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Passing arguments to scripts
Relation between commands and programs
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.
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.
Passing arguments
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.
To pass an argument we simply add the values after the script in the command. For example:
./arguments.sh Thomas 33
The script is as follows:
#!/bin/bash
echo "File is called $0"
echo "The arguments provided are $@"
echo "The first argument is $1"
echo "The second argument is $2"
echo "Your name is $1 and you are $2 years old"
This outputs:
File is called ./arguments.sh
The arguments provided are Thomas 33
The first argument is Thomas
The second argument is 33
Your name is Thomas and you are 33 years old
Some points to note on syntax. The $ is used to individuate the script itself and its arguments.
- Each argument passed is accessible from an index starting at
1($1) - The script itself occupies the
0position, hence we are able to log the name of the script at line 1$0) - To log the arguments as a group (for instance to later loop through them) we use
$@. - To get the number of arguments use
$#