What 开发者_如何学运维is the purpose of a command that does nothing, being little more than a comment leader, but is actually a shell builtin in and of itself?

It's slower than inserting a comment into your scripts by about 40% per call, which probably varies greatly depending on the size of the comment. The only possible reasons I can see for it are these:

# 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

I guess what I'm really looking for is what historical application it might have had.

Historically, Bourne shells didn't have true and false as built-in commands. true was instead simply aliased to :, and false to something like let 0.

: is slightly better than true for portability to ancient Bourne-derived shells. As a simple example, consider having neither the ! pipeline operator nor the || list operator (as was the case for some ancient Bourne shells). This leaves the else clause of the if statement as the only means for branching based on exit status:

if command; then :; else ...; fi

Since if requires a non-empty then clause and comments don't count as non-empty, : serves as a no-op.

Nowadays (that is: in a modern context) you can usually use either : or true. Both are specified by POSIX, and some find true easier to read. However there is one interesting difference: : is a so-called POSIX special built-in, whereas true is a regular built-in.

Special built-ins are required to be built into the shell; Regular built-ins are only "typically" built in, but it isn't strictly guaranteed. There usually shouldn't be a regular program named : with the function of true in PATH of most systems.
Probably the most crucial difference is that with special built-ins, any variable set by the built-in - even in the environment during simple command evaluation - persists after the command completes, as demonstrated here using ksh93:
```
$ unset x; ( x=hi :; echo "$x" )
hi
$ ( x=hi true; echo "$x" )

$
```
Note that Zsh ignores this requirement, as does GNU Bash except when operating in POSIX compatibility mode, but all other major "POSIX sh derived" shells observe this including dash, ksh93, and mksh.
Another difference is that regular built-ins must be compatible with exec - demonstrated here using Bash:
```
$ ( exec : )
-bash: exec: :: not found
$ ( exec true )
$
```
POSIX also explicitly notes that : may be faster than true, though this is of course an implementation-specific detail.

I use it to easily enable/disable variable commands:

#!/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

This makes for a clean script. This cannot be done with '#'.

Also,

: >afile

is one of the simplest ways to guarantee that 'afile' exists but is 0 length.

A useful application for : is if you're only interested in using parameter expansions for their side-effects rather than actually passing their result to a command.

In that case, you use the parameter expansion as an argument to either : or false depending upon whether you want an exit status of 0 or 1. An example might be

: "${var:=$1}"

Since : is a builtin, it should be pretty fast.

: can also be for block comment (similar to /* */ in C language). For example, if you want to skip a block of code in your script, you can do this:

: << 'SKIP'

your code block here

SKIP

Two more uses not mentioned in other answers:

Logging

Take this example script:

set -x
: Logging message here
example_command

The first line, set -x, makes the shell print out the command before running it. It's quite a useful construct. The downside is that the usual echo Log message type of statement now prints the message twice. The colon method gets round that. Note that you'll still have to escape special characters just like you would for echo.

Cron job titles

I've seen it being used in cron jobs, like this:

45 10 * * * : Backup for database ; /opt/backup.sh

This is a cron job that runs the script /opt/backup.sh every day at 10:45. The advantage of this technique is that it makes for better looking email subjects when the /opt/backup.sh prints some output.

It's similar to pass in Python.

One use would be to stub out a function until it gets written:

future_function () { :; }

If you'd like to truncate a file to zero bytes, useful for clearing logs, try this:

:> file.log

You could use it in conjunction with backticks (``) to execute a command without displaying its output, like this:

: `some_command`

Of course you could just do some_command > /dev/null, but the :-version is somewhat shorter.

That being said I wouldn't recommend actually doing that as it would just confuse people. It just came to mind as a possible use-case.

It's also useful for polyglot programs:

#!/usr/bin/env sh
':' //; exec "$(command -v node)" "$0" "$@"
~function(){ ... }

This is now both an executable shell-script and a JavaScript program: meaning ./filename.js, sh filename.js, and node filename.js all work.

(Definitely a little bit of a strange usage, but effective nonetheless.)

Some explication, as requested:

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.)

Self-documenting functions

You can also use : to embed documentation in a function.

Assume you have a library script mylib.sh, providing a variety of functions. You could either source the library (. mylib.sh) and call the functions directly after that (lib_function1 arg1 arg2), or avoid cluttering your namespace and invoke the library with a function argument (mylib.sh lib_function1 arg1 arg2).

Wouldn't it be nice if you could also type mylib.sh --help and get a list of available functions and their usage, without having to manually maintain the function list in the help text?

#!/bin/bash

# all "public" functions must start with this prefix
LIB_PREFIX='lib_'

# "public" library functions
lib_function1() {
    : This function does something complicated with two arguments.
    :
    : Parameters:
    : '   arg1 - first argument ($1)'
    : '   arg2 - second argument'
    :
    : Result:
    : "   it's complicated"

    # actual function code starts here
}

lib_function2() {
    : Function documentation

    # function code here
}

# help function
--help() {
    echo MyLib v0.0.1
    echo
    echo Usage: mylib.sh [function_name [args]]
    echo
    echo Available functions:
    declare -f | sed -n -e '/^'$LIB_PREFIX'/,/^}$/{/\(^'$LIB_PREFIX'\)\|\(^[ \t]*:\)/{
        s/^\('$LIB_PREFIX'.*\) ()/\n=== \1 ===/;s/^[ \t]*: \?['\''"]\?/    /;s/['\''"]\?;\?$//;p}}'
}

# main code
if [ "${BASH_SOURCE[0]}" = "${0}" ]; then
    # the script was executed instead of sourced
    # invoke requested function or display help
    if [ "$(type -t - "$1" 2>/dev/null)" = function ]; then
        "$@"
    else
        --help
    fi
fi

A few comments about the code:

All "public" functions have the same prefix. Only these are meant to be invoked by the user, and to be listed in the help text.
The self-documenting feature relies on the previous point, and uses declare -f to enumerate all available functions, then filters them through sed to only display functions with the appropriate prefix.
It is a good idea to enclose the documentation in single quotes, to prevent undesired expansion and whitespace removal. You'll also need to be careful when using apostrophes/quotes in the text.
You could write code to internalize the library prefix, i.e. the user only has to type mylib.sh function1 and it gets translated internally to lib_function1. This is an exercise left to the reader.
The help function is named "--help". This is a convenient (i.e. lazy) approach that uses the library invoke mechanism to display the help itself, without having to code an extra check for $1. At the same time, it will clutter your namespace if you source the library. If you don't like that, you can either change the name to something like lib_help or actually check the args for --help in the main code and invoke the help function manually.

I saw this usage in a script and thought it was a good substitute for invoking basename within a script.

oldIFS=$IFS  
IFS=/  
for basetool in $0 ; do : ; done  
IFS=$oldIFS

... this is a replacement for the code: basetool=$(basename $0)

Another way, not yet mentioned here is the initialisation of parameters in infinite while-loops. Below is not the cleanest example, but it serves it's purpose.

#!/usr/bin/env bash
[ "$1" ] && foo=0 && bar="baz"
while : "${foo=2}" "${bar:=qux}"; do
    echo "$foo"
    (( foo == 3 )) && echo "$bar" && break
    (( foo=foo+1 ))
done