Showing 178 of 178 total issues
Double quote to prevent globbing and word splitting. Open
eval shellspec_syntax_dispatch modifier ${1+'"$@"'}
- Read upRead up
- Exclude checks
Double quote to prevent globbing and word splitting.
Problematic code:
echo $1
for i in $*; do :; done # this done and the next one also applies to expanding arrays.
for i in $@; do :; done
Correct code:
echo "$1"
for i in "$@"; do :; done # or, 'for i; do'
Rationale
The first code looks like "print the first argument". It's actually "Split the first argument by IFS (spaces, tabs and line feeds). Expand each of them as if it was a glob. Join all the resulting strings and filenames with spaces. Print the result."
The second one looks like "iterate through all arguments". It's actually "join all the arguments by the first character of IFS (space), split them by IFS and expand each of them as globs, and iterate on the resulting list". The third one skips the joining part.
Quoting variables prevents word splitting and glob expansion, and prevents the script from breaking when input contains spaces, line feeds, glob characters and such.
Strictly speaking, only expansions themselves need to be quoted, but for stylistic reasons, entire arguments with multiple variable and literal parts are often quoted as one:
$HOME/$dir/dist/bin/$file # Unquoted (bad)
"$HOME"/"$dir"/dist/bin/"$file" # Minimal quoting (good)
"$HOME/$dir/dist/bin/$file" # Canonical quoting (good)
When quoting composite arguments, make sure to exclude globs and brace expansions, which lose their special meaning in double quotes: "$HOME/$dir/src/*.c"
will not expand, but "$HOME/$dir/src"/*.c
will.
Note that $( )
starts a new context, and variables in it have to be quoted independently:
echo "This $variable is quoted $(but this $variable is not)"
echo "This $variable is quoted $(and now this "$variable" is too)"
Exceptions
Sometimes you want to split on spaces, like when building a command line:
options="-j 5 -B"
make $options file
Just quoting this doesn't work. Instead, you should have used an array (bash, ksh, zsh):
options=(-j 5 -B) # ksh: set -A options -- -j 5 -B
make "${options[@]}" file
or a function (POSIX):
make_with_flags() { make -j 5 -B "$@"; }
make_with_flags file
To split on spaces but not perform glob expansion, Posix has a set -f
to disable globbing. You can disable word splitting by setting IFS=''
.
Similarly, you might want an optional argument:
debug=""
[[ $1 == "--trace-commands" ]] && debug="-x"
bash $debug script
Quoting this doesn't work, since in the default case, "$debug"
would expand to one empty argument while $debug
would expand into zero arguments. In this case, you can use an array with zero or one elements as outlined above, or you can use an unquoted expansion with an alternate value:
debug=""
[[ $1 == "--trace-commands" ]] && debug="yes"
bash ${debug:+"-x"} script
This is better than an unquoted value because the alternative value can be properly quoted, e.g. wget ${output:+ -o "$output"}
.
As always, this warning can be [[ignore]]d on a case-by-case basis.
this is especially relevant when BASH many not be available for the array work around. For example, use in eval or in command options where script has total control of the variables...
FLAGS="-av -e 'ssh -x' --delete --delete-excluded"
...
# shellcheck disable=SC2086
eval rsync $FLAGS ~/dir remote_host:dir
Notice
Original content from the ShellCheck https://github.com/koalaman/shellcheck/wiki.
In POSIX sh, read -d is undefined. Open
IFS= read -r -d "" "$1" < "$2" ||:
- Read upRead up
- Exclude checks
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.
Can't follow non-constant source. Use a directive to specify location. Open
. "$SHELLSPEC_SUPPORT_BIN"
- Read upRead up
- Exclude checks
Can't follow non-constant source. Use a directive to specify location.
Problematic code:
. "$(find_install_dir)/lib.sh"
Correct code:
# shellcheck source=src/lib.sh
. "$(find_install_dir)/lib.sh"
Rationale:
ShellCheck is not able to include sourced files from paths that are determined at runtime. The file will not be read, potentially resulting in warnings about unassigned variables and similar.
