Bash shell basics — inline execution

In the Bash shell, to obtain the value of an environment variable we use the dollar (ie $) character.

echo $PATH

A similar technique can be used in a pipeline, that has multiple commands.

# output from pgrep is passed back to the ps command
ps -o pid,euser $(pgrep -u root systemctl)

Similarly, inside a Bash script, to get the output of a function we can use the dollar character with parenthesis (ie $() ). This feature is called an inline execution.

In the Bash shell, inline execution is used to pass output back to the calling function, or calling statement and can be a useful feature for your next Bash shell script.

Inline execution

To control the flow of a script, we can use a For statement, and inline execution can be used.

#!/bin/bash

# flow control involving inline execution
for pid in $(ps --no-headers -o pid,euser $(pgrep -u root)|awk '{print $1}')
do
    echo $pid
    # do something here
    # kill $pid
done

exit

So the pipeline returns the output, via an inline execution, to the For statement.

The same technique can be performed when calling a function, and you want to return the standard output of the function to the calling function.

#!/bin/bash

function func {
    local a='func: a'

    # echo the standard output back to the calling function
    echo $a
}

main() {
    # call a function
    local z="$(func)"

    # echo the output of the function
    echo $z
}

main

exit

Using an inline execution, the output of the function is returned to the calling function (ie main).

Using the same technique, the output of a function can be retuned to an If statement.

#!/bin/bash

function func {
    local a=$(ps --no-headers -o pid,euser $(pgrep -u $1 $2)|awk '{print $1}')

    # echo the standard output back to the calling function
    echo $a
}

main() {
    # process the whole list of pipelines
    # but exit when a pipeline status returns an error (ie >0)
    if func oracle java && func postfix java
    then
        echo 'list execution was a success: pipestatus='$PIPESTATUS
    else
        echo 'list execution failed: pipestatus='$PIPESTATUS
    fi
}

main

exit

Using the inline execution, the output of the function is returned to an If statement.

The If statement treats the output as a pipeline, so each pipeline is executed.

The If statement evaluates the list of pipelines, and if the pipestatus is zero, then the execution of the list of pipelines is a success.

Reference

https://www.gnu.org/software/bash/manual/bash.html

https://google.github.io/styleguide/shellguide.html

Checkout some other Bash articles here:

Bash shell basics — lists of pipelines asynchronously

Bash shell basics — scoping

Bash shell basics — synchronous or asynchronous

Bash shell basics — pipe and redirection