Portability Concerns With Shell Variable Expansions
Potential Pitfalls of Shell Variables
Shell variables are used to store data in bash and other Unix shell scripts. However, there are some key portability concerns and pitfalls to be aware of when working with shell variables across different systems and environments.
Environment Differences Across Systems
A major portability consideration with shell variables is differences in shell environments across operating systems. Variables set in a script on Linux, for example, may not be defined or have the same value on other Unix-like systems like Solaris or macOS.
Additionally, default shell environments can vary across Linux distributions. A variable that exists in Debian may not exist in CentOS. Even different versions of the same distribution can have slight environment discrepancies.
These environmental differences mean scripts relying on certain shell variables may not work as expected when run in a different context. Testing scripts across target platforms is important to identify such issues.
Understanding Variable Scoping Rules
The scoping rules around shell variables can also pose portability issues. Variables defined in global scope are available to subshells and child processes. However, variables set inside a bash function or subshell are not visible in the parent shell environment.
For example, setting a variable in a subprocess launched with $( ) won’t retain that variable after the subprocess exits. The subtle scoping rules around shell variables can lead to unexpected behavior when moving code between systems.
Handling Unset Variables
Attempting to expand an unset variable in bash will typically substitute an empty string. However, propagating an unset variable to other commands can result in error messages or unexpected outcomes.
For portability reasons, code should handle unset variables gracefully. Check if variables are set before expansion. Provide default values when variables are unset. Validate variables have the expected form before passing to other commands.
Variable Expansion Operations
In addition to basic variable assignment and expansion, bash provides special expansion operations. These can be useful, but also introduce portability considerations.
Parameter Expansion vs Command Substitution
Two common expansions are parameter expansion with ${ } and command substitution with $( ). It’s important to understand the differences for writing portable code.
Parameter expansions substitute the value of a variable into a string. Command substitutions run a command and replace the expression with the command’s standard output.
For example, ${VAR} expands the VALUE of VAR, while $(echo $VAR) runs echo to print the value. The former is portable, while the latter depends on the echo command.
Word Splitting and Pathname Expansion
By default, variable expansions undergo word splitting, treating white space as delimiters, and pathname expansion, replacing expressions like *.txt with matching filenames.
These expansion features change how variables are interpreted. To avoid bugs, it’s best to quote expansions unless word splitting or globbing is explicitly needed.
Quote Characters for Preventing Expansions
Bash offers various quote characters to control how variables and commands are interpreted. Double quotes ” prevent word splitting and globbing. Single quotes ‘ avoid all expansions and interpretation.
Escaping variables with backslashes \$ prevents expansion entirely. Using the appropriate quotes prevents unintended behavior during expansion operations.
Writing Portable Shell Scripts
Following shell scripting best practices helps avoid many portability pitfalls related to variables and expansion operations.
Using Standard sh Instead of Bashisms
Scripts using bash-specific features and expansions may not work on systems using other default shells like dash or ash.
Restricting scripts to POSIX sh features guarantees better portability across different Unix-like environments. Test scripts with sh instead of bash to identify bashisms.
Testing Scripts Across Platforms
Unit testing scripts locally is not enough to catch all portability bugs. Scripts should be tested on all target platforms to validate functionality and error handling.
Utilize CI/CD pipelines to integrate cross-platform testing. Manual testing on different OSes also helps uncover environment-specific issues before reaching production.
Abstracting System Differences
Instead of scattering system-specific code throughout a script, centralize environment and platform differences into modular functions.
Abstract calls to external commands, shell facilities, and tools into wrapper functions. This isolates portability code from the main script logic for simpler testing and maintenance.
Recommendations for Robust Code
Following defensive coding best practices prevents bugs and unintended behavior from variable expansions.
Validate Inputs and Environments
Validate all external inputs and data before passing to variable expansions or external commands. Scan for invalid data, injection attacks, incorrect argument counts or formats.
Also check the surrounding environment – are expected directories present, system facilities available, correct utilities installed? Adding guards prevents surprises.
Check Return Values and Handle Errors
Instead of assuming success, check return codes from commands and handle errors appropriately. This applies to command substitutions, exit statuses, as well as expansions.
Handle unset variables and empty expansions gracefully. Capture errors from utilities and external programs to stop issues from propagating system-wide.
Follow Best Practices for Defensive Coding
Apply standard defensive coding techniques to shell scripts:
- Initialize variables before use
- Quote expansions by default
- Use strict conditional checking – == not just =
- Beware hidden gotchas with shell special variables like $?
- Use functions over external commands when possible
These practices prevent surprises during expansions across systems.
Example Code Snippets
Here are some concrete examples of portable approaches to common shell variable operations:
Conditionally Set Default Values
#!/bin/sh
# Set default value if var is unset
: ${VAR:=default}
# Only set default if var is null
[ -z "$VAR" ] && VAR="default"
Wrap Expansions to Prevent Splitting
#!/bin/sh
# Join on colon prevents splitting on spaces
vars=$(echo "${USER}:${HOME}" | tr ':' ' ')
# Force string context keeps words together
string="$VAR"
parsed=( $string )
Portably Get Script’s Directory
#!/bin/sh # Directory containing script Even if symlinked DIR=$(dirname "$(readlink -f "$0")") # Assign CDPATH to avoid globs OLD_CDPATH="$CDPATH" CDPATH='' DIR=$(cd "$(dirname "$0")"; pwd) CDPATH="$OLD_CDPATH"
