Default Shell vs Script Shell: The First Trap

macOS Catalina (10.15) switched the default interactive shell to Zsh in 2019. Most Linux distributions still default to Bash. This means developers writing scripts on macOS are often writing Zsh without knowing it, because `#!/bin/sh` on macOS now resolves to a POSIX-compliant subset of Zsh, not Bash.

On Linux, `/bin/sh` is almost always `dash` or `bash --posix`, never Zsh. A script that works on macOS with `#!/bin/sh` can fail on Linux if it depends on Zsh-specific behavior that happens to be available in the macOS `sh` compatibility layer.

Always use an explicit shebang. If you need Bash 5 features, write `#!/usr/bin/env bash`. If you need Zsh, write `#!/usr/bin/env zsh`. Never rely on `#!/bin/sh` for anything beyond strict POSIX compliance.

# Check which shell /bin/sh actually points to
ls -la /bin/sh
file /bin/sh

# On Ubuntu 24.04:
# /bin/sh -> dash

# On macOS 14 Sonoma:
# /bin/sh -> zsh (POSIX mode)

# Check installed versions
bash --version | head -1
zsh --version

Array Indexing: The Most Common Silent Bug

Bash arrays are zero-indexed. Zsh arrays are one-indexed by default. This single difference breaks more cross-shell scripts than any other feature. In our test environment, a deployment script that parsed a list of server hostnames into an array worked perfectly in Zsh during development, then silently skipped the first server in production because Bash started reading from index 0 while the script logic assumed index 1.

In Bash, `${array[0]}` is the first element. In Zsh, `${array[0]}` is empty by default because Zsh arrays start at 1, making `${array[1]}` the first element. You can force Zsh to use zero-based indexing with `setopt KSH_ARRAYS`, which also changes several other array behaviors to match ksh93.

Associative arrays (hash maps) are available in both shells but with different declaration syntax. Bash requires `declare -A`. Zsh uses `typeset -A`, though `declare -A` also works in Zsh 5.x as an alias.

The practical rule: if your script touches arrays with numeric indexes and needs to run on both shells, add `setopt KSH_ARRAYS` at the top of Zsh scripts, or better, just target one shell explicitly and document which one.

# Bash - zero-indexed
bash -c '
  arr=("alpha" "beta" "gamma")
  echo "Index 0: ${arr[0]}"  # alpha
  echo "Index 1: ${arr[1]}"  # beta
  echo "Length:  ${#arr[@]}"
'

# Zsh - one-indexed (default)
zsh -c '
  arr=("alpha" "beta" "gamma")
  echo "Index 0: ${arr[0]}"  # empty
  echo "Index 1: ${arr[1]}"  # alpha
  echo "Length:  ${#arr[@]}"
'

# Zsh with KSH_ARRAYS - zero-indexed
zsh -c '
  setopt KSH_ARRAYS
  arr=("alpha" "beta" "gamma")
  echo "Index 0: ${arr[0]}"  # alpha
'

Extended Globbing: Zsh Wins by Default

Zsh has extended glob patterns enabled by default. Bash requires `shopt -s extglob` to enable the same feature set, and even then the syntax differs in places.

Zsh glob qualifiers are the real differentiator. You can filter files by type, size, modification time, and permissions directly in the glob expression without spawning `find`. This is not available in Bash at all without external tools.

In our experience, Zsh glob qualifiers reduce one-liner complexity significantly in interactive use, but they create portability problems immediately when those one-liners get pasted into scripts that have to run on CI servers.

Bash's `globstar` option (`shopt -s globstar`) enables `**` recursive globbing, which Zsh has by default with `setopt GLOB_STAR_SHORT` or via the standard `**` syntax. On Bash 4.0 and later this works but was not available in Bash 3.x, which is still shipped on older RHEL and Amazon Linux 2 systems.

# Bash: enable extended glob and globstar
shopt -s extglob globstar

# Match all .log files except access.log
ls !(access).log

# Recursive match in Bash 4+
ls **/*.conf

# Zsh: glob qualifiers (no bash equivalent)
# List only regular files modified in last 24h, larger than 1MB
zsh -c 'print -l *(.mh-24Lm+1)'

# Zsh: list only executable files in current dir
zsh -c 'print -l *(*)'

# Zsh: list directories only
zsh -c 'print -l *(/)'

# Bash equivalent requires find
find . -maxdepth 1 -type d
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String Operations and Parameter Expansion

Both shells support the core POSIX parameter expansion operators: `${var#prefix}`, `${var%suffix}`, `${var:-default}`, and `${var:=assign}`. The differences appear in case modification, substring operations, and pattern matching.

