如何理解 : 命令?

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代码:

SHELL BUILTIN COMMANDS
       Unless otherwise noted, each builtin command documented in this section
       as accepting options preceded by - accepts -- to signify the end of the
       options.
       : [arguments]
              No  effect;  the command does nothing beyond expanding arguments
              and performing any specified redirections.  A zero exit code  is
              returned.

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代码:

[color=blue]-(user@host:tty)-(tmp)-
[301 0] $ [/color]cat foo.sh
N=10
i=1
while :; do
    ((sum += i++))
    [ $i -gt $N ] && break
done
echo "sum[1..$N] = $sum"
[color=blue]-(user@host:tty)-(tmp)-
[301 0] $ [/color]bash foo.sh
sum[1..10] = 55
[color=blue]-(user@host:tty)-(tmp)-
[301 0] $ [/color]