Use a [[Directive]] to point shellcheck to a fixed location it can read instead.
Exceptions:
If you don't care that ShellCheck is unable to account for the file, specify # shellcheck source=/dev/null
.
Notice
Original content from the ShellCheck https://github.com/koalaman/shellcheck/wiki.
In POSIX sh, 'shopt' is undefined. Open
[ "${SHELLSPEC_FAILGLOB_AVAILABLE:-}" ] && shopt -u failglob
- Read upRead up
- Exclude checks
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.
Can't follow non-constant source. Use a directive to specify location. Open
. "$SHELLSPEC_SUPPORT_BIN"
- Read upRead up
- Exclude checks
Can't follow non-constant source. Use a directive to specify location.
Problematic code:
. "$(find_install_dir)/lib.sh"
Correct code:
# shellcheck source=src/lib.sh
. "$(find_install_dir)/lib.sh"
Rationale:
ShellCheck is not able to include sourced files from paths that are determined at runtime. The file will not be read, potentially resulting in warnings about unassigned variables and similar.
Use a [[Directive]] to point shellcheck to a fixed location it can read instead.
Exceptions:
If you don't care that ShellCheck is unable to account for the file, specify # shellcheck source=/dev/null
.
Notice
Original content from the ShellCheck https://github.com/koalaman/shellcheck/wiki.
Double quote to prevent globbing and word splitting. Open
shellspec_clone_typeset() { eval typeset ${1+'"$@"'} 2>/dev/null; }
- Read upRead up
- Exclude checks
Double quote to prevent globbing and word splitting.
Problematic code:
echo $1
for i in $*; do :; done # this done and the next one also applies to expanding arrays.
for i in $@; do :; done
Correct code:
echo "$1"
for i in "$@"; do :; done # or, 'for i; do'
Rationale
The first code looks like "print the first argument". It's actually "Split the first argument by IFS (spaces, tabs and line feeds). Expand each of them as if it was a glob. Join all the resulting strings and filenames with spaces. Print the result."
The second one looks like "iterate through all arguments". It's actually "join all the arguments by the first character of IFS (space), split them by IFS and expand each of them as globs, and iterate on the resulting list". The third one skips the joining part.
Quoting variables prevents word splitting and glob expansion, and prevents the script from breaking when input contains spaces, line feeds, glob characters and such.
Strictly speaking, only expansions themselves need to be quoted, but for stylistic reasons, entire arguments with multiple variable and literal parts are often quoted as one:
$HOME/$dir/dist/bin/$file # Unquoted (bad)
"$HOME"/"$dir"/dist/bin/"$file" # Minimal quoting (good)
"$HOME/$dir/dist/bin/$file" # Canonical quoting (good)
When quoting composite arguments, make sure to exclude globs and brace expansions, which lose their special meaning in double quotes: "$HOME/$dir/src/*.c"
will not expand, but "$HOME/$dir/src"/*.c
will.
Note that $( )
starts a new context, and variables in it have to be quoted independently:
echo "This $variable is quoted $(but this $variable is not)"
echo "This $variable is quoted $(and now this "$variable" is too)"
Exceptions
Sometimes you want to split on spaces, like when building a command line:
options="-j 5 -B"
make $options file
Just quoting this doesn't work. Instead, you should have used an array (bash, ksh, zsh):
options=(-j 5 -B) # ksh: set -A options -- -j 5 -B
make "${options[@]}" file
or a function (POSIX):
make_with_flags() { make -j 5 -B "$@"; }
make_with_flags file
To split on spaces but not perform glob expansion, Posix has a set -f
to disable globbing. You can disable word splitting by setting IFS=''
.