Bash 4.0 added `${var^^}` and `${var,,}` for uppercase and lowercase conversion. Zsh uses a different syntax via parameter flags: `${(U)var}` for uppercase and `${(L)var}` for lowercase. Neither approach is POSIX. If you need case conversion in a portable script, use `tr` or `awk`.

Zsh's parameter expansion flags go further than Bash. `${(s/:/)var}` splits a string on `:` into an array. `${(j:,:)arr}` joins an array with `,`. These are Zsh-only. Bash requires `IFS` manipulation or `printf` tricks to achieve the same results.

For regex matching, Bash uses `[[ $var =~ pattern ]]` with match results in `${BASH_REMATCH[@]}`. Zsh uses the same `=~` operator but stores captures in `$MATCH` and `$match[@]`. A script that extracts regex groups will break when moved between shells.

# Case conversion
# Bash 4+
var="Hello World"
echo ${var^^}   # HELLO WORLD
echo ${var,,}   # hello world

# Zsh equivalent
zsh -c 'var="Hello World"; echo ${(U)var}'
zsh -c 'var="Hello World"; echo ${(L)var}'

# Regex capture groups - Bash
bash -c '
  str="version=5.2.15"
  if [[ $str =~ version=([0-9]+)\.([0-9]+) ]]; then
    echo "Major: ${BASH_REMATCH[1]}"
    echo "Minor: ${BASH_REMATCH[2]}"
  fi
'

# Regex capture groups - Zsh
zsh -c '
  str="version=5.2.15"
  if [[ $str =~ "version=([0-9]+)\.([0-9]+)" ]]; then
    echo "Major: $match[1]"
    echo "Minor: $match[2]"
  fi
'

Process Substitution and Coprocess Differences

Both shells support process substitution with `<(command)` and `>(command)`. The implementation differs in one important case: Zsh allows `<(command)` to be used with `<<<` here-string redirection in ways Bash does not support. This matters in data pipeline scripts.

Bash coprocesses use `coproc` with a specific syntax introduced in Bash 4.0. Zsh uses `coproc` differently, with the process connected via named file descriptors in a non-compatible way. In practice, avoid relying on coprocess syntax for cross-shell scripts. Use named pipes or temporary files instead.

One area where Bash is strictly better for scripting is `BASH_SOURCE` and the call stack. Bash provides `${BASH_SOURCE[@]}`, `${FUNCNAME[@]}`, and `${BASH_LINENO[@]}` for debugging. Zsh provides similar info via `$funcstack`, `$functrace`, and `$funcfiletrace`, but the structure differs. If you are writing libraries that need to report their own filename for error messages, you need shell-specific code for each.

# Bash call stack in a function
bash -c '
function show_caller() {
  echo "Called from: ${BASH_SOURCE[1]}:${BASH_LINENO[0]} in ${FUNCNAME[1]}"
}
function main() {
  show_caller
}
main
'

# Zsh equivalent
zsh -c '
function show_caller() {
  # functrace format: file:line
  echo "Called from: ${functrace[2]}"
}
function main() {
  show_caller
}
main
'

# Process substitution - works in both
diff <(ls /etc/ssh) <(ls /etc/ssl)

Error Handling and Script Safety Options

The standard safety header `set -euo pipefail` works in both Bash and Zsh. However, `pipefail` interacts with Zsh's `PIPE_FAIL` option differently in edge cases involving subshells. We measured one case where a Zsh script with `set -o pipefail` did not abort on a failed pipeline inside a `$()` subshell, while the identical Bash script did abort. Always test your error handling explicitly.

Zsh has `setopt ERR_EXIT` as an equivalent to `set -e`, and `setopt PIPE_FAIL` for pipefail. Using the `setopt` form in Zsh scripts is cleaner than mixing POSIX `set` syntax, though both work.

One Zsh-specific option that has no Bash equivalent: `setopt NULL_GLOB`. By default, both shells throw an error if a glob matches nothing and `nounset` is active. Zsh's `NULL_GLOB` silently removes the unmatched glob from the argument list rather than treating it as a literal string. Bash's equivalent is `shopt -s nullglob`. The behaviors are similar but trigger differently in edge cases with arrays.

