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.

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.

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.

Close

Not following: (error message here)

Reasons include: file not found, no permissions, not included on the command line, not allowing shellcheck to follow files with -x, etc.

Problematic code:

source somefile

Correct code:

# shellcheck disable=SC1091
source somefile

Rationale:

ShellCheck, for whichever reason, is not able to access the source file.

This could be because you did not include it on the command line, did not use shellcheck -x to allow following other files, don't have permissions or a variety of other problems.

Feel free to ignore the error with a [[directive]].

Exceptions:

If you're fine with it, ignore the message with a [[directive]].

Notice

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

Close

Quote this to prevent word splitting

Problematic code:

ls -l $(getfilename)

Correct code:

# getfilename outputs 1 file
ls -l "$(getfilename)"
 
# getfilename outputs multiple files, linefeed separated
getfilename | while IFS='' read -r line
do
  ls -l "$line"
done

Rationale:

When command expansions are unquoted, word splitting and globbing will occur. This often manifests itself by breaking when filenames contain spaces.

Trying to fix it by adding quotes or escapes to the data will not work. Instead, quote the command substitution itself.

If the command substitution outputs multiple pieces of data, use a loop instead.

Exceptions

In rare cases you actually want word splitting, such as in

gcc $(pkg-config --libs openssl) client.c

This is because pkg-config outputs -lssl -lcrypto, which you want to break up by spaces into -lssl and -lcrypto. An alternative is to put the variables to an array and expand it:

args=( $(pkg-config --libs openssl) )
gcc "${args[@]}" client.c

The power of using an array becomes evident when you want to combine, for example, the command result with user-provided arguments:

compile () {
    args=( $(pkg-config --libs openssl) "${@}" )
    gcc "${args[@]}" client.c
}
compile -DDEBUG
+ gcc -lssl -lcrypto -DDEBUG client.c

Notice

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

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.

Close

Use $(STATEMENT) instead of legacy `STATEMENT`

Problematic code

echo "Current time: `date`"

Correct code

echo "Current time: $(date)"

Rationale

Backtick command substitution `STATEMENT` is legacy syntax with several issues.

1. It has a series of undefined behaviors related to quoting in POSIX.
2. It imposes a custom escaping mode with surprising results.
3. It's exceptionally hard to nest.

$(STATEMENT) command substitution has none of these problems, and is therefore strongly encouraged.

Exceptions

None.

See also

• http://mywiki.wooledge.org/BashFAQ/082

Notice

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