Similarly, you might want an optional argument:
debug=""
[[ $1 == "--trace-commands" ]] && debug="-x"
bash $debug script
Quoting this doesn't work, since in the default case, "$debug"
would expand to one empty argument while $debug
would expand into zero arguments. In this case, you can use an array with zero or one elements as outlined above, or you can use an unquoted expansion with an alternate value:
debug=""
[[ $1 == "--trace-commands" ]] && debug="yes"
bash ${debug:+"-x"} script
This is better than an unquoted value because the alternative value can be properly quoted, e.g. wget ${output:+ -o "$output"}
.
As always, this warning can be [[ignore]]d on a case-by-case basis.
this is especially relevant when BASH many not be available for the array work around. For example, use in eval or in command options where script has total control of the variables...
FLAGS="-av -e 'ssh -x' --delete --delete-excluded"
...
# shellcheck disable=SC2086
eval rsync $FLAGS ~/dir remote_host:dir
Notice
Original content from the ShellCheck https://github.com/koalaman/shellcheck/wiki.
Can't follow non-constant source. Use a directive to specify location. Open
. "$SHELLSPEC_SUPPORT_BIN"
- Read upRead up
- Exclude checks
Can't follow non-constant source. Use a directive to specify location.
Problematic code:
. "$(find_install_dir)/lib.sh"
Correct code:
# shellcheck source=src/lib.sh
. "$(find_install_dir)/lib.sh"
Rationale:
ShellCheck is not able to include sourced files from paths that are determined at runtime. The file will not be read, potentially resulting in warnings about unassigned variables and similar.
Use a [[Directive]] to point shellcheck to a fixed location it can read instead.
Exceptions:
If you don't care that ShellCheck is unable to account for the file, specify # shellcheck source=/dev/null
.
Notice
Original content from the ShellCheck https://github.com/koalaman/shellcheck/wiki.
In POSIX sh, 'shopt' is undefined. Open
-o) shopt -s "$2" 2>/dev/null || set -o "$2" ;;
- Read upRead up
- Exclude checks
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.
Can't follow non-constant source. Use a directive to specify location. Open
. "$SHELLSPEC_SUPPORT_BIN"
- Read upRead up
- Exclude checks
Can't follow non-constant source. Use a directive to specify location.
Problematic code:
. "$(find_install_dir)/lib.sh"
Correct code:
# shellcheck source=src/lib.sh
. "$(find_install_dir)/lib.sh"
Rationale:
ShellCheck is not able to include sourced files from paths that are determined at runtime. The file will not be read, potentially resulting in warnings about unassigned variables and similar.
Use a [[Directive]] to point shellcheck to a fixed location it can read instead.
Exceptions:
If you don't care that ShellCheck is unable to account for the file, specify # shellcheck source=/dev/null
.
Notice
Original content from the ShellCheck https://github.com/koalaman/shellcheck/wiki.
Can't follow non-constant source. Use a directive to specify location. Open
. "$SHELLSPEC_SUPPORT_BIN"
- Read upRead up
- Exclude checks
Can't follow non-constant source. Use a directive to specify location.
Problematic code:
. "$(find_install_dir)/lib.sh"
Correct code:
# shellcheck source=src/lib.sh
. "$(find_install_dir)/lib.sh"
Rationale:
ShellCheck is not able to include sourced files from paths that are determined at runtime. The file will not be read, potentially resulting in warnings about unassigned variables and similar.
Use a [[Directive]] to point shellcheck to a fixed location it can read instead.
Exceptions:
If you don't care that ShellCheck is unable to account for the file, specify # shellcheck source=/dev/null
.
Notice
Original content from the ShellCheck https://github.com/koalaman/shellcheck/wiki.
Can't follow non-constant source. Use a directive to specify location. Open
. "$SHELLSPEC_SUPPORT_BIN"
- Read upRead up
- Exclude checks
Can't follow non-constant source. Use a directive to specify location.
Problematic code:
. "$(find_install_dir)/lib.sh"
Correct code:
# shellcheck source=src/lib.sh
. "$(find_install_dir)/lib.sh"
Rationale:
ShellCheck is not able to include sourced files from paths that are determined at runtime. The file will not be read, potentially resulting in warnings about unassigned variables and similar.
