51 lines
2.2 KiB
Markdown
51 lines
2.2 KiB
Markdown
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---
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tags:
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- Programming_Languages
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- shell
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---
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## Relation between commands and programs
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Whenever we issue a command in bash we are really running an executable program that is associated with the command. This is why when we create our own bash scripts we must run `chmod` to make them executables. When we issue a command like `./file.sh` we are running an executable program.
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How come, however that when we use a program like `cd` or `npm` we don’t have to type `./cd.sh` or `./npm.sh` ? Remember from our discussion of the `PATH` environment variable that whenever we use inbuilt commands like `ls` and `cd` we are automatically sourcing them from the binary directory because we have these directories in our `PATH` . Hence the shell knows in advance what these commands mean. In the case of custom scripts, these aren’t typically added to the `PATH` so we have to source them in order to run them.
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## Passing arguments
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If you think about it, a script is really just a function that runs when you source it. As such there needs to be a way for you to pass data to the function so that it can actually act like a function and take arguments. When we use for example `cd ./Desktop` we are passing a directory name as an argument to the `cd` program. We can do the same thing with our custom bash scripts.
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To pass an argument we simply add the values after the script in the command. For example:
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````bash
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./arguments.sh Thomas 33
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````
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The script is as follows:
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````bash
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#!/bin/bash
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echo "File is called $0"
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echo "The arguments provided are $@"
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echo "The first argument is $1"
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echo "The second argument is $2"
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echo "Your name is $1 and you are $2 years old"
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````
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This outputs:
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````
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File is called ./arguments.sh
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The arguments provided are Thomas 33
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The first argument is Thomas
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The second argument is 33
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Your name is Thomas and you are 33 years old
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````
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Some points to note on syntax. The `$` is used to individuate the script itself and its arguments.
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* Each argument passed is accessible from an index starting at `1` (`$1`)
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* The script itself occupies the `0` position, hence we are able to log the name of the script at line 1 `$0` )
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* To log the arguments as a group (for instance to later loop through them) we use `$@` .
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* To get the number of arguments use `$#`
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