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Control Operators: Another Way To Branch
bash provides two control operators that can perform branching. The && (AND) and || (OR) operators work like the logical operators in the [[ ]] compound command. This is the syntax:
command1 && command2
and
command1 || command2
It is important to understand the behavior of these. With the && operator, command1 is executed and command2 is executed if, and only if, command1 is successful. With the || operator, command1 is executed and command2 is executed if, and only if, command1 is unsuccessful.
In practical terms, it means that we can do something like this:
[me@linuxbox ~]$ mkdir temp && cd temp
[me@linuxbox ~]$ mkdir temp && cd temp
This will create a directory named temp, and if it succeeds, the current working directory will be changed to temp. The second command is attempted only if the mkdir com- mand is successful. Likewise, a command like this:
[me@linuxbox ~]$ [[ -d temp ]] || mkdir temp
[me@linuxbox ~]$ [[ -d temp ]] || mkdir temp
will test for the existence of the directory temp, and only if the test fails, will the direc- tory be created. This type of construct is very handy for handling errors in scripts, a sub- ject we will discuss more in later chapters. For example, we could do this in a script:
[ -d temp ] || exit 1
[ -d temp ] || exit 1
If the script requires the directory temp, and it does not exist, then the script will termi- nate with an exit status of one.