在我上一篇文章《更改数字基数的奇妙技巧》中，我探讨了 Linux shell 令人惊讶的即时转换数字基数的能力，包括这个将十六进制 FF 转换为十进制表示的简洁代码片段

$ echo $(( 0xFF ))
255

并且，我还讨论了如何在脚本中使用方便的 printf 命令，例如这个命令可以将十进制数字显示为八进制和十六进制

$ printf "octal: %o\nhex: %x\n" 42 42
octal: 52
hex: 2a

这非常酷，但老实说，我很少发现自己需要转换数字基数，所以这真的属于“有趣的 shell 技巧”的范畴。您的经验可能不同，所以无论如何都值得学习。

在本文中，我认为深入了解 printf 命令会很有趣，因为它非常强大，但在开始之前，这里有一个快速提示：一些可以让您的 if-then 语句更简洁的巧妙方法。

如果您像我一样，那么您会发现自己经常在 shell 脚本中编写条件语句块。嗯，我的意思是

if [ you're like me ] ; then
   you find yourself...

好吧，你明白了。事实上，无论条件表达式是只有六行还是数百行，它们都是代码序列转变为更复杂程序的关键。

一个典型的条件表达式实际上可能看起来像这样

if [ $(date +%w) -eq 0 ]; then
   echo "It's Sunday"
else
   echo "It's not Sunday"
fi

这很清晰易读，但它确实在 shell 脚本中占用了大量的垂直空间。

幸运的是，有一些方法可以使用 shell 脚本中的 && 和 || 符号来简化代码。

&& 符号表示如果 && 之前调用的命令以成功返回代码结束，则执行后续操作——例如

test $(date +%w) -eq 0 && echo "Sunday"




If it's Monday afternoon when I run this code, I'll get no output,
and the echo statement isn't even evaluated. But if it's Sunday,
the above command will output appropriately.



The || notation offers the same basic functionality but with the opposite
logic: if the return code of the command prior to the || returns a fail
(non-zero) return code, then the subsequent command will be invoked:


test $(date +%w) -eq 0 || echo "It's not Sunday yet"





You also can make this even more succinct by using the [] notational
shortcut for a test—just remember to include the closing ] to ensure
it's all well formed:


[ $(date +%w -eq 0 ] || echo "it's not Sunday yet"




The biggest limitation with this notation is that there's really no
reliable and properly interpreted way to add an else clause.



You can try something like this:


cmd1 && cmd2 || cmd3





But because of precedence interpretation, it's likely to have cmd3
invoked if either cmd1 or cmd2 have a non-zero return code, which makes it
functionality different from this:


if cmd1 ; then
  cmd2
else
  cmd3
fi




All is not lost, however, because you always can use a lot of semicolons to
move that onto a single line:


if cmd1 ; then cmd2 ; else cmd3 ; fi





But, is it more readable? Is it really how you want to write your commands?
Maybe. At least now you know!



The Ever-Helpful printf Command


Now, let's look at a completely different type of command,
a command that is a built-in C programming language function that's so
darn useful, it's now included in Linux as a standalone command.



In C and its brethren, the command shows up like this:


printf(formatstring, arg, arg);





This actually is a shortcut for the more general
fprintf() command, which
prepends the file handle and would look more like the following:


fprintf(stdio, formatstring, arg, arg);





It's not really relevant to this discussion, but hey, you should know this C
programming nuance just so you know what's going on, right?



Okay, okay, back to the shell.



The printf command is basically the same, just without the parentheses and
commas:


printf formatstring arg arg





Unlike the echo command,
printf doesn't automatically append a
carriage-return line-feed sequence, so you can end up with odd results like this:


$ printf "hello"
hello$





The format string allows a number of backslash-escaped sequences to
alleviate this problem, notably \n to produce the end-of-line carriage
return.



Indeed, go back to the first few paragraphs of this column, and you'll
notice I included this sequence:


printf "octal: %o\nhex: %x\n" 42 42





Now you know what those \n sequences mean: each
produces an end-of-line
sequence.



Additional escape sequences include \a for a bell
(try it!), \b for a
backspace, \t for a tab and
\\ for a backslash character itself.



Where things get more interesting is with the specifics of the format
string. All of these are denoted with the % symbol followed by the specific
letter that specifies how the associated argument should be interpreted and
displayed. Give it a decimal value but use %o, and it'll be output as
octal (as shown earlier).



The most important sequences are:

• %c 用于字符。

• %s 用于字符串（字符序列）。

• %d 用于十进制值。

• %f 用于浮点非整数值。

当然，这里有很多细微之处，特别是显示浮点数可能会非常复杂，因为使用了各种表示约定。您可以阅读 printf 的 man 手册以获取更多详细信息。

几乎每个格式序列也允许您指定字段宽度和精度，这使得所有这些都变得既复杂又有趣。

让我们考虑浮点数 3.141597 以及 printf 可能以不同方式显示它的方式

$ pi=3.141597
$ printf "%d\n" $pi
-bash: printf: 3.141597: invalid number
0

这不应该令人惊讶；您不能将浮点数解释为整数。请改用 %f

$ printf "%f\n" $pi
3.141597

那是默认值，printf 显示了浮点值的默认精度。

让我们看看如果您指定零精度（即小数点后零位数字）会发生什么

$ printf "%.0f\n" $pi
3

这是有道理的。但是，如果您实际处理的是货币，并且您希望能够确保不会得到像 $20.4342434 这样的怪异值，该怎么办？

$ printf "%.2f\n" $pi
3.14

真正有趣的地方在于，当您想要在列中对齐值时，为每个字段分配 10、15、20 甚至更多字符的空间。这就是字段宽度，它出现在格式字符串说明符的小数点之前，或者在没有小数点的情况下单独出现

$ printf "X%15fX\n" $pi
X       3.141597X

您也可以组合使用

$ printf "X%10.2fX\n" $pi
X      3.14X

您还可以将字段宽度说明符与字符串一起使用，这尤其有趣

$ printf "|%20s|%20s|\n" "one" "two"; printf "|%20s|%20s|\n"
"three" "four"
|                 one|                 two|
|               three|                four|
$

我的篇幅快用完了，但我鼓励您查看 printf 命令及其许多技巧，以帮助您从 shell 脚本创建更具吸引力的输出！

别忘了，如果您有任何关于我应该处理的 shell 脚本的想法，或者我应该考虑的游戏，请随时发送电子邮件至 dave@linuxjournal.com。