While zsh markets itself as "fully backwards-compatible," there are subtle but critical differences that matter in production environments:

# Bash handles array indices differently
arr=(a b c)
echo ${arr[-1]}  # Works in zsh but fails in bash (pre-4.0)

# Process substitution behavior differs
diff <(sort file1) <(sort file2) # More reliable in bash for large files

Bash consistently outperforms zsh in:

• Startup time (critical for short-lived processes)
• Memory usage (important for embedded systems)
• Pipeline execution (noticeable in complex scripts)

# Test startup time comparison
time for i in {1..1000}; do bash -c ":"; done
time for i in {1..1000}; do zsh -c ":"; done

Bash maintains stricter POSIX compliance which is crucial for:

# Portable shebang lines
#!/bin/sh vs #!/bin/zsh
# This Dockerfile snippet demonstrates strict POSIX needs
FROM alpine
RUN apk add bash
COPY script.sh # Requires POSIX-compliant syntax

In corporate IT ecosystems:

• Default shell on RHEL/CentOS remains bash
• Security tools often assume bash behavior
• Existing infrastructure scripts (CI/CD) frequently require bash

# Example of security-sensitive bashism
restricted_shell() {
  set -o restricted
  # This protection works differently in zsh
}

Surprisingly, bash includes functionality not in zsh:

# Associative arrays before zsh implemented them
declare -A arr=([key1]=val1 [key2]=val2)

# Process substitution with coprocesses
coproc myproc { sleep 5; echo done; }

The shell landscape evolves constantly, but bash remains the pragmatic choice for system-level scripting where performance, compatibility, and predictability outweigh interactive convenience.

While zsh markets itself as "fully backward-compatible" with bash, there are critical edge cases where this breaks down in real system administration scenarios:

#!/bin/bash
# This will fail in zsh due to different array handling
declare -A associative_array=([key1]="value1" [key2]="value2")
for key in "${!associative_array[@]}"; do
    echo "$key - ${associative_array[$key]}"
done

Bash consistently outperforms zsh in:

• Startup time (critical for short-lived scripts)
• Memory footprint (important in container environments)
• Subshell execution speed

Bash adheres closer to POSIX standards, making it the safer choice for:

#!/bin/sh
# This portable shebang works better with bash
while IFS= read -r line; do
    process "$line"
done < "input.txt"

Bash remains the default shell on:

• Most Linux distributions (including RHEL/CentOS)
• BSD systems
• Docker base images
• CI/CD environments

Surprisingly, bash has some unique capabilities:

#!/bin/bash
# Process substitution works more reliably
diff <(sort file1) <(sort file2)

# Better handling of SIGCHLD traps
trap 'cleanup' CHLD

Zsh shines in interactive use cases with:

• Better tab completion
• Theming capabilities
• Interactive globbing