一个什么都不做的命令，只不过是一个注释引导，但实际上是一个内置的shell，它的目的是什么?
它比每次调用在脚本中插入注释要慢40%左右，这可能取决于注释的大小。我认为唯一可能的原因是:
# poor man's delay function
for ((x=0;x<100000;++x)) ; do : ; done
# inserting comments into string of commands
command ; command ; : we need a comment in here for some reason ; command
# an alias for `true'
while : ; do command ; done
我想我真正想要的是它在历史上的应用。
当前回答
我用它来轻松地启用/禁用变量命令:
#!/bin/bash
if [[ "$VERBOSE" == "" || "$VERBOSE" == "0" ]]; then
vecho=":" # no "verbose echo"
else
vecho=echo # enable "verbose echo"
fi
$vecho "Verbose echo is ON"
Thus
$ ./vecho
$ VERBOSE=1 ./vecho
Verbose echo is ON
这有助于编写干净的脚本。这不能用'#'完成。
同时,
: >afile
是保证'afile'存在但长度为0的最简单方法之一。
其他回答
如果你想把一个文件截断为零字节，这对清除日志很有用，试试这个:
:> file.log
我在一个脚本中看到了这种用法，并认为它可以很好地替代在脚本中调用basename。
oldIFS=$IFS
IFS=/
for basetool in $0 ; do : ; done
IFS=$oldIFS
.．. 这是替换代码:baseool =$(basename $0)
:的一个有用的应用程序是，如果您只对使用参数展开的副作用感兴趣，而不是实际将其结果传递给命令。
在这种情况下，您使用参数展开作为参数:或false，这取决于您想要的退出状态是0还是1。一个例子是
: "${var:=$1}"
由于:是内置的，它应该相当快。
我用它来轻松地启用/禁用变量命令:
#!/bin/bash
if [[ "$VERBOSE" == "" || "$VERBOSE" == "0" ]]; then
vecho=":" # no "verbose echo"
else
vecho=echo # enable "verbose echo"
fi
$vecho "Verbose echo is ON"
Thus
$ ./vecho
$ VERBOSE=1 ./vecho
Verbose echo is ON
这有助于编写干净的脚本。这不能用'#'完成。
同时,
: >afile
是保证'afile'存在但长度为0的最简单方法之一。
它对多语言程序也很有用:
#!/usr/bin/env sh
':' //; exec "$(command -v node)" "$0" "$@"
~function(){ ... }
现在这既是一个可执行的shell脚本，也是一个JavaScript程序:这意味着。/filename.js、sh filename.js和node filename.js都可以工作。
(这种用法确实有点奇怪，但还是很有效。)
请作一些说明:
Shell-scripts are evaluated line-by-line; and the exec command, when run, terminates the shell and replaces it's process with the resultant command. This means that to the shell, the program looks like this: #!/usr/bin/env sh ':' //; exec "$(command -v node)" "$0" "$@" As long as no parameter expansion or aliasing is occurring in the word, any word in a shell-script can be wrapped in quotes without changing its' meaning; this means that ':' is equivalent to : (we've only wrapped it in quotes here to achieve the JavaScript semantics described below) ... and as described above, the first command on the first line is a no-op (it translates to : //, or if you prefer to quote the words, ':' '//'. Notice that the // carries no special meaning here, as it does in JavaScript; it's just a meaningless word that's being thrown away.) Finally, the second command on the first line (after the semicolon), is the real meat of the program: it's the exec call which replaces the shell-script being invoked, with a Node.js process invoked to evaluate the rest of the script. Meanwhile, the first line, in JavaScript, parses as a string-literal (':'), and then a comment, which is deleted; thus, to JavaScript, the program looks like this: ':' ~function(){ ... } Since the string-literal is on a line by itself, it is a no-op statement, and is thus stripped from the program; that means that the entire line is removed, leaving only your program-code (in this example, the function(){ ... } body.)
