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Bash Reference Manual

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3.2.4 Looping Constructs

Bash supports the following looping constructs.

Note that wherever a `;' appears in the description of a command's syntax, it may be replaced with one or more newlines.

until
The syntax of the until command is:
 
until test-commands; do consequent-commands; done
Execute consequent-commands as long as test-commands has an exit status which is not zero. The return status is the exit status of the last command executed in consequent-commands, or zero if none was executed.

while
The syntax of the while command is:
 
while test-commands; do consequent-commands; done

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

for
The syntax of the for command is:

 
for name [in words ...]; do commands; done
Expand words, and execute commands once for each member in the resultant list, with name bound to the current member. If `in words' is not present, the for command executes the commands once for each positional parameter that is set, as if `in "$@"' had been specified (see section 3.4.2 Special Parameters). The return status is the exit status of the last command that executes. If there are no items in the expansion of words, no commands are executed, 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 expression expr1 is evaluated according to the rules described below (see section 6.5 Shell Arithmetic). The arithmetic expression expr2 is then evaluated repeatedly until it evaluates to zero. Each time expr2 evaluates to a non-zero value, commands are executed and the arithmetic expression expr3 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 in list that is executed, or false if any of the expressions is invalid.

The break and continue builtins (see section 4.1 Bourne Shell Builtins) may be used to control loop execution.


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