Use a [[Directive]] to point shellcheck to a fixed location it can read instead.
Exceptions:
If you don't care that ShellCheck is unable to account for the file, specify # shellcheck source=/dev/null
.
Notice
Original content from the ShellCheck https://github.com/koalaman/shellcheck/wiki.
Quote this to prevent word splitting. Open
shellmetrics $(sources)
- Read upRead up
- Exclude checks
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.
Quote this to prevent word splitting. Open
count helper $(helpers)
- Read upRead up
- Exclude checks
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 "${args[@]}" client.c
}
compile -DDEBUG
+ gcc -lssl -lcrypto -DDEBUG client.c
Notice
Original content from the ShellCheck https://github.com/koalaman/shellcheck/wiki.
Shell functions can't be passed to external commands. Open
find examples -name '*.sh'
- Read upRead up
- Exclude checks
Shell functions can't be passed to external commands.
Problematic code:
foo() { bar --baz "$@"; frob --baz "$@"; };
find . -exec foo {} +
Correct code:
find . -exec sh -c 'bar --baz "$@"; frob --baz "$@";' -- {} +
Rationale:
Shell functions are only known to the shell. External commands like find
, xargs
, su
and sudo
do not recognize shell functions.
Instead, the function contents can be executed in a shell, either through sh -c
or by creating a separate shell script as an executable file.
Exceptions
If you're intentionally passing a word that happens to have the same name as a declared function, you can quote it to make shellcheck ignore it, e.g.
nobody() {
sudo -u "nobody" "$@"
}
Notice
Original content from the ShellCheck https://github.com/koalaman/shellcheck/wiki.
In POSIX sh, 'type' is undefined. Open
if type "$cmd" >/dev/null 2>&1; then
- Read upRead up
- Exclude checks
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.
In POSIX sh, 'typeset' is undefined. Open
typeset +x PATH
- Read upRead up
- Exclude checks
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.
In POSIX sh, += is undefined. Open
if string+="concat" 2>/dev/null; then
- Read upRead up
- Exclude checks
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.
Double quote to prevent globbing and word splitting. Open
eval trans control ${1+'"$@"'}
- Read upRead up
- Exclude checks
Double quote to prevent globbing and word splitting.
Problematic code:
echo $1
for i in $*; do :; done # this done and the next one also applies to expanding arrays.
for i in $@; do :; done
Correct code:
echo "$1"
for i in "$@"; do :; done # or, 'for i; do'
Rationale
The first code looks like "print the first argument". It's actually "Split the first argument by IFS (spaces, tabs and line feeds). Expand each of them as if it was a glob. Join all the resulting strings and filenames with spaces. Print the result."
The second one looks like "iterate through all arguments". It's actually "join all the arguments by the first character of IFS (space), split them by IFS and expand each of them as globs, and iterate on the resulting list". The third one skips the joining part.
Quoting variables prevents word splitting and glob expansion, and prevents the script from breaking when input contains spaces, line feeds, glob characters and such.
Strictly speaking, only expansions themselves need to be quoted, but for stylistic reasons, entire arguments with multiple variable and literal parts are often quoted as one:
$HOME/$dir/dist/bin/$file # Unquoted (bad)
"$HOME"/"$dir"/dist/bin/"$file" # Minimal quoting (good)
"$HOME/$dir/dist/bin/$file" # Canonical quoting (good)
When quoting composite arguments, make sure to exclude globs and brace expansions, which lose their special meaning in double quotes: "$HOME/$dir/src/*.c"
will not expand, but "$HOME/$dir/src"/*.c
will.
Note that $( )
starts a new context, and variables in it have to be quoted independently:
echo "This $variable is quoted $(but this $variable is not)"
echo "This $variable is quoted $(and now this "$variable" is too)"
Exceptions
Sometimes you want to split on spaces, like when building a command line:
options="-j 5 -B"
make $options file
Just quoting this doesn't work. Instead, you should have used an array (bash, ksh, zsh):
options=(-j 5 -B) # ksh: set -A options -- -j 5 -B
make "${options[@]}" file
or a function (POSIX):
make_with_flags() { make -j 5 -B "$@"; }
make_with_flags file
To split on spaces but not perform glob expansion, Posix has a set -f
to disable globbing. You can disable word splitting by setting IFS=''
.
