Resolving Variable Scope Issues In Bash Pipelines
Defining The Problem: Variable Values Lost Across Pipelines
Bash pipelines allow commands to be chained together by connecting the standard output of one command to the standard input of another. This enables efficient data processing flows. However, pipelines also create subshells for each command. As a result, variables set in one part of a pipeline may lose their values further down the chain.
Understanding what is happening behind the scenes in bash pipelines explains why values get lost and how to preserve variables across pipeline stages.
Why Variable Values Are Lost in Pipelines
Background on how pipelines work in Bash
A pipeline chains multiple commands by redirecting the standard output stream of one command into the standard input of another. This connects the commands. However, under the hood, bash also creates a subshell to execute each command in the pipeline.
These subshells cannot access variables set in other subshells. Rather, they get copies of variables from their parent environment. The parent environment sees updated variable values, but child subshells do not.
Explanation of how this impacts variable scoping
The key thing to understand is that pipelines trigger subshells, and each subshell gets its own copy of variables. Any changes one subshell makes to variables will not be seen by others. This is what causes the variable scoping issues.
For example, if you set a variable MY_VAR in the first command in a pipeline, the updated value is not available to later commands. Those later subshells start with a copy of MY_VAR taken when the pipeline launched.
Understanding this, we can now look at strategies for preserving variable values across the subshells in a pipeline.
Preserving Variables With export
Example showing variables lost across pipeline
Here is a pipeline demonstrating the variable scoping issue:
MY_VAR="foo" echo $MY_VAR | read MY_VAR; echo $MY_VAR
We would expect this to output “foo” twice. However, it actually outputs:
foo
The second subshell never receives the updated MY_VAR value set in the first command.
Using export
to make variable available to subshells
The export command flags a variable to be exported into child subshell environments. Exported variables get passed down into pipes and commands executed later.
By using export we can make MY_VAR persist across the pipeline:
export MY_VAR="foo" echo $MY_VAR | read MY_VAR; echo $MY_VAR
Now this prints MY_VAR properly twice:
foo foo
Code examples demonstrating export
Exporting variables is an easy way to maintain their visibility across pipelines. Here are some additional examples:
# Single command pipeline export COUNT=0 for i in {1..10}; do echo $i; done | read; ((COUNT++)) echo $COUNT # Prints 1 # Chained pipeline export LANG=en_US.UTF-8 echo "my string" | tr '[:lower:]' '[:upper:]' | md5sum | cut -d' ' -f1 echo $LANG # Prints en_US.UTF-8
So using export provides an easy way to maintain variable scopes across pipelines.
Alternative: Storing Output In Variables
Example showing variables lost across pipeline
As a reminder, here is an example pipeline with a lost variable:
VAR=init echo "hello" | read VAR; echo $VAR
This prints:
hello
Despite setting VAR to init, the second command does not see this updated value.
Storing command output in a variable
An alternative is to store pipeline output in a variable that gets passed down the chain:
OUT="$(echo "hello")" read VAR <<< "$OUT"; echo $VAR
By capturing echo output in $OUT, we can later set VAR from it reliably:
hello
Code examples showing improved scopes
This variable capture approach also works across longer pipelines:
OUT="$(ls -l)" echo "$OUT" | grep -q .txt if [ $? -eq 0 ]; then echo "Directory contains .txt files" fi
So saving output in variables is another way to maintain state across pipelines.
Passing Variables Explicitly
Example showing variables lost across pipeline
Our problematic pipeline example yet again:
VAR=init echo "hello" | read VAR; echo $VAR
Producing:
hello
Passing variables explicitly to commands in pipeline
We can also pass variables directly to pipeline stages:
VAR=init echo "hello" | VAR=$VAR read; echo $VAR
By setting VAR explicitly for the read command, we maintain visibility:
init
Code examples showing explicit variable passing
Explicitly passing variables enables clearer scopes:
DATE=`date +%F` touch "$DATE-report.txt" ls *report* | wc -l | VAR=$VAR read; echo $VAR # 1
The VAR value is maintained and accessible outside the pipeline.
When To Redesign for Readability
Benefits of readable and modular scripts
While these techniques enable variable passing in pipelines, complex pipelines can become difficult to understand.
In these cases, it can be better to break the pipeline into reusable functions. This improves understandability, debugging, and reusability of logic blocks.
Ways to break up pipelines for improved readability
Some ways to modularize lengthy pipelines include:
- Wrapping stages in helper functions or scripts
- Using temporary files to pass data between steps
- Setting control variables rather than pipes
Designing scripts this way leads to clearer code, though it does take more up-front effort.
Example rewrite for clarity over minimalism
Here is an example rewrite:
# Harder to understand pipeline VAR=init cat file.txt | grep foo | wc -l | read VAR; echo $VAR # Clearer implementation with functions getNumFoo() { grep foo "$1" | wc -l } file=file.txt count=$(getNumFoo "$file") echo $count
The functional approach improves readability while eliminating the variable scope trickiness.