Java Joy: Using JShell With Standard Input

The Java Development Kit (JDK) includes a tool called jshell that can be used to interactively test Java code. Normally we run jshell and type Java code to be executed from an interactive shell. But we can also use jshell as a tool on the command line to accept standard input containing Java code to be executed. The output of the code can be used as input for another tool on the command line. We run jshell - to run jshell and accept standard input. The simplest way to pass Java code to jshell is to use echo to print the Java code to standard output and pipe it to jshell.

The following example shows how to use jshell and echo to get the default time zone of our system:

$ echo 'System.out.println(TimeZone.getDefault().getID());' | jshell -
Europe/Amsterdam

In the following example we fetch all Java system properties using jshell and then use grep to find the system properties with file in the name:

$ echo 'System.getProperties().list(System.out)' | jshell - | grep file
file.separator=/
file.encoding=UTF-8

The next example uses multi-line standard input:

$ jshell - << EOF
int square(int n) {
    return n * n;
}

IntStream.range(1, 10).map(n -> square(n)).forEach(System.out::println);
EOF

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Written with Java 21.0.4.