Shell script Programming Loop statement _linux Shell

It is common to repeat a series of commands while running a script, when we need to use a loop statement to do this.

One, for command

Format: for variable in list;

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Done

The for command traverses each value in the list and exits the loop after the traversal completes.

The list is in the following ways:

1. A series of values defined in the order;
2, variable;
3, order;
4, catalogue;
5, documents.

Example 1:

The previous example uses the {initial value ... End value} format to represent the numeric value of an interval.

Example 2: Take a variable as a list:

Example 3: Generate a list with commands:

In the example above, the SEQ command is used to generate the list, and the SEQ command is in the format: SEQ [initial value step] end value

Using inverted quotes in the above two examples will result in a command run.

Example 4: Use a directory as a list:

Second, C language style for command

Format: for ((variable assignment; cyclic termination condition; step size); Todo

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Done

Example: Ask for the plus from 1 to 100:

Iii. until order

Format: Until condition; Todo

Statement
...
Done

The loop is exited when the condition in the until command is met.

Instance:

Four, while command

Format: While condition; Todo

Statement
...
Done

The while command enters the loop when the condition is satisfied.

Instance:

V. Control cycle

1. Break command

Function: Exit the loop in progress.

Instance: If the variable i is 3, exit the loop.

2, Continue command

Function: Advance into the next round of circulation.
Instance: Adds the number between 1 to 20 and 51 to 100:

Vi. the use of while

1, Dead Cycle

Format: while:; Todo
Statement
Done

Instance:

2. Read the contents of the file into the loop

Format: while read variable name; Todo

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Done < documents

Instance: reads the contents of the/etc/passwd file line by row: