Released in December 2020, Bash 5.1 introduced several notable features and improvements for modern shell scripting.
New Shell Variables
EPOCHSECONDS and EPOCHREALTIME
Two new special variables for accessing current time:
# Get current Unix timestamp in seconds
echo "Current timestamp: ${EPOCHSECONDS}"
# Get current Unix timestamp with microsecond precision
echo "High precision timestamp: ${EPOCHREALTIME}"
start="${EPOCHREALTIME}"
... some operation ...sleep 0.5 end="${EPOCHREALTIME}" duration=$(awk "BEGIN {print ${end} - ${start}}") echo "Operation took ${duration} seconds"
Example: Log with precise timestampslog_message() {
printf '[%s.%06d] %s\n'
"$(date -d "@${EPOCHSECONDS}" '+%Y-%m-%d %H:%M:%S')"
"${EPOCHREALTIME#.}"
"$"
}
log_message "Starting process"
**Use cases:**
- Precise performance measurement
- High-resolution timestamps for logging
- Avoiding external `date` command calls
- Microsecond-precision timing in scripts
### SRANDOM Variable
Cryptographically strong random numbers:
```bash
# Generate secure random number (if available)
if [[ -n "${SRANDOM}" ]]; then
secure_random="${SRANDOM}"
echo "Secure random: ${secure_random}"
else
echo "SRANDOM not available (requires getrandom/getentropy)"
fi
# Practical example: Generate temporary file with secure random name
temp_file="/tmp/secure_${SRANDOM}_${EPOCHSECONDS}.tmp"
Note: SRANDOM requires system support for getrandom() or getentropy() syscalls.
Improved Redirection Syntax
{varname} Redirection
Assign file descriptor to variable for better file handle management:
# Open file descriptor and store in variable
exec {fd}< input.txt
while IFS= read -r -u "${fd}" line; do
echo "Read: ${line}"
done
exec {fd}<&- # Close the file descriptor
# Practical example: Multiple file handles
exec {input_fd}< data.txt
exec {output_fd}> results.txt
exec {error_fd}> errors.txt
process_data() {
while IFS= read -r -u "${input_fd}" line; do
if validate_line "${line}"; then
echo "Processed: ${line}" >&"${output_fd}"
else
echo "Error: ${line}" >&"${error_fd}"
fi
done
}
process_data
exec {input_fd}<&-
exec {output_fd}>&-
exec {error_fd}>&-
Benefits:
- Named file descriptors instead of magic numbers
- Automatic FD allocation avoids conflicts
- More readable and maintainable code
- Easier tracking of open file handles
Array Enhancements
Improved Associative Array Handling
Better support for complex array operations:
# Declare associative array with typeset
declare -A config=(
[host]="localhost"
[port]="8080"
[debug]="true"
)
# Enhanced array expansion
for key in "${!config[@]}"; do
printf '%s=%s\n' "${key}" "${config[${key}]}"
done
# Practical example: Configuration parser
parse_config() {
declare -gA app_config
local line key value
while IFS='=' read -r key value; do
[[ "${key}" =~ ^[[:space:]]*# ]] && continue # Skip comments
[[ -z "${key}" ]] && continue # Skip empty lines
# Trim whitespace
key="${key#"${key%%[![:space:]]*}"}"
key="${key%"${key##*[![:space:]]}"}"
value="${value#"${value%%[![:space:]]*}"}"
value="${value%"${value##*[![:space:]]}"}"
app_config["${key}"]="${value}"
done < config.ini
}
Multidimensional Array Support
Improved handling of nested array structures.
Readline 8.1 Integration
Enhanced text editing and command-line interaction:
Key Bindings and History
# Configure readline in ~/.inputrc
# Enable case-insensitive completion
set completion-ignore-case on
# Enable visible stats for completions
set visible-stats on
# Show all completions immediately
set show-all-if-ambiguous on
# Use colors for completion matching
set colored-completion-prefix on
Improved History Search
# In your script or .bashrc
# Bind Ctrl+R for reverse incremental search (default, but enhanced in 5.1)
bind '"\C-r": reverse-search-history'
# Bind Ctrl+S for forward incremental search
bind '"\C-s": forward-search-history'
# Improved history handling
shopt -s histappend # Append to history
shopt -s cmdhist # Multi-line commands as one entry
HISTCONTROL=ignoreboth # Ignore duplicates and leading spaces
HISTSIZE=10000
HISTFILESIZE=20000
Signal Handling Improvements
Better signal propagation in subshells and process substitutions:
# Enhanced trap handling in subshells
cleanup() {
echo "Cleaning up..." >&2
rm -f "${temp_file}"
exit 130 # 128 + SIGINT
}
trap cleanup SIGINT SIGTERM
temp_file=$(mktemp)
# Signal properly propagates through pipelines
long_running_task | process_output &
pid=$!
# Wait for background job with proper signal handling
wait "${pid}" 2>/dev/null
exit_code=$?
if [[ ${exit_code} -gt 128 ]]; then
signal=$((exit_code - 128))
echo "Process terminated by signal ${signal}" >&2
fi
Bug Fixes and Edge Cases
Subshell Handling
Improved behavior when spawning subshells:
# More reliable subshell variable inheritance
outer_var="parent"
(
# Subshell now more reliably inherits parent variables
echo "In subshell: ${outer_var}"
inner_var="child"
)
# outer_var still accessible, inner_var is not
echo "After subshell: ${outer_var}"
Pattern Matching Edge Cases
Fixed edge cases in glob pattern matching:
# More reliable glob matching with special characters
shopt -s nullglob # Empty expansion for non-matching globs
shopt -s extglob # Extended pattern matching
# Example: Match files but handle no matches gracefully
files=(*.txt)
if [[ ${#files[@]} -eq 0 ]]; then
echo "No .txt files found"
else
printf 'Found: %s\n' "${files[@]}"
fi
Performance Improvements
- Faster variable expansion in loops
- Optimized array operations
- Reduced memory usage for large arrays
- Improved efficiency in pattern matching
Compatibility Notes
Upgrading from Bash 5.0
Most scripts are compatible, but note:
EPOCHSECONDSandEPOCHREALTIMEare new - check for existence if targeting older versionsSRANDOMmay not be available on all systems- Some obscure edge cases in expansion behavior were fixed
Version Check
# Check Bash version before using 5.1 features
if [[ "${BASH_VERSINFO[0]}" -ge 5 ]] && [[ "${BASH_VERSINFO[1]}" -ge 1 ]]; then
# Safe to use Bash 5.1 features
timestamp="${EPOCHSECONDS}"
else
# Fallback for older versions
timestamp=$(date +%s)
fi
References
- Bash NEWS file - Official release notes
- GNU Bash 5.1 Release - Source distribution
- Bash man page:
man bash(section on version-specific features)
Additional Resources
For broader Bash development patterns and best practices, see:
- ../bash-development/SKILL.md - Core Bash development patterns
- ../bash-portability/SKILL.md - Cross-version compatibility