Combining UNIX Commands using && and ||

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.




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Thanks for reading, and happy scripting!!!


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