Similarly, you might want an optional argument:
debug=""
[[ $1 == "--trace-commands" ]] && debug="-x"
bash $debug script
Quoting this doesn't work, since in the default case, "$debug"
would expand to one empty argument while $debug
would expand into zero arguments. In this case, you can use an array with zero or one elements as outlined above, or you can use an unquoted expansion with an alternate value:
debug=""
[[ $1 == "--trace-commands" ]] && debug="yes"
bash ${debug:+"-x"} script
This is better than an unquoted value because the alternative value can be properly quoted, e.g. wget ${output:+ -o "$output"}
.
As always, this warning can be [[ignore]]d on a case-by-case basis.
this is especially relevant when BASH many not be available for the array work around. For example, use in eval or in command options where script has total control of the variables...
FLAGS="-av -e 'ssh -x' --delete --delete-excluded"
...
# shellcheck disable=SC2086
eval rsync $FLAGS ~/dir remote_host:dir
Notice
Original content from the ShellCheck https://github.com/koalaman/shellcheck/wiki.
Double quote to prevent globbing and word splitting. Open
eval shellspec_syntax_dispatch verb should_not ${1+'"$@"'}
- Read upRead up
- Exclude checks
Double quote to prevent globbing and word splitting.
Problematic code:
echo $1
for i in $*; do :; done # this done and the next one also applies to expanding arrays.
for i in $@; do :; done
Correct code:
echo "$1"
for i in "$@"; do :; done # or, 'for i; do'
Rationale
The first code looks like "print the first argument". It's actually "Split the first argument by IFS (spaces, tabs and line feeds). Expand each of them as if it was a glob. Join all the resulting strings and filenames with spaces. Print the result."
The second one looks like "iterate through all arguments". It's actually "join all the arguments by the first character of IFS (space), split them by IFS and expand each of them as globs, and iterate on the resulting list". The third one skips the joining part.
Quoting variables prevents word splitting and glob expansion, and prevents the script from breaking when input contains spaces, line feeds, glob characters and such.
Strictly speaking, only expansions themselves need to be quoted, but for stylistic reasons, entire arguments with multiple variable and literal parts are often quoted as one:
$HOME/$dir/dist/bin/$file # Unquoted (bad)
"$HOME"/"$dir"/dist/bin/"$file" # Minimal quoting (good)
"$HOME/$dir/dist/bin/$file" # Canonical quoting (good)
When quoting composite arguments, make sure to exclude globs and brace expansions, which lose their special meaning in double quotes: "$HOME/$dir/src/*.c"
will not expand, but "$HOME/$dir/src"/*.c
will.
Note that $( )
starts a new context, and variables in it have to be quoted independently:
echo "This $variable is quoted $(but this $variable is not)"
echo "This $variable is quoted $(and now this "$variable" is too)"
Exceptions
Sometimes you want to split on spaces, like when building a command line:
options="-j 5 -B"
make $options file
Just quoting this doesn't work. Instead, you should have used an array (bash, ksh, zsh):
options=(-j 5 -B) # ksh: set -A options -- -j 5 -B
make "${options[@]}" file
or a function (POSIX):
make_with_flags() { make -j 5 -B "$@"; }
make_with_flags file
To split on spaces but not perform glob expansion, Posix has a set -f
to disable globbing. You can disable word splitting by setting IFS=''
.
Similarly, you might want an optional argument:
debug=""
[[ $1 == "--trace-commands" ]] && debug="-x"
bash $debug script
Quoting this doesn't work, since in the default case, "$debug"
would expand to one empty argument while $debug
would expand into zero arguments. In this case, you can use an array with zero or one elements as outlined above, or you can use an unquoted expansion with an alternate value:
debug=""
[[ $1 == "--trace-commands" ]] && debug="yes"
bash ${debug:+"-x"} script
This is better than an unquoted value because the alternative value can be properly quoted, e.g. wget ${output:+ -o "$output"}
.
