## Always use double quotes around variable substitutions and command substitutions: `"$foo"`, `"$(foo)"`
If you use `$foo` unquoted, your script will choke on input or parameters (or command output, with `$(foo)`) containing whitespace or `\[*?`.
There, you can stop reading. Well, ok, here are a few more:
* `read` — **To read input line by line with the `read` builtin, use `while IFS= read -r line; do …`**
Plain `read` treats backslashes and whitespace specially.
* `xargs` — **Avoid `xargs`**. If you must use `xargs`, make that `xargs -0`. Instead of `find … | xargs`, **prefer `find … -exec …`**.
`xargs` treats whitespace and the characters `\"'` specially.
This answer applies to Bourne/POSIX-style shells (`sh`, `ash`, `dash`, `bash`, `ksh`, `mksh`, …). Zsh users should skip it and read the end of http://unix.stackexchange.com/questions/68694/when-is-double-quoting-necessary/68748#68748 instead. If you want the whole nitty-gritty, [read the standard](http://pubs.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html#tag_02_06) or your shell's manual.
----
Note that the explanations below contains a few approximations (statements that are true in most conditions but can be affected by the surrounding context or by configuration).
## Why do I need to write `"$foo"`? What happens without the quotes?
`$foo` does not mean “take the value of the variable `foo`”. It means something much more complex:
* First, take the value of the variable.
* Field splitting: treat that value as a whitespace-separated list of fields, and build the resulting list. For example, if the variable contains `foo * bar ` then the result of this step is the 3-element list `foo`, `*`, `bar`.
* Filename generation: treat each field as a glob, i.e. as a wildcard pattern, and replace it by the list of file names that match this pattern. If the pattern doesn't match any files, it is left unmodified. In our example, this results in the list containing `foo`, following by the list of files in the current directory, and finally `bar`. If the current directory is empty, the result is `foo`, `*`, `bar`.
Note that the result is a list of strings. There are two contexts in shell syntax: list context and string context. Field splitting and filename generation only happen in list context, but that's most of the time. Double quotes delimit a string context: the whole double-quoted string is a single string, not to be split. (Exception: `"$@"` to expand to the list of positional parameters, e.g. `"$@` is equivalent to `"$1" "$2" "$3"` if there are three positional parameters. See http://unix.stackexchange.com/questions/41571/what-is-the-difference-between-and/94200#94200)
The same happens to command substitution with `$(foo)` or with `` `foo` ``. On a side note, don't use `` `foo` ``: its quoting rules are weird and non-portable, and all modern shells support `$(foo)` which is absolutely equivalent except for having intuitive quoting rules.
The output of arithmetic substitution also undergoes the same expansions, but that isn't normally a concern as it only contains non-expandable characters (assuming `IFS` doesn't contain digits or `-`).
See http://unix.stackexchange.com/questions/68694/when-is-double-quoting-necessary for more details about the cases when you can leave out the quotes.
Unless you mean for all this rigmarole to happen, just remember to always use double quotes around variable and command substitutions. Do take care: leaving out the quotes can lead not just to errors but to [security holes](http://unix.stackexchange.com/questions/171346/security-implications-of-forgetting-to-quote-a-variable-in-bash-posix-shells/171347#171347).
### How do I process a list of file names?
If you write `myfiles="file1 file2"`, with spaces to separate the files, this can't work with file names containing spaces. Unix file names can contain any character other than `/` (which is always a directory separator) and null bytes (which you can't use in shell scripts with most shells).
Same problem with `myfiles=*.txt; … process $myfiles`. When you do this, the variable `myfiles` contains the 5-character string `*.txt`, and it's when you write `$myfiles` that the wildcard is expanded. This example will actually work, until you change your script to be `myfiles="$someprefix*.txt"; … process $myfiles`. If `someprefix` is set to `final report`, this won't work.
To process a list of any kind (such as file names), put it in an array. This requires mksh, ksh93 or bash (or zsh, which doesn't have all these quoting issues); a plain POSIX shell (such as ash, dash or yash) doesn't have array variables.
myfiles=("$someprefix"*.txt)
process "${myfiles[@]}"
Ksh88 has array variables with a different assignment syntax `set -A myfiles "someprefix"*.txt` (see [assignation variable under different ksh environment](http://stackoverflow.com/questions/22862038/assignation-variable-under-different-ksh-environment) if you need ksh88/bash portability). Bourne/POSIX-style shells have a single one array, the array of positional parameters `"$@"` which you set with `set` and which is local to a function:
set -- "$someprefix"*.txt
process -- "$@"
### What about file names that begin with `-`?
On a related note, keep in mind that file names can begin with a `-` (dash/minus), which most commands interpret as denoting an option. If you have a file name that begins with a variable part, be sure to pass `--` before it, as in the snippet above. This indicates to the command that it has reached the end of options, so anything after that is a file name even if it starts with `-`.
Alternatively, you can make sure that your file names begin with a character other than `-`. Absolute file names begin with `/`, and you can add `./` at the beginning of relative names. The following snippet turns the content of the variable `f` into a “safe” way of refering to the same file that's guaranteed not to start with `-`.
case "$f" in -*) "f=./$f";; esac
On a final note on this topic, beware that some commands interpret `-` as meaning standard input or standard output, even after `--`. If you need to refer to an actual file named `-`, or if you're calling such a program and you don't want it to read from stdin or write to stdout, make sure to rewrite `-` as above. See http://unix.stackexchange.com/questions/110750/what-is-the-difference-between-du-sh-and-du-sh/110756#110756 for further discussion.
## What's up with `read`?
Without `-r`, `read` allows continuation lines — this is a single logical line of input:
hello \
world
`read` splits the input line into fields delimited by characters in `$IFS` (without `-r`, backslash also escapes those). For example, if the input is a line containing three words, then `read first second third` sets `first` to the first word of input, `second` to the second word and `third` to the third word. If there are more words, the last variable contains everything that's left after setting the preceding ones. Leading and trailing whitespace are trimmed.
Setting `IFS` to the empty string avoids any trimming. See http://unix.stackexchange.com/questions/18886/why-is-while-ifs-read-used-so-often-instead-of-ifs-while-read/18936#18936 for a longer explanation.
## What's wrong with `xargs`?
The input format of `xargs` is whitespace-separated strings which can optionally be single- or double-quoted. No standard tool outputs this format.
The input to `xargs -L1` or `xargs -l` is almost a list of lines, but not quite — if there is a space at the end of a line, the following line is a continuation line.
You can use `xargs -0` where applicable (and where available: GNU (Linux, Cygwin), BusyBox, BSD, OSX, but it isn't in POSIX). That's safe, because null bytes can't appear in most data, in particular in file names. To produce a null-separated list of file names, use `find … -print0` (or you can use `find … -exec …` as explained below).
### How do I process files found by `find`?
find … -exec some_command a_parameter another_parameter {} +
`some_command` needs to be an external command, it can't be a shell function or alias. If you need to invoke a shell to process the files, call `sh` explicitly.
find … -exec sh -c '
for x do
… # process the file "$x"
done
' find-sh {} +
## I have some other question
Browse the [tag:quoting] tag on this site, or [tag:shell] or [tag:shell-script]. (Click on “learn more…” to see some general tips and a hand-selected list of common questions.) If you've searched and you can't find an answer, [ask away](http://unix.stackexchange.com/questions/ask).