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In POSIX sh, something is undefined.

You have declared that your script works with /bin/sh, but you are using features that have undefined behavior according to the POSIX specification.

It may currently work for you, but it can or will fail on other OS, the same OS with different configurations, from different contexts (like initramfs/chroot), or in different versions of the same OS, including future updates to your current system.

Either declare that your script requires a specific shell like #!/bin/bash or #!/bin/dash, or rewrite the script in a portable way.

For help with rewrites, the Ubuntu wiki has a list of portability issues that broke people's #!/bin/sh scripts when Ubuntu switched from Bash to Dash. See also Bashism on wooledge's wiki. ShellCheck may not warn about all these issues.

$'c-style-escapes'

bash, ksh:

a=$' \t\n'

POSIX:

a="$(printf '%b_' ' \t\n')"; a="${a%_}" # protect trailing \n

Want some good news? See http://austingroupbugs.net/view.php?id=249#c590.

$"msgid"

Bash:

echo $"foo $(bar) baz"

POSIX:

. gettext.sh # GNU Gettext sh library
# ...
barout=$(bar)
eval_gettext 'foo $barout baz' # See GNU Gettext doc for more info.

Or you can change them to normal double quotes so you go without gettext.

Arithmetic for loops

Bash:

for ((init; test; next)); do foo; done

POSIX:

: $((init))
while [ $((test)) -ne 0 ]; do foo; : $((next)); done

Arithmetic exponentiation

Bash:

printf "%s\n" "$(( 2**63 ))"

POSIX:

The POSIX standard does not allow for exponents. However, you can replicate them completely built-in using a POSIX compatible function. As an example, the pow function from here.

pow () {
    set "$1" "$2" 1
    while [ "$2" -gt 0 ]; do
      set "$1" $(($2-1)) $(($1*$3))
    done
    # %d = signed decimal, %u = unsigned decimal
    # Either should overflow to 0
    printf "%d\n" "$3"
}

To compare:

$ echo "$(( 2**62 ))"
4611686018427387904
$ pow 2 62
4611686018427387904

Alternatively, if you don't mind using an external program, you can use bc. Be aware though: bash and other programs may abide by a certain maximum integer that bc does not (for bash that's: 64-bit signed long int, failing back to 32-bit signed long int).

Example:

# Note the overflow that gives a negative number
$ echo "$(( 2**63 ))"
-9223372036854775808

# No such problem
$ echo 2^63 | bc
9223372036854775808

# 'bc' just keeps on going
$ echo 2^1280 | bc
20815864389328798163850480654728171077230524494533409610638224700807\
21611934672059602447888346464836968484322790856201558276713249664692\
98162798132113546415258482590187784406915463666993231671009459188410\
95379622423387354295096957733925002768876520583464697770622321657076\
83317005651120933244966378183760369413644440628104205339687097746591\
6057756101739472373801429441421111406337458176

standalone ((..))

Bash:

((a=c+d))
((d)) && echo d is true.

POSIX:

: $((a=c+d)) # discard the output of the arith expn with `:` command
[ $((d)) -ne 0 ] && echo d is true. # manually check non-zero => true

select loops

It takes extra care over terminal columns to make select loop look like bash's, which generates a list with multiple items on one line, or like ls.

It is, however, still possible to make a naive translation for select foo in bar baz; do eat; done:

while
  _i=0 _foo= foo=
  for _name in bar baz; do echo "$((_i+=1))) $_name"; done
  printf '$# '; read _foo
do
  case _foo in 1) foo=bar;; 2) foo=baz;; *) continue;; esac
  eat
done

Here-strings

Bash, ksh:

grep aaa <<< "$g"

POSIX:

# not exactly the same -- <<< adds a trailing \n if $g doesn't end with \n
printf '%s' "$g" | grep aaa

echo flags

See https://unix.stackexchange.com/tags/echo/info.

${var/pat/replacement}

Bash:

echo "${TERM/%-256*}"

POSIX:

echo "$TERM" | sed -e 's/-256.*$//g'
# Special case for this since we are matching the end:
echo "${TERM%-256*}"

printf %q

Bash:

printf '%q ' "$@"

POSIX:

# TODO: Interpret it back to printf escapes for hard-to-copy chars like \t?
# See also: http://git.savannah.gnu.org/cgit/libtool.git/tree/gl/build-aux/funclib.sh?id=c60e054#n1029
reuse_quote()(
  for i; do
    __i_quote=$(printf '%s\n' "$i" | sed -e "s/'/'\\\\''/g"; echo x)
    printf "'%s'" "${__i_quote%x}"
  done
)
reuse_quote "$@"

Exception

Depends on what your expected POSIX shell providers would use.

Notice

Original content from the ShellCheck https://github.com/koalaman/shellcheck/wiki.