Portability and POSIX sh

Prerequisites: Conditionals: if and tests, Arrays

POSIX sh is the standard shell language that every Unix-like system provides. Bash implements it and adds many extensions ("bashisms"). Scripts written in plain POSIX sh run anywhere: in minimal Docker images (Alpine uses BusyBox ash), on Debian and Ubuntu (where /bin/sh is dash), on BSDs, and in embedded systems.

When portability matters

Scripts in Docker images that may not include Bash.
Installation scripts (curl ... | sh) that must run on any system.
Git hooks and system scripts shared across different machines.

For your own tools on machines where you control the software, Bash's extra features are usually worth it — just use #!/usr/bin/env bash, not #!/bin/sh, so the right shell runs.

Common bashisms and their POSIX equivalents

Bash	POSIX sh
`[[ $a == "$b" ]]`	`[ "$a" = "$b" ]`
`[[ $s == prefix* ]]`	`case $s in prefix*) ... ;; esac`
`(( n > 5 ))`	`[ "$n" -gt 5 ]`
`(( n++ ))`	`n=$((n + 1))`
arrays `arr=(a b)`	`set -- a b` and `"$@"` (one list only)
`function name { }`	`name() { }`
`echo -e "a\tb"`	`printf 'a\tb\n'`
`source file`	`. file`
`$'\n'`	a literal newline in quotes
`local var`	not in POSIX (widely supported anyway)
`<<< "string"`	`printf '%s\n' "string" \|`

A portable script

cat > count.sh <<'EOF'
#!/bin/sh
set -eu

if [ "$#" -lt 1 ]; then
  printf 'Usage: %s <dir>\n' "$0" >&2
  exit 2
fi

count=0
for f in "$1"/*; do
  [ -f "$f" ] || continue
  count=$((count + 1))
done
printf '%s contains %d file(s)\n' "$1" "$count"
EOF

mkdir -p box && touch box/a box/b
sh count.sh box

box contains 2 file(s)

Checking portability

Run the script with dash or busybox sh, which reject most bashisms.
ShellCheck checks against the shebang: with #!/bin/sh, it warns about every Bash-only feature.
checkbashisms (from Debian's devscripts) is another checker.

Other portability traps

Tools differ too, not just shells: GNU (Linux) versus BSD (macOS) versions of sed -i, date -d, grep -P, readlink -f, and stat take different options. Test on the systems you support, or install GNU tools on macOS with Homebrew's coreutils, gnu-sed, and grep packages.