Handling positional parameters [Bash Hackers Wiki]

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====== Handling positional parameters ======

{{keywords>bash shell scripting arguments positional parameters options}}


===== Intro =====

The day will come when you want to give arguments to your scripts. These arguments are known as **positional parameters**. Some relevant special parameters are described below:
^Parameter(s)^Description^
|''$0''|the first positional parameter, equivalent to ''argv[0]'' in C, see [[scripting:posparams#the_first_argument | the first argument]]|
|''$FUNCNAME''| the function name (__**attention**__: inside a function, ''$0'' is still the ''$0'' of the shell, **not** the function name)|
|''$1 ... $9''|the argument list elements from 1 to 9|
|''${10} ... ${N}''|the argument list elements beyond 9 (note the [[syntax:pe | parameter expansion]] syntax!)|
|''$*''|all positional parameters except ''$0'', see [[scripting:posparams#mass_usage | mass usage]]|
|''$@''|all positional parameters except ''$0'', see [[scripting:posparams#mass_usage | mass usage]]|
|''$#''|the number of arguments, not counting ''$0''|


These positional parameters reflect exactly what was given to the
script when it was called.

Option-switch parsing (e.g. ''-h'' for displaying help) is not performed at
this point.

See also [[dict:terms:parameter | the dictionary entry for "parameter"]].


===== The first argument =====

The very first argument you can access is referenced as ''$0''. It
is usually set to the script's name exactly as called, and it's
set on shell initialization:

__Testscript__ - it just echos ''$0'':
<code>
#!/bin/bash
echo "$0"
</code>
You see, ''$0'' is always set to the name the script is called with (''>'' is the prompt...):
<code>
> ./testscript 
./testscript
</code>
<code>
> /usr/bin/testscript
/usr/bin/testscript
</code>

However, this isn't true for login shells:
<code>
> echo "$0"
-bash
</code>

In other terms, ''$0'' is not a positional parameter, it's a
special parameter independent from the positional parameter list. It
can be set to anything. In the **ideal** case it's the pathname
of the script, but since this gets set on invocation, the invoking
program can easily influence it (the ''login'' program does that for
login shells, by prefixing a dash, for example).

Inside a function, ''$0'' still behaves as described above. To
get the function name, use ''$FUNCNAME''.

===== Shifting =====

The builtin command ''shift'' is used to change the positional parameter values:
  * ''$1'' will be discarded
  * ''$2'' will become ''$1''
  * ''$3'' will become ''$2''
  * ...
  * in general: ''$N'' will become ''$N-1''

The command can take a number as argument: Number of positions to shift.
 e.g. ''shift 4'' shifts ''$5'' to ''$1''.

===== Using them =====

Enough theory, you want to access your script-arguments. Well, here we go.

==== One by one ====

One way is to access specific parameters:
<code>
#!/bin/bash
echo "Total number of arguments: $#"
echo "Argument 1: $1"
echo "Argument 2: $2"
echo "Argument 3: $3"
echo "Argument 4: $4"
echo "Argument 5: $5"
</code>

While useful in another situation, this way is lacks flexibility.
The maximum number of arguments is a fixedvalue
- which is a bad idea if you write a script that takes many filenames
as arguments.

=> forget that one

==== Loops ====

There are several ways to loop through the positional parameters.

----

You can code a [[syntax:ccmd:c_for | C-style for-loop]] using ''$#''
as the end value. On every iteration, the ''shift''-command is used to
shift the argument list:

<code>
numargs=$#
for ((i=1 ; i <= numargs ; i++))
do
    echo "$1"
    shift
done
</code>

Not very stylish, but usable. The ''numargs'' variable is used
to store the initial value of ''$#'' because the shift command
will change it as the script runs.

----

Another way to iterate one argument at a time is the ''for'' loop
without a given wordlist. The loop uses the positional parameters as a wordlist:

<code>
for arg
do
    echo "$arg"
done
</code>
__Advantage:__ The positional parameters will be preserved

----

The next method is similar to the first example (the ''for'' loop), but
it doesn't test for reaching ''$#''. It shifts and checks if ''$1''
still expands to something, using the [[commands:classictest | test command]]:

<code>
while [ "$1" ]
do
    echo "$1"
    shift
done
</code>

Looks nice, but has the disadvantage of stopping when ''$1'' is empty
(null-string). Let's modify it to run as long as ''$1'' is defined
(but may be null), using [[syntax:pe#use_an_alternate_value | parameter expansion for an alternate value]]:

<code>
while [ "${1+defined}" ]; do
  echo "$1"
  shift
done
</code>

==== Getopts ====

There is a [[howto:getopts_tutorial|small tutorial dedicated to ''getopts'']] (//under construction//).

===== Mass usage =====

==== All Positional Parameters ====

Sometimes it's necessary to just "relay" or "pass" given
arguments to another program. It's very inefficient to do that in one
of these loops, as you will destroy integrity, most likely
(spaces!).

The shell developers created ''$*'' and ''$@'' for this purpose.

As overview:

^Syntax      ^Effective result                 ^
|  ''$*''    |  ''$1 $2 $3 ... ${N}''          |
|  ''$@''    |  ''$1 $2 $3 ... ${N}''          |
|  ''"$*"''  |  ''"$1c$2c$3c...c${N}"''        |
|  ''"$@"''  |  ''"$1" "$2" "$3" ... "${N}"''  |

Without being quoted (double quotes), both have the same
effect: All positional parameters from ''$1'' to the last one used are
expanded without any special handling.

When the ''$*'' special parameter is double quoted, it expands to the
equivalent of: ''"$1c$2c$3c$4c........$N"'', where 'c' is the first
character of ''IFS''.

