git_remote_tags "${PRE:+--pre}"

Close

var was modified in a subshell. That change might be lost.

Problematic code:

There are many ways of accidentally creating subshells, but a common one is piping to a loop:

n=0
printf "%s\n" {1..10} | while read i; do (( n+=i )); done
echo $n

Correct code:

# Bash specific: process substitution. Also try shopts like lastpipe.
n=0
while read i; do (( n+=i )); done < <(printf "%s\n" {1..10})
echo $n

In sh, a temp file (better if fifo or fd) can be used instead of process substitution. And if it's acceptable to do it with waiting, try Here Documents.

Rationale:

Variables set in subshells are not available outside the subshell. This is a wide topic, and better described on the Wooledge Bash Wiki.

Here are some constructs that cause subshells (shellcheck may not warn about all of them). In each case, you can replace subshell1 by a command or function that sets a variable, e.g. simply var=foo, and the variable will appear to be unset after the command is run. Similarly, you can replace regular with var=foo, and it will be set afterwards:

Pipelines:

subshell1 | subshell2 | subshell3    # Bash, Dash, Ash
subshell1 | subshell2 | regular      # Ksh, Zsh

Command substitution:

regular "$(subshell1)" "`subshell2`"

Process substitution:

regular <(subshell1) >(subshell2)

Some forms of grouping:

( subshell )
{ regular; }

Backgrounding:

subshell1 &
subshell2 &

Anything executed by external processes:

find . -exec subshell1 {} \;
find . -print0 | xargs -0 subshell2
sudo subshell3
su -c subshell4

This applies not only to setting variables, but also setting shell options and changing directories.

Exceptions

You can ignore this error if you don't care that the changes aren't reflected, because work on the value branches and shouldn't be recombined.

Notice

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

  type "$1" >/dev/null 2>&1 && return 0

Close

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.

    tag=${line##*/} && pre=${tag#${tag%%[-+]*}}

Close

Modification of var is local (to subshell caused by pipeline).

See companion warning [[SC2031]].

Notice

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