Powershell – How to pass an argument to a PowerShell script

automationcommand lineitunesparameter-passingpowershell

There's a PowerShell script named itunesForward.ps1 that makes iTunes fast forward 30 seconds:

$iTunes = New-Object -ComObject iTunes.Application

if ($iTunes.playerstate -eq 1)
{
  $iTunes.PlayerPosition = $iTunes.PlayerPosition + 30
}

It is executed with a prompt line command:

powershell.exe itunesForward.ps1

Is it possible to pass an argument from the command line and have it applied in the script instead of the hardcoded 30 seconds value?

Best Answer

Tested as working:

#Must be the first statement in your script (not coutning comments)
param([Int32]$step=30) 

$iTunes = New-Object -ComObject iTunes.Application

if ($iTunes.playerstate -eq 1)
{
  $iTunes.PlayerPosition = $iTunes.PlayerPosition + $step
}

Call it with

powershell.exe -file itunesForward.ps1 -step 15

Multiple parameters syntax (comments are optional, but allowed):

<#
    Script description.

    Some notes.
#>
param (
    # height of largest column without top bar
    [int]$h = 4000,
    
    # name of the output image
    [string]$image = 'out.png'
)

And some example for advanced parameters, e.g. Mandatory:

<#
    Script description.

    Some notes.
#>
param (
    # height of largest column without top bar
    [Parameter(Mandatory=$true)]
    [int]$h,
    
    # name of the output image
    [string]$image = 'out.png'
)

Write-Host "$image $h"

A default value will not work with a mandatory parameter. You can omit the =$true for advanced parameters of type boolean [Parameter(Mandatory)].

Related Solutions

Java – Is Java “pass-by-reference” or “pass-by-value”

Java is always pass-by-value. Unfortunately, when we deal with objects we are really dealing with object-handles called references which are passed-by-value as well. This terminology and semantics easily confuse many beginners.

It goes like this:

public static void main(String[] args) {
    Dog aDog = new Dog("Max");
    Dog oldDog = aDog;

    // we pass the object to foo
    foo(aDog);
    // aDog variable is still pointing to the "Max" dog when foo(...) returns
    aDog.getName().equals("Max"); // true
    aDog.getName().equals("Fifi"); // false
    aDog == oldDog; // true
}

public static void foo(Dog d) {
    d.getName().equals("Max"); // true
    // change d inside of foo() to point to a new Dog instance "Fifi"
    d = new Dog("Fifi");
    d.getName().equals("Fifi"); // true
}

In the example above aDog.getName() will still return "Max". The value aDog within main is not changed in the function foo with the Dog "Fifi" as the object reference is passed by value. If it were passed by reference, then the aDog.getName() in main would return "Fifi" after the call to foo.

Likewise:

public static void main(String[] args) {
    Dog aDog = new Dog("Max");
    Dog oldDog = aDog;

    foo(aDog);
    // when foo(...) returns, the name of the dog has been changed to "Fifi"
    aDog.getName().equals("Fifi"); // true
    // but it is still the same dog:
    aDog == oldDog; // true
}

public static void foo(Dog d) {
    d.getName().equals("Max"); // true
    // this changes the name of d to be "Fifi"
    d.setName("Fifi");
}

In the above example, Fifi is the dog's name after call to foo(aDog) because the object's name was set inside of foo(...). Any operations that foo performs on d are such that, for all practical purposes, they are performed on aDog, but it is not possible to change the value of the variable aDog itself.

For more information on pass by reference and pass by value, consult the following SO answer: https://stackoverflow.com/a/430958/6005228. This explains more thoroughly the semantics and history behind the two and also explains why Java and many other modern languages appear to do both in certain cases.

Bash – How to parse command line arguments in Bash

Bash Space-Separated (e.g., `--option argument`)

cat >/tmp/demo-space-separated.sh <<'EOF'
#!/bin/bash

POSITIONAL=()
while [[ $# -gt 0 ]]; do
  key="$1"

  case $key in
    -e|--extension)
      EXTENSION="$2"
      shift # past argument
      shift # past value
      ;;
    -s|--searchpath)
      SEARCHPATH="$2"
      shift # past argument
      shift # past value
      ;;
    -l|--lib)
      LIBPATH="$2"
      shift # past argument
      shift # past value
      ;;
    --default)
      DEFAULT=YES
      shift # past argument
      ;;
    *)    # unknown option
      POSITIONAL+=("$1") # save it in an array for later
      shift # past argument
      ;;
  esac
done

set -- "${POSITIONAL[@]}" # restore positional parameters

echo "FILE EXTENSION  = ${EXTENSION}"
echo "SEARCH PATH     = ${SEARCHPATH}"
echo "LIBRARY PATH    = ${LIBPATH}"
echo "DEFAULT         = ${DEFAULT}"
echo "Number files in SEARCH PATH with EXTENSION:" $(ls -1 "${SEARCHPATH}"/*."${EXTENSION}" | wc -l)
if [[ -n $1 ]]; then
    echo "Last line of file specified as non-opt/last argument:"
    tail -1 "$1"
fi
EOF

chmod +x /tmp/demo-space-separated.sh

/tmp/demo-space-separated.sh -e conf -s /etc -l /usr/lib /etc/hosts

Output from copy-pasting the block above

FILE EXTENSION  = conf
SEARCH PATH     = /etc
LIBRARY PATH    = /usr/lib
DEFAULT         =
Number files in SEARCH PATH with EXTENSION: 14
Last line of file specified as non-opt/last argument:
#93.184.216.34    example.com

