Parameters in Linux Bash Scripts

When writing a bash script in Linux there might come a time where I might want to pass one or more arguments for a script. With that said there is starting out with positional parameters, but it is important to refer to positional parameters as such. That is because parameters can refer to a number of things in bash scripts other than just simple basic positional parameters.

There is knowing how to access arguments for a call of a script, and there is also knowing how to find out how many arguments where given when the script was called in total. There is also doing something for all arguments that are given like how the Linux cat command works when giving file names as arguments. So on top of positional parameters there are also a number of special parameters for tasks such as getting all the parameters, and the count of parameters.

There is also the bash command itself, and any arguments that might be based to the bash command before the script or command that is called. So then there is yet another set of parameters or arguments to be aware of. In addition there is also parsing the positional arguments into a form that can be described as named parameters. That is making a script that will except arguments in a similar way to that of many of the Linux commands that one might be familiar with.

So then in this post I will be going over some bash script examples where I am going over each of the kinds of parameters to be aware of.

1 - Positional Parameters in bash scripts

To start off with how about just a simple bash script that takes just one argument that is an example of a positional parameter. A sort of Bash Script Parameter Hello world for what it is worth. With that said there are a few Variables to work with inside a bash script that are references to each argument that is passed when the script is called. They all start off with the dollar sign symbol for starters followed by a number from zero upwards. Zero will be a reference to the command or script that is called, and then one forwards will be all the arguments for the call of the script.

So then Say I just want to make a simple hello world example of parameters in bash scripts. I would start off with the bash shebang as always for a script. Followed by just using the echo command to echo out hello followed by whatever is passed as the first argument by way of using $1.

So Something like this:

#!/bin/bash
echo "Hello $1"

That I would then save as something like basic.sh, after that I just need to make it executable, and then call it passing what I want as the first argument.

$ chmod 755 basic.sh
$ ./basic.sh "World"
Hello World

So then there is a basic hello world example of a positioned rather than flagged parameter in bash. However there are at least a few more things to cover beyond this when it comes to finding out how many arguments are passed, how to loop over all arguments, and how to work with flags. There are a few other related topics that might come up in the process of working with parameters so lets look at a few more examples of these kinds of bash scripts.

2 - Special parameters

On top of positional parameters there are a number of special parameters also.

2.1 - Get a count of the number of arguments that where given

What if I want to get a count of how many arguments there are when a script is called? For this there is the $# variable which will given the number of arguments given. This will not count the command or script itself, so if no arguments are given beyond just calling the script then the value will be zero.

#!/bin/bash
echo "num of arguments: ${#}"

$ chmod 755 count.sh
$ ./count.sh a b c d e f
num of arguments: 6

3 - Sum example using variable indirection ${!varname} to loop over arguments

When I took a moment to look over the man page on bash, there is a section on Parameter Expansion. In this section it explains something this is called variable indirection by making use of an exclamation point inside the body of a pair of curly brackets with the dollar sign symbol.

In other words this is a way to make the value of a variable be part of the variable name. In other sections in this post I have outlines that the variables of interest that hold values for arguments are $0, $1, $2, and so forth. However how do I go about making the value of a variable that is stepped by a rate of one be part of a variable name? The answer to this problem in bash is variable indirection.

Here I have a sum.sh file that makes use of $. I am then using the value of $i to resolve to a variable name by making user of ${!i}, the value of which I am storing in $n. I can then tabulate a sum of all the arguments given by way of adding $sum and $n to $sum which starts at 0.

#!/bin/bash
i=1
sum=0
while [ $i -le ${#} ]
do
  n=${!i}
  sum=$(( $sum + $n))
  i=$(( $i + 1 ))
done
echo "$sum"

When I test this out it works as expected, I always get the sum of all the number arguments given.

chmod 755 sum.sh
$ ./sum.sh 10 5 7
22

SO then this is one way to go about looping over all the arguments that are given, the total number of which can change from one call to another. However maybe there is at least one other way to go about doing this that might be worth writing about. In any case I touched base on another topic of interest when it comes to bash scripts which is how to go about using a variable for a variable name. If that is not used when working with parameters I am sure that it will come in handy elsewhere.

4 - Another sum example making use of $@ to loop over arguments

Another way to make a script that will loop over all arguments of a script will be to use $@. This will give an array of the arguments that can then be used with a for in loop.

#!/bin/bash
sum=0
for foo in "$@"
do
  sum=$(( $sum + foo))
done
echo "$sum"

$ chmod 755 sum_at.sh
$ ./sum_at.sh 5 10
15

So then on top of using variable indirection This would prove to be another way to loop over the arguments that are given when calling a bash script.

5 - Conclusion

SO that is it for now when it comes to parameters in bash scripts. I am still fairly new to writing bash scripts though, and I still have a lot more to learn about until I can start to make some real solid examples of bash scripts. In the future I will want to edit this post to expand it with more examples as needed.