SC2039

In POSIX sh, something is undefined.

Note: This warning has been retired in favor of individual SC3xxx warnings for each individual issue.

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.

Testing Equality

== operator is not supported in POSIX sh

Bash:

if [ "$a" == "$b" ]; then
    echo "equal"
fi

POSIX:

if [ "$a" = "$b" ]; then
    echo "equal"
fi

$"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.

${var:1} (substring expansion)

https://wiki.ubuntu.com/DashAsBinSh#A.24.7Bfoo:3.5B:1.5D.7D

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:

read aaa bbb <<< $(grep foo bar)

POSIX:

read aaa bbb << EOF
$(grep foo bar)
EOF

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 (the start [#] also works):
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'" "${__i_quote%x}"
  done
)
reuse_quote "$@"

jobs flags

The only acceptable flags under POSIX sh for jobs are -l and -p (see spec). Common flags supported by other shells are -s and -r, to check for stopped/suspended jobs and running jobs. A portable alternative is using grep or awk:

"$(jobs | awk '/(S|s)(topped|uspended)/')" # instead of jobs -s
"$(jobs | awk '/(R|r)(unning)/')"          # instead of jobs -r

Although the state of stopped jobs is Stopped in Bash and dash, and it's the one specified by POSIX, Suspended is also a valid alternative (but Zsh happens to not respect the capitalization, that's why we try to match suspended). Similarly, the state of running jobs is Running according to POSIX. Bash and dash respect this, but Zsh uses running.

Redirect both stdout and stderr

Change:

>& and &>

To:

command > file 2>&1 or command 2>&1 | othercommand

No Comments / Exceptions

Prefixing signal names with SIG

Instead of e.g.:

trap my_handler SIGTERM

use:

trap my_handler TERM
# or (`trap -l` for a list of signal numbers; not every one is portable!)
trap my_handler 15

disown

Bash:

<command>
disown %<command>

POSIX:

nohup <command>

Note that while nohup can be used to achieve the same result, their semantics is different. Also note that nohup will, by default, redirect input and output.

Errors with changed codes

Some errors, currently having their own pages, were linked to this page in older ShellCheck versions.

• SC3026: In POSIX sh, ^ in place of ! in glob bracket expressions is undefined.

Exception

Depends on what your expected POSIX shell providers would use.

Some features have POSIX proposals:

• local: https://www.austingroupbugs.net/bug_view_page.php?bug_id=767
• $'c-style-escape': see above