if [ -z "$BOSH_CA_CERT" -a -z "$BOSH_CERT_FILE" ]

Close

Prefer [ p ] && [ q ] as [ p -a q ] is not well defined.

And likewise, prefer [ p ] || [ q ] over [ p -o q ].

Problematic code:

[ "$1" = "test" -a -z "$2" ]

Correct code:

[ "$1" = "test" ] && [ -z "$2" ]

Rationale:

-a and -o to mean AND and OR in a [ .. ] test expression is not well defined. They are obsolescent extensions in POSIX and the lack of additional quoting levels means that expressions involving these operators are often ambiguous. Values that begin with hyphens and the ! string are the usual issues.

Using multiple [ .. ] expressions with shell AND/OR operators && and || is well defined and therefore preferred (but note that they have equal precedence, while -a/-o is unspecified but usually implemented as -a having higher precedence).

Exceptions:

If the shell variant being used is ksh derived (such as the bash shell) it will have the shell builtin command [[ ... ]]. This has the operators &&, ||, (, ), ! which safely avoid the ambiguity by noting which arguments were quoted and requiring the operators to be unquoted (except by the [[ ... ]] construct itself).

Notice

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

my_realpath() { echo $(cd $(dirname $1); pwd)/$(basename $1); }

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.

    export BOSH_CA_CERT=$(my_realpath $BOSH_CA_CERT)

Close

Declare and assign separately to avoid masking return values.

Problematic code:

export foo="$(mycmd)"

Correct code:

foo=$(mycmd)
export foo

Rationale:

In the original code, the return value of mycmd is ignored, and export will instead always return true. This may prevent conditionals, set -e and traps from working correctly.

When first marked for export and assigned separately, the return value of the assignment will be that of mycmd. This avoids the problem.

Exceptions:

If you intend to ignore the return value of an assignment, you can either ignore this warning or use

foo=$(mycmd) || true
export foo

Shellcheck does not warn about export foo=bar because bar is a literal and not a command substitution with an independent return value. It also does not warn about local -r foo=$(cmd), where declaration and assignment must be in the same command.

Notice

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

DIRECTOR_IP_NO_SPACE=$(echo $DIRECTOR_IP |tr -d [:blank:])

Close

Quote parameters to tr to prevent glob expansion.

Problematic code:

tr -cd [:digit:]

Correct code:

tr -cd '[:digit:]'

Rationale:

[:digit:] is a shell glob that matches any single character file named e.g. g or t in the current directory. Quoting it prevents the script from breaking in directories with files like this.

Exceptions

None

Notice

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

my_realpath() { echo $(cd $(dirname $1); pwd)/$(basename $1); }

Close

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.

    export BOSH_CA_CERT=$(my_realpath $BOSH_CA_CERT)

Close

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.

my_realpath() { echo $(cd $(dirname $1); pwd)/$(basename $1); }

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.

bosh log-in $CA_CERT_FILE

Close

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.

DIRECTOR_IP_NO_SPACE=$(echo $DIRECTOR_IP |tr -d [:blank:])

Close

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.

my_realpath() { echo $(cd $(dirname $1); pwd)/$(basename $1); }

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.

my_realpath() { echo $(cd $(dirname $1); pwd)/$(basename $1); }

Close

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.

    ERROR_COUNT=$(($ERROR_COUNT+1))

Close

$/${} is unnecessary on arithmetic variables.

Problematic code:

echo $(($n+1))

Correct code:

echo $((n+1))

Rationale:

The $ on regular variables in arithmetic contexts is unnecessary, and can even lead to subtle bugs. This is because the contents of $((..)) is first expanded into a string, and then evaluated as an expression:

$ a='1+1'
$ echo $(($a * 5))    # becomes 1+1*5
6
$ echo $((a * 5))     # evaluates as (1+1)*5
10

The $ is unavoidable for special variables like $1 vs 1, $# vs #. It's also required when adding modifiers to parameters expansions, like ${#var} or ${var%-}. ShellCheck does not warn about these cases.

Notice

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

    ERROR_COUNT=$(($ERROR_COUNT+1))

Close

$/${} is unnecessary on arithmetic variables.

Problematic code:

echo $(($n+1))

Correct code:

echo $((n+1))

Rationale:

The $ on regular variables in arithmetic contexts is unnecessary, and can even lead to subtle bugs. This is because the contents of $((..)) is first expanded into a string, and then evaluated as an expression:

$ a='1+1'
$ echo $(($a * 5))    # becomes 1+1*5
6
$ echo $((a * 5))     # evaluates as (1+1)*5
10

The $ is unavoidable for special variables like $1 vs 1, $# vs #. It's also required when adding modifiers to parameters expansions, like ${#var} or ${var%-}. ShellCheck does not warn about these cases.

Notice

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

    ERROR_COUNT=$(($ERROR_COUNT+1))

Close

$/${} is unnecessary on arithmetic variables.

Problematic code:

echo $(($n+1))

Correct code:

echo $((n+1))

Rationale:

The $ on regular variables in arithmetic contexts is unnecessary, and can even lead to subtle bugs. This is because the contents of $((..)) is first expanded into a string, and then evaluated as an expression:

$ a='1+1'
$ echo $(($a * 5))    # becomes 1+1*5
6
$ echo $((a * 5))     # evaluates as (1+1)*5
10

The $ is unavoidable for special variables like $1 vs 1, $# vs #. It's also required when adding modifiers to parameters expansions, like ${#var} or ${var%-}. ShellCheck does not warn about these cases.

Notice

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