Looping Constructs(from bash manual)_bash looping-优快云博客

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本文详细介绍了Shell脚本中的三种循环结构：until、while和for循环的语法及使用方法，并解释了它们的执行逻辑和返回状态。此外，还特别说明了for循环的一种替代形式及其算术表达式的评估规则。

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until

The syntax of theuntil command is:

            until test-commands; do consequent-commands; done

Execute consequent-commands as long astest-commands has an exit status which is not zero. The return status is the exit status of the last command executedinconsequent-commands, or zero if none was executed.

while

The syntax of thewhile command is:

            while test-commands; do consequent-commands; done

Execute consequent-commands as long astest-commands has an exit status of zero. The return status is the exit status of the last command executedinconsequent-commands, or zero if none was executed.

for

The syntax of thefor command is:

            for name [ [in [words ...] ] ; ] do commands; done

Expand words, and executecommands once for each memberin the resultant list, with name bound to the current member. If ‘inwords’ is not present, the for commandexecutes thecommands once for each positional parameter that isset, as if ‘in "$@"’ had been specified. The return status is the exit status of the last command that executes. If there are no items in the expansion ofwords, no commands areexecuted, and the return status is zero.

An alternate form of the for command is also supported:

            for (( expr1 ; expr2 ; expr3 )) ; do commands ; done

First, the arithmetic expressionexpr1 is evaluated accordingto the rules described below . The arithmetic expressionexpr2 is then evaluated repeatedlyuntil it evaluates to zero. Each timeexpr2 evaluates to a non-zero value, commands areexecuted and the arithmetic expressionexpr3 is evaluated. If any expression is omitted, it behaves as if it evaluates to 1. The return value is the exit status of the last command inlistthat is executed, or false if any of the expressions is invalid.