Achieving Posix/Unix Compliance: Configuring Echo In Different Shells
The Core Issue – Inconsistent Echo Output
The echo command is used ubiquitously in shells and scripts to print output. However, the default behavior of echo is often inconsistent across different shells and environments. Specifically, echo by default does not fully comply with the POSIX standard, which requires echo to not interpret backslash escapes and special characters in its arguments. Instead, most shells like bash, zsh, ksh and others print the given arguments as-is by default, interpreting escapes and special chars. This leads to portability issues when writing scripts expected to run across different POSIX compliant shells. Additionally, the default echo behavior can result in security issues when printing untrusted input that contains exploits.
Understanding Echo Behavior
To understand the core issue better, we will overview what echo outputs by default in common shells, why this behavior causes problems, and what the POSIX standard expects instead:
What echo outputs by default in bash, zsh, ksh, etc
In bash, zsh, and ksh, echo by default prints its arguments directly without any interpretation. This means backslash escapes like \n will be translated to a newline character. Special characters like $ will be interpreted as environment variable substitutions if they match a defined variable. And options like -n to suppress newline will be processed as flags to control echo behavior. For example:
# Bash echo -n "Hello \n $PATH" > Hello > /usr/bin:/usr/local/bin # Zsh echo -n "Hello \n $PATH" > Hello > /usr/bin:/usr/local/bin # Ksh echo -n "Hello \n $PATH" > Hello > /usr/bin:/usr/local/bin
As seen above, in all shells the \n escape is printed as a newline. And $PATH prints the value of the PATH variable. The -n flag also works to suppress the trailing newline.
Why default echo behavior is problematic
While convenient, this default interpretation of arguments causes two core issues:
- Portability problems – Scripts assuming this behavior may break if run under shells strictly following POSIX
- Security issues – Untrusted input passed to echo could trigger code execution or data leakage by injecting variable expansions, escapes etc when printed
Overview of POSIX requirement for echo
The POSIX standard specifies that echo should only print its arguments, without any interpretation or transformation. Backslashes, escapes, special chars should be printed literally. Environment variable substitutions, flags like -n etc. should be ignored. This ensures scripts using echo have consistent, portable behavior across POSIX compliant shells. And untrusted input passed to echo will not result in any code execution or data leakage. For example, a POSIX compliant echo would work like:
# POSIX echo echo -n "Hello \n $PATH" > Hello \n $PATH
As seen above, the arguments are printed unchanged, without interpreting the escapes or variable substitution.
Enabling Safe Echo in Different Shells
Since most common shells do not use a POSIX compliant echo by default, steps need to be taken to configure echo appropriately in bash, zsh, ksh etc. to address portability and security concerns. This section covers options to make echo conform with POSIX expectations in different shells.
Setting xpg_echo option in bash
Bash provides a shell option called xpg_echo that changes the default echo behavior to be POSIX compliant when set. This prevents interpretation of escapes and special characters in the arguments passed to echo.
To enable xpg_echo simply run:
shopt -s xpg_echo
Or add the command to your bash profile. Once enabled, echo will be POSIX compliant:
# XPG echo enabled shopt -s xpg_echo echo -n "Hello \n $PATH" > Hello \n $PATH
Using “print” built-in instead of echo in zsh
Zsh does not have an option to make echo POSIX compliant. Instead, the print built-in can used, which behaves akin to a POSIX echo by default:
# Zsh print print -n "Hello \n $PATH" > Hello \n $PATH
Simply replace all echo calls with print and zsh will now print arguments without interpretation.
The echo flags -e, -n, -E in ksh/mksh
In ksh and mksh, using various flags can control echo behavior:
- -e – Enable interpretation of backslash escapes
- -E – Explicitly disable interpretation of escapes (POSIX mode)
- -n – Remove trailing newline
So POSIX behavior can be achieved via:
# Ksh echo -E -n "Hello \n $PATH" > Hello \n $PATH
This prints the given arguments literally without any interpretation.
Example Echo Calls
To demonstrate portable and POSIX compliant echo usage across shells, here are some common examples.
Print string without newline
# Bash shopt -s xpg_echo; echo -n "Hello" # Zsh print -n "Hello" # Ksh echo -E -n "Hello"
Disable interpretation of backslash escapes
# Bash shopt -s xpg_echo; echo "Hello \n" # Zsh print "Hello \n" # Ksh echo -E "Hello \n"
Output environment variables safely
# Bash shopt -s xpg_echo; echo "PATH is set to $PATH" # Zsh print "PATH is set to $PATH" # Ksh echo -E "PATH is set to $PATH"
As seen above, with the various options to configure echo to be POSIX compliant, we can achieve consistent and secure echo output across different shells using portable calls.
Achieving Portable and Secure Echo Usage
Based on the behavior overview and configuration settings covered, these are best practices for cross-shell echo usage:
Recommendations for cross-shell echo behavior
- In bash, always enable xpg_echo option via shopt -s xpg_echo
- In zsh, use print rather than echo for calls
- In ksh/mksh, use echo -E to disable argument interpretation
- Only pass trusted strings from verified sources as echo arguments
Sticking to these practices will prevent portability issues or security vulnerabilities from incorrect echo behavior.
Alternatives to echo for printing output
In some cases using dedicated cross-platform printing functions can also avoid echo inconsistencies:
- printf – Formatted printing like in C, takes a format string and arguments
- print – Variant in some shells that behaves like POSIX echo
- perl, python, ruby – Using their printing functions if available
So in summary, either always enabling POSIX echo where possible, or using portable print alternatives can let you reliably output data in shell scripts.
