pip3 $pip_options install --upgrade pipsource $BASE_DIR/$VENV_DIR_NSTAT/bin/activatepip3 $pip_options install -r $BASE_DIR/requirements.txtecho $1for i in $*; do :; done # this done and the next one also applies to expanding arrays.for i in $@; do :; doneecho "$1"for i in "$@"; do :; done # or, 'for i; do'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)"Sometimes you want to split on spaces, like when building a command line:
options="-j 5 -B"make $options fileJust quoting this doesn't work. Instead, you should have used an array (bash, ksh, zsh):
options=(-j 5 -B) # ksh: set -A options -- -j 5 -Bmake "${options[@]}" fileor a function (POSIX):
make_with_flags() { make -j 5 -B "$@"; }make_with_flags fileTo 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 $debug scriptQuoting this doesn't work, since in the default case, "$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"} scriptThis 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=SC2086eval rsync $FLAGS ~/dir remote_host:dirOriginal content from the ShellCheck https://github.com/koalaman/shellcheck/wiki.
. "$(find_install_dir)/lib.sh"# shellcheck source=src/lib.sh. "$(find_install_dir)/lib.sh"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.
If you don't care that ShellCheck is unable to account for the file, specify # shellcheck source=/dev/null.
Original content from the ShellCheck https://github.com/koalaman/shellcheck/wiki.
echo $1for i in $*; do :; done # this done and the next one also applies to expanding arrays.for i in $@; do :; doneecho "$1"for i in "$@"; do :; done # or, 'for i; do'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)"Sometimes you want to split on spaces, like when building a command line:
options="-j 5 -B"make $options fileJust quoting this doesn't work. Instead, you should have used an array (bash, ksh, zsh):
options=(-j 5 -B) # ksh: set -A options -- -j 5 -Bmake "${options[@]}" fileor a function (POSIX):
make_with_flags() { make -j 5 -B "$@"; }make_with_flags fileTo 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 $debug scriptQuoting this doesn't work, since in the default case, "$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"} scriptThis 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=SC2086eval rsync $FLAGS ~/dir remote_host:dirOriginal content from the ShellCheck https://github.com/koalaman/shellcheck/wiki.