But when the ''$@'' special parameter is used inside double quotes, it
expands to the equivanent of...

''"$1" "$2" "$3" "$4" ..... "$N"''

...which **reflects all positional parameters as they were
set initially** and passed to the script or function. If you want
to re-use your positional parameters to **call another program** (for
example in a wrapper-script), then this is the choice for you, use
double quoted ''"$@"''.

Well, let's just say: **You almost always want a quoted ''"$@"''!**

==== Range Of Positional Parameters ====

Another way to mass expand the positional parameters is similar to
what is possible for a range of characters using
[[syntax:pe#substring_expansion | substring expansion]] on normal
parameters and the mass expansion range of [[syntax:arrays | arrays]].

''${@:START:COUNT}''

''${*:START:COUNT}''

''"${@:START:COUNT}"''

''"${*:START:COUNT}"''

The rules for using ''@'' or ''*'' and quoting are the same as
above. This will expand ''COUNT'' number of positional parameters
beginning at ''START''. ''COUNT'' can be omitted (''${@:START}''), in
which case, all positional parameters beginning at ''START'' are
expanded.

If ''START'' is negative, the positional parameters are numbered in reverse
starting with the last one.

''COUNT'' may not be negative, i.e. the element count may not be decremented.

__**Example:**__
START at the last positional parameter:
<code>
echo "${@: -1}"
</code>

__**Attention**__: As of Bash 4, a ''START'' of ''0'' includes the special parameter ''$0'', i.e. the shell name or whatever $0 is set to, when the positional parameters are in use. A ''START'' of ''1'' begins at ''$1''. In Bash 3 and older, both ''0'' and ''1'' began at ''$1''.

===== Setting Positional Parameters =====

Setting positional parameters with command line arguments,
is not the only way to set them.
The [[ commands:builtin:set | builtin command, set ]]
may be used to "artificially" change the positional parameters from
inside the script or function:

<code>
set "This is" my new "set of" positional parameters

# RESULTS IN
# $1: This is
# $2: my
# $3: new
# $4: set of
# $5: positional
# $6: parameters
</code>

It's wise to signal "end of options" when setting positional
parameters this way. If not, the dashes might be interpreted as an option switch
by ''set'' itself:

<code>
# both ways work, but behave differently. See the article about the set command!
set -- ...
set - ...
</code>

Alternately this will also preserve any verbose (-v) or tracing (-x) flags, which may otherwise be reset by ''set''
<code>
set -$- ...
</code>

FIXME continue

===== Production examples =====

==== Using a while loop ====

To make your program accept options as standard command syntax:

''COMMAND [options] <params>''  # Like 'cat -A file.txt'

See simple option parsing code below. It's not that flexible. It
doesn't auto-interpret combined options (-fu USER) but it works and is
a good rudimentary way to parse your arguments.

<code>
#!/bin/sh
# Keeping options in alphabetical order makes it easy to add more.

while :
do
    case "$1" in
      -f | --file)
	  file="$2"   # You may want to check validity of $2
	  shift 2
	  ;;
      -h | --help)
	  display_help  # Call your function
	  # no shifting needed here, we're done.
	  exit 0
	  ;;
      -u | --user)
	  username="$2" # You may want to check validity of $2
	  shift 2
	  ;;
      -v | --verbose)
          #  It's better to assign a string, than a number like "verbose=1"
	  #  because if you're debugging the script with "bash -x" code like this:
	  #
	  #    if [ "$verbose" ] ...
	  #
	  #  You will see:
	  #
	  #    if [ "verbose" ] ...
	  #
          #  Instead of cryptic
	  #
	  #    if [ "1" ] ...
	  #
	  verbose="verbose"
	  shift
	  ;;
      --) # End of all options
	  shift
	  break;
      -*)
	  echo "Error: Unknown option: $1" >&2
	  exit 1
	  ;;
      *)  # No more options
	  break
	  ;;
    esac
done

# End of file
</code>

==== Filter unwanted options with a wrapper script ====

This simple wrapper enables filtering unwanted options (here: ''-a''
and ''--all'' for ''ls'') out of the command line. It reads the
positional parameters and builds a filtered array consisting of them, then
calls ''ls'' with the new option set. It also respects the ''--''
as "end of options" for ''ls'' and doesn't change anything after it:

<code>
#!/bin/bash

# simple ls(1) wrapper that doesn't allow the -a option

options=()  # the buffer array for the parameters
eoo=0       # end of options reached

while [[ $1 ]]
do
    if ! ((eoo)); then
	case "$1" in
	  -a)
	      shift
	      ;;
	  --all)
	      shift
	      ;;
	  -[^-]*a*|-a?*)
	      options+=("${1//a}")
	      shift
	      ;;
	  --)
	      eoo=1
	      options+=("$1")
	      shift
	      ;;
	  *)
	      options+=("$1")
	      shift
	      ;;
	esac
    else
	options+=("$1")

	# Another (worse) way of doing the same thing:
	# options=("${options[@]}" "$1")
	shift
    fi
done

/bin/ls "${options[@]}"
</code>

==== Using getopts ====

There is a [[howto:getopts_tutorial|small tutorial dedicated to ''getopts'']] (//under construction//).

===== See also =====

  * Internal: [[howto:getopts_tutorial]]
  * Internal: [[syntax:ccmd:while_loop]]
  * Internal: [[syntax:ccmd:c_for]]
  * Internal: [[syntax:arrays]] (for equivalent syntax for mass-expansion)
  * Internal: [[syntax:pe#substring_expansion | Substring expansion on a parameter]]  (for equivalent syntax for mass-expansion)
  * Dictionary, internal: [[dict:terms:parameter]]