Shell – How to unset the positional parameters

A function cannot affect its caller's positional parameters. This is by design: positional parameters are meant to be private to the function.

Make your function work on an array.

myfunction () {
  local _myfunction_arrayname=$1
  shift
  … # work on the positional parameters
  eval "$_myfunction_arrayname=(\"\$@\")"
}
myfunction foo "$@"
set -- "${foo[@]}"

In ksh93 and bash, there's a roundabout way to do something approaching by combining an alias and the . (source) builtin with a process substitution. Example.

alias myfunction='. <(echo myfunction_body \"\$@\"; echo set -- "\"\${new_positional_parameters[@]}\"")'

Put the meat of the work of the function in myfunction_body and make it set the array new_positional_parameters. After a call to myfunction, the positional parameters are set to the values that myfunction_body puts in new_positional_parameters.

Let's suppose that the script generating the output is called generate. Then, to display the number of seconds it takes for it to generate each line:

$ generate | ( t0=$(date +%s); while read -r line; do t1=$(date +%s); echo " $((t1-t0)) $line"; t0=$t1; done )
 2 foo
 0 foo
 5 foo
 3 foo

The same commands spread out over multiple lines looks like:

generate | ( t0=$(date +%s)
    while read -r line
    do
        t1=$(date +%s)
        echo " $((t1-t0)) $line"
        t0=$t1
    done
    )

Alternatively, for convenience, we can define a shell function that contains this code:

timer() { t0=$(date +%s); while read -r line; do t1=$(date +%s); echo " $((t1-t0)) $line"; t0=$t1; done; }

We can use this function as follows:

$ generate | timer
 0 foo
 2 foo
 4 foo
 3 foo

How it works

• t0=$(date +%s)

  This captures the current time at the start of the script in seconds-since-epoch.

• while read -r line; do

  This starts a loop which reads from standard input

• t1=$(date +%s)

  This captures the time in seconds-since-epoch at which the current line was captured.

• echo " $((t1-t0)) $line"

  This prints out the time in seconds that it took for the current line.

• t0=$t1

  This updates t0 for the next line.

• done

  This signals the end of the while loop.