As always, this warning can be [[ignore]]d on a case-by-case basis.
this is especially relevant when BASH many not be available for the array work around. For example, use in eval or in command options where script has total control of the variables...
FLAGS="-av -e 'ssh -x' --delete --delete-excluded"
...
# shellcheck disable=SC2086
eval rsync $FLAGS ~/dir remote_host:dir
Notice
Original content from the ShellCheck https://github.com/koalaman/shellcheck/wiki.
Double quote to prevent globbing and word splitting. Open
eval shellspec_syntax_dispatch modifier ${1+'"$@"'}
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Double quote to prevent globbing and word splitting.
Problematic code:
echo $1
for i in $*; do :; done # this done and the next one also applies to expanding arrays.
for i in $@; do :; done
Correct code:
echo "$1"
for i in "$@"; do :; done # or, 'for i; do'
Rationale
The first code looks like "print the first argument". It's actually "Split the first argument by IFS (spaces, tabs and line feeds). Expand each of them as if it was a glob. Join all the resulting strings and filenames with spaces. Print the result."
The second one looks like "iterate through all arguments". It's actually "join all the arguments by the first character of IFS (space), split them by IFS and expand each of them as globs, and iterate on the resulting list". The third one skips the joining part.
Quoting variables prevents word splitting and glob expansion, and prevents the script from breaking when input contains spaces, line feeds, glob characters and such.
Strictly speaking, only expansions themselves need to be quoted, but for stylistic reasons, entire arguments with multiple variable and literal parts are often quoted as one:
$HOME/$dir/dist/bin/$file # Unquoted (bad)
"$HOME"/"$dir"/dist/bin/"$file" # Minimal quoting (good)
"$HOME/$dir/dist/bin/$file" # Canonical quoting (good)
When quoting composite arguments, make sure to exclude globs and brace expansions, which lose their special meaning in double quotes: "$HOME/$dir/src/*.c"
will not expand, but "$HOME/$dir/src"/*.c
will.
Note that $( )
starts a new context, and variables in it have to be quoted independently:
echo "This $variable is quoted $(but this $variable is not)"
echo "This $variable is quoted $(and now this "$variable" is too)"
Exceptions
Sometimes you want to split on spaces, like when building a command line:
options="-j 5 -B"
make $options file
Just quoting this doesn't work. Instead, you should have used an array (bash, ksh, zsh):
options=(-j 5 -B) # ksh: set -A options -- -j 5 -B
make "${options[@]}" file
or a function (POSIX):
make_with_flags() { make -j 5 -B "$@"; }
make_with_flags file
To split on spaces but not perform glob expansion, Posix has a set -f
to disable globbing. You can disable word splitting by setting IFS=''
.
Similarly, you might want an optional argument:
debug=""
[[ $1 == "--trace-commands" ]] && debug="-x"
bash $debug script
Quoting this doesn't work, since in the default case, "$debug"
would expand to one empty argument while $debug
would expand into zero arguments. In this case, you can use an array with zero or one elements as outlined above, or you can use an unquoted expansion with an alternate value:
debug=""
[[ $1 == "--trace-commands" ]] && debug="yes"
bash ${debug:+"-x"} script
This is better than an unquoted value because the alternative value can be properly quoted, e.g. wget ${output:+ -o "$output"}
.
As always, this warning can be [[ignore]]d on a case-by-case basis.
this is especially relevant when BASH many not be available for the array work around. For example, use in eval or in command options where script has total control of the variables...
FLAGS="-av -e 'ssh -x' --delete --delete-excluded"
...
# shellcheck disable=SC2086
eval rsync $FLAGS ~/dir remote_host:dir
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Original content from the ShellCheck https://github.com/koalaman/shellcheck/wiki.