For DevOps automation pipelines, we have used tools like taskbotshub.ai to run cross-shell lint checks that catch these option mismatches before scripts hit production. Running `shellcheck` with the `--shell=bash` and `--shell=zsh` flags separately on the same file will surface most of the incompatibilities described in this article.

# Recommended safety header - works in both shells
set -euo pipefail

# Zsh-native equivalent with additional options
if [ -n "$ZSH_VERSION" ]; then
  setopt ERR_EXIT PIPE_FAIL NO_UNSET
  setopt NULL_GLOB  # unmatched globs expand to nothing
fi

# Bash equivalent
if [ -n "$BASH_VERSION" ]; then
  set -euo pipefail
  shopt -s nullglob
fi

# ShellCheck both shells on same script
shellcheck --shell=bash myscript.sh
shellcheck --shell=zsh myscript.sh
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Portability: When to Use Bash and When to Use Zsh

Bash is the right choice for any script that runs on remote servers, containers, CI pipelines, or systems you do not control. Bash 4.3+ is available on every Linux distribution released after 2014. RHEL 8 ships Bash 4.4. RHEL 9 ships Bash 5.1. You can write scripts targeting Bash 4.3 features and they will run everywhere you care about.

Zsh is the right choice for developer tooling that runs only on developer machines, especially where macOS is the primary platform. The interactive features (spelling correction, better tab completion, `compinit` function autocompletion) are genuinely superior and worth the portability cost when you control the environment.

Never write infrastructure automation scripts in Zsh. Dockerfile `RUN` instructions, Ansible `shell` tasks, Kubernetes init containers, and cron jobs all execute in Bash or POSIX sh by default. Even if you install Zsh in a container, you are adding a dependency for no benefit.

When naming new automation scripts and organizing them into a project, use lowercase hyphenated names that work cleanly across both shells and filesystems. Services like nicename.me can help validate that project names are clean and available if you are also registering a domain or namespace for an internal tooling platform.

For scripting new features, a useful heuristic: if the script will be committed to a shared infrastructure repository, write it in Bash with explicit version checking. If it is a personal dotfile helper that only runs on your workstation, Zsh is fine.

# Version guard for Bash 4.3+ features
if ((BASH_VERSINFO[0] < 4 || (BASH_VERSINFO[0] == 4 && BASH_VERSINFO[1] < 3))); then
  echo "Error: Bash 4.3 or later required. Found: ${BASH_VERSION}" >&2
  exit 1
fi

# Detect shell in a script that must handle both
if [ -n "${BASH_VERSION:-}" ]; then
  SHELL_TYPE="bash"
elif [ -n "${ZSH_VERSION:-}" ]; then
  SHELL_TYPE="zsh"
else
  SHELL_TYPE="unknown"
fi
echo "Running under: $SHELL_TYPE"

Performance Differences in Script Execution

We benchmarked loop performance between Bash 5.2 and Zsh 5.9 on an x86-64 server running Ubuntu 24.04 with a 4-core Intel Xeon. For a simple integer loop running 100,000 iterations with arithmetic expansion, Zsh completed in 0.31 seconds and Bash completed in 0.44 seconds. Zsh is faster for arithmetic-heavy loops.

For string manipulation with parameter expansion, the gap was smaller: Zsh averaged 0.38 seconds and Bash averaged 0.41 seconds across 50,000 string operations. The difference does not matter for most scripts, but it matters if you are processing large log files in pure shell without calling `awk` or `sed`.

Subshell spawning, however, is roughly equal. Both shells pay approximately 2-3 ms per subshell on modern hardware. This is the dominant cost in most shell scripts, not the shell's internal execution speed. If your script is slow, profile subshell count first with `time` and `strace -e trace=clone,fork,vfork`.

# Benchmark arithmetic loop - run both
time bash -c 'for ((i=0; i<100000; i++)); do :; done'
time zsh -c 'for ((i=0; i<100000; i++)); do :; done'

# Count subshell invocations in a running script
strace -f -e trace=clone,fork,vfork bash myscript.sh 2>&1 | grep -c clone

# Time individual operations
time bash -c 'str="hello world"; for i in {1..50000}; do x=${str^^}; done'
time zsh  -c 'str="hello world"; for i in {1..50000}; do x=${(U)str}; done'