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关于shell命令中嵌入if判断

2012-03-31 16:26 190 查看

Almost all programming languages provide an if statement for controlling the flow of a program based upon the result of a test. This simple example illustrates one acceptable format of the UNIX shell's if statement:

if [ -f unixfile ]

then

rm unixfile

fi

This snippet of code will remove (delete) the file named unixfile if it exists and is a regular file. The -f enclosed within brackets performs this test. These two UNIX commands, the -f test and the remove statement, can be combined and compacted into
a single line of code by using the && shell construct:

[ -f unixfile ] && rm unixfile

This statement is read, if command1 is true (has an exit status of zero) then perform command2.

The || construct will do the opposite:

command1 || command 2

If the exit status of command1 is false (non-zero), command2 will be executed.

UNIX pipelines may be used on either side of && or ||, and both constructs can be used on the same line to simulate the logic of an if-else statement. Consider the following lines of code:

if [ -f unixfile ]

then

rm unixfile

else

print "unixfile was not found, or is not a regular file"

fi

Using && and || together, this block of code can be reduced to:

[ -f unixfile ] && rm unixfile || print "unixfile was not found, or is not a regular file"

Finally, multiple commands can be executed based on the result of command1 by incorporating braces and semi-colons:

command1 && { command2 ; command3 ; command4 ; }

If the exit status of command1 is true (zero), commands 2, 3, and 4 will be performed.

These two UNIX shell constructs are very powerful tools for any shell script programmer's arsenal.

These two UNIXshell constructs are very powerful tools for any shell script programmer'sarsenal.

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标签： shell command unix tools delete file

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