回答by David W.
I'm not interested in outputting it to the screen (that's what printf is mainly used for, right?) The variable $num is going to be used as a parameter for another program but let me see what I can do with this.
我对将它输出到屏幕不感兴趣（这就是 printf 的主要用途，对吗？）变量 $num 将用作另一个程序的参数，但让我看看我可以用它做什么。
You can still use printf:
您仍然可以使用printf：
for num in {1..5}
do
value=$(printf "%02d" $num)
... Use $value for your purposes
done
回答by Henk Langeveld
From bash 4.0 onward, you can use Brace Expansionwith fixed length strings. See below for the original announcement.
从 bash 4.0 开始，您可以将Brace Expansion与固定长度的字符串一起使用。请参阅下面的原始公告。
It will do just what you need, and does not require anything external to the shell.
它会做你所需要的，不需要外壳外部的任何东西。
$ echo {01..05}
01 02 03 04 05
for num in {01..05}
do
echo $num
done
01
02
03
04
05
CHANGES, release bash-4.0, section 3
This is a terse description of the new features added to bash-4.0 since the release of bash-3.2.
. . .
z. Brace expansion now allows zero-padding of expanded numeric values and will add the proper number of zeroes to make sure all values contain the same number of digits.
这是自 bash-3.2 发布以来添加到 bash-4.0 的新特性的简要描述。
. . .
z。大括号扩展现在允许对扩展数值进行零填充，并将添加适当数量的零以确保所有值包含相同数量的数字。
回答by Benoit
why not printf '%02d' $num? See help printffor this internal bash command.
为什么不printf '%02d' $num呢？请参阅help printf此内部 bash 命令。
回答by jd_doubley
Just a note: I have experienced different behaviours on different versions of bash:
请注意：我在不同版本的 bash 上经历了不同的行为：
- version 3.1.17(1)-release-(x86_64-suse-linux) and
- Version 4.1.17(9)-release (x86_64-unknown-cygwin))
- 版本 3.1.17(1)-release-(x86_64-suse-linux) 和
- 版本 4.1.17(9)-release (x86_64-unknown-cygwin))
for the former (3.1) for nn in (00..99) ; do ...works but for nn in (000..999) ; do ...does not work
both will work on version 4.1 ; haven't tested printf behaviour
(bash --versiongave the version info)
对于前者 (3.1)for nn in (00..99) ; do ...有效但for nn in (000..999) ; do ...不起作用，两者都适用于 4.1 版；尚未测试 printf 行为（bash --version提供版本信息）
Cheers, Jan
干杯，简
