Do multiple shell scripts sourcing a common other script interfere with each other when they modify a "sourced" variable?

Question

If I source the same shell script from multiple other scripts, which I run in parallel, and modify a shell variable defined in the sourced script, will the sourcing scripts mess up each other's value of that variable?

Example :

• source.sh

  #!/bin/bash
  
  some_value=$1+10
  

• file-1.sh

  #!/bin/bash
  
  source ./source.sh 5
  ./some-script $some_value
  

• file-2.sh

  #!/bin/bash
  
  source ./source.sh 10
  ./some-script.sh $some_value
  

Answer 1

You have as many environment or shell variables $some_value as you have processes which define them.

Then if you launch at the same time file-1.sh and file-2.sh you will have different values since you have different processes. Note that source doesn’t create process, the sourced script works with the file-1.sh and with the file-2.sh process environments.

Answer 2

When you source a shell-script, that means the current shell instance is reading the script line-by-line and executing the commands on the spot, as if you had entered them on the command-line (if you are executing the source call from the prompt) or as if you had copy-and-pasted them into the script (if you are sourcing from a script).

Therefore, the different shell scripts file-1.sh and file-2.sh both behave as having a locally defined shell variable that only coincidentally has the same name some_value in both cases. Even if you ran the two scripts in parallel, they would not know anything about each other, and so there are no cross-interactions between them - the sub-shell in which file-1.sh is running does not know that the sub-shell in which file-2.sh is running also happens to have a variable some_value defined.

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As a side note, your attempt at creating an arithmetic expression as

some_value=$1+10

will not have the effect you may imagine. For file-1.sh it will yield 5+10 literally, and for file-2.sh it will yield 10+10 literally. In order to perform arithmetic evaluation, you have to place it in an arithmetic expansion, as in

some_value=$(($1+10))

I would recommend checking shell-scripts with shellcheck, also available as standalone tool on many Linux distributions, to catch syntax (and some logical) errors.