Usage

demo-space-separated.sh -e conf -s /etc -l /usr/lib /etc/hosts

Bash Equals-Separated (e.g., `--option=argument`)

cat >/tmp/demo-equals-separated.sh <<'EOF'
#!/bin/bash

for i in "$@"; do
  case $i in
    -e=*|--extension=*)
      EXTENSION="${i#*=}"
      shift # past argument=value
      ;;
    -s=*|--searchpath=*)
      SEARCHPATH="${i#*=}"
      shift # past argument=value
      ;;
    -l=*|--lib=*)
      LIBPATH="${i#*=}"
      shift # past argument=value
      ;;
    --default)
      DEFAULT=YES
      shift # past argument with no value
      ;;
    *)
      # unknown option
      ;;
  esac
done
echo "FILE EXTENSION  = ${EXTENSION}"
echo "SEARCH PATH     = ${SEARCHPATH}"
echo "LIBRARY PATH    = ${LIBPATH}"
echo "DEFAULT         = ${DEFAULT}"
echo "Number files in SEARCH PATH with EXTENSION:" $(ls -1 "${SEARCHPATH}"/*."${EXTENSION}" | wc -l)
if [[ -n $1 ]]; then
    echo "Last line of file specified as non-opt/last argument:"
    tail -1 $1
fi
EOF

chmod +x /tmp/demo-equals-separated.sh

/tmp/demo-equals-separated.sh -e=conf -s=/etc -l=/usr/lib /etc/hosts

Output from copy-pasting the block above

FILE EXTENSION  = conf
SEARCH PATH     = /etc
LIBRARY PATH    = /usr/lib
DEFAULT         =
Number files in SEARCH PATH with EXTENSION: 14
Last line of file specified as non-opt/last argument:
#93.184.216.34    example.com

Usage

demo-equals-separated.sh -e=conf -s=/etc -l=/usr/lib /etc/hosts

To better understand ${i#*=} search for "Substring Removal" in this guide. It is functionally equivalent to `sed 's/[^=]*=//' <<< "$i"` which calls a needless subprocess or `echo "$i" | sed 's/[^=]*=//'` which calls two needless subprocesses.

Using bash with getopt[s]

getopt(1) limitations (older, relatively-recent getopt versions):

can't handle arguments that are empty strings
can't handle arguments with embedded whitespace

More recent getopt versions don't have these limitations. For more information, see these docs.

POSIX getopts

Additionally, the POSIX shell and others offer getopts which doen't have these limitations. I've included a simplistic getopts example.

cat >/tmp/demo-getopts.sh <<'EOF'
#!/bin/sh

# A POSIX variable
OPTIND=1         # Reset in case getopts has been used previously in the shell.

# Initialize our own variables:
output_file=""
verbose=0

while getopts "h?vf:" opt; do
  case "$opt" in
    h|\?)
      show_help
      exit 0
      ;;
    v)  verbose=1
      ;;
    f)  output_file=$OPTARG
      ;;
  esac
done

shift $((OPTIND-1))

[ "${1:-}" = "--" ] && shift

echo "verbose=$verbose, output_file='$output_file', Leftovers: $@"
EOF

chmod +x /tmp/demo-getopts.sh

/tmp/demo-getopts.sh -vf /etc/hosts foo bar

Output from copy-pasting the block above

verbose=1, output_file='/etc/hosts', Leftovers: foo bar

Usage

demo-getopts.sh -vf /etc/hosts foo bar

The advantages of getopts are:

It's more portable, and will work in other shells like dash.
It can handle multiple single options like -vf filename in the typical Unix way, automatically.

The disadvantage of getopts is that it can only handle short options (-h, not --help) without additional code.

There is a getopts tutorial which explains what all of the syntax and variables mean. In bash, there is also help getopts, which might be informative.

Best Answer

Related Solutions

Java – Is Java “pass-by-reference” or “pass-by-value”

Bash – How to parse command line arguments in Bash

Bash Space-Separated (e.g., --option argument)

Output from copy-pasting the block above

Usage

Bash Equals-Separated (e.g., --option=argument)

Output from copy-pasting the block above

Usage

Using bash with getopt[s]

POSIX getopts

Output from copy-pasting the block above

Usage

Related Topic

Bash Space-Separated (e.g., `--option argument`)

Bash Equals-Separated (e.g., `--option=argument`)