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I'm familiar with creating 'temporary files' for command input in bash, e.g.
cat file_1 <(echo hello) file_2
I want to read more about the subject, but I don't know the name of the <( ) operator. I suspect it's a kind of IO redirect.
Is there a name for this notation/operator?
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Bash has a large set of logical operators that can be used in conditional expressions. The most basic form of the if control structure tests for a condition and then executes a list of program statements if the condition is true. There are three types of operators: file, numeric, and non-numeric operators.
$1 means an input argument and -z means non-defined or empty. You're testing whether an input argument to the script was defined when running the script. Follow this answer to receive notifications.
The [[ ... ]] part allows to test a condition using operators. Think of it as an if statement. In your example, you're using the -s operator, which tests that the referenced file is not empty. Copy link CC BY-SA 3.0.
Using and Setting Default Values There is another syntax, ":=", which sets the variable to the default if it is undefined: echo "Your name is : ${myname:=John Doe}" This technique means that any subsequent access to the $myname variable will always get a value, either entered by the user, or "John Doe" otherwise.
This is called process substitution:
Process substitution is a form of redirection where the input or output of a process (some sequence of commands) appear as a temporary file.
Also from Bash Reference Manual → 3.5.6 Process Substitution:
Process substitution allows a process’s input or output to be referred to using a filename. It takes the form of
<(list)or
>(list)The process list is run asynchronously, and its input or output appears as a filename. This filename is passed as an argument to the current command as the result of the expansion. If the >(list) form is used, writing to the file will provide input for list. If the <(list) form is used, the file passed as an argument should be read to obtain the output of list. Note that no space may appear between the < or > and the left parenthesis, otherwise the construct would be interpreted as a redirection. Process substitution is supported on systems that support named pipes (FIFOs) or the /dev/fd method of naming open files.
When available, process substitution is performed simultaneously with parameter and variable expansion, command substitution, and arithmetic expansion.
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