Almquist Shell (ash / /bin/sh) — Fleet Reference

1. History and Lineage

Kenneth Almquist wrote ash in the late 1980s as a clean reimplementation of the UNIX System V shell. The first public release was posted to Usenet on 30 May 1989. Almquist's stated goals: fast, small, and POSIX-compatible — original binary was 92 KB.

BSD adoption timeline:

Variants in active use:

• FreeBSD sh — extended ash in FreeBSD base, supports pipefail, $'...' quoting
• NetBSD sh — ash in NetBSD base, strictly POSIX-oriented
• dash — Debian/Ubuntu /bin/sh, smallest and strictest
• busybox sh — embedded systems; limited subset

2. License

FreeBSD sh (and the original ash) is licensed under the 3-clause BSD license. This is explicitly compatible with the project's MIT/BSD/Apache-2.0 policy.

Contrast with bash, which is GPL-3.0 — copyleft, incompatible with project license policy, and therefore not allowed in fleet base scripts.

3. Where It Runs in Our Fleet

TinyBSD v1 defaults contract: shell = /bin/sh, no bash, no zsh, no additional shell packages. Scripts for TinyBSD MUST be pure POSIX sh with no bashisms.

Ansible remote execution against these hosts uses raw module or ansible_shell_executable: /bin/sh. Never set ansible_shell_executable: /bin/bash on BSD fleet nodes — bash requires ports/pkg install.

4. Key Differences from bash — What DOES NOT Work

These constructs are bash-specific and will fail silently or fatally on ash:

Arrays

# BASH ONLY — will fail on ash:
arr=(one two three)
echo "${arr[1]}"

# ASH portable alternatives:
set -- one two three          # use positional parameters
echo "$2"                     # second element

# or IFS trick to iterate:
items="one two three"
for item in $items; do echo "$item"; done

[[ ]] double-bracket conditionals

# BASH ONLY:
if [[ "$x" == foo* ]]; then ...

# ASH — use [ ] or case:
if [ "$x" = "foo" ]; then ...
case "$x" in foo*) ...; esac

$RANDOM

# BASH ONLY:
n=$RANDOM

# ASH portable:
n=$(od -An -N2 -tu2 /dev/urandom | tr -d ' ')
# or: awk 'BEGIN{srand(); print int(rand()*32768)}'

$PIPESTATUS

# BASH ONLY:
cmd1 | cmd2
echo "${PIPESTATUS[0]}"  # exit status of cmd1

# ASH with pipefail (FreeBSD sh only):
set -o pipefail
cmd1 | cmd2    # pipeline fails if any stage fails

# Fully portable: use temp files or process substitution workaround

Process substitution

# BASH ONLY:
diff <(cmd1) <(cmd2)

# ASH portable:
cmd1 > /tmp/a.$$ && cmd2 > /tmp/b.$$
diff /tmp/a.$$ /tmp/b.$$
rm -f /tmp/a.$$ /tmp/b.$$

Here-strings

# BASH ONLY:
read var <<< "value"

# ASH portable:
var=$(printf '%s' "value")
# or:
read var << 'EOF'
value
EOF

Brace expansion

# BASH ONLY:
for i in {1..10}; do ...

# ASH portable:
i=1; while [ "$i" -le 10 ]; do
  echo "$i"
  i=$((i + 1))
done

function keyword

# BASH ONLY:
function myfunc() { echo hi; }

# ASH — omit keyword, parentheses only:
myfunc() { echo hi; }

echo -e behavior

# Inconsistent across ash variants — use printf instead:
printf 'line1\nline2\n'
printf '%s\n' "$var"

References

• references/posix-features.md — POSIX sh features that work (sections 5–6)
• references/fleet-patterns.md — fleet script patterns + why not bash (sections 7–8)

9. Quick Reference Card

DOES WORK (ash/POSIX sh)        DOES NOT WORK (bash-only)
─────────────────────────────   ──────────────────────────
[ ] test                        [[ ]] conditional
$()  command substitution       <() process substitution
$(( )) arithmetic               <<< here-string
${var##pat} expansion           ${arr[n]} array index
for/while/until/case            {1..10} brace expansion
name() { } functions            function name() {}
trap ... EXIT/INT/TERM          $RANDOM
set -eu pipefail (FreeBSD)      $PIPESTATUS
local inside functions          declare -a/-A arrays
printf (preferred over echo)    echo -e (unreliable)
getopts                         bash-specific getopts extensions
IFS tricks for splitting        mapfile / readarray

10. Useful External Tools (Available on BSD Base)

See Also

• freebsd skill — FreeBSD host management, pkg, rc, pf
• netbsd skill — NetBSD host management, pkgsrc, rc
• ansible-fleet skill — fleet-wide script deployment
• unicode-glyphs skill — BMP/SMP token selection for filenames