## 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`, `yash`…). Zsh users should skip it and read the end of https://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" "$3"` if there are three positional parameters. See https://unix.stackexchange.com/questions/41571/what-is-the-difference-between-and/94200#94200)
The same happens to command substitution with `$(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 https://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](https://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 $myfiles`. If `someprefix` is set to `final report`, this won't work.
To process a list of any kind (such as file names), put it in an array. This requires mksh, ksh93, yash or bash (or zsh, which doesn't have all these quoting issues); a plain POSIX shell (such as ash or dash) doesn't have array variables.
myfiles=("$someprefix"*.txt)
process "${myfiles[@]}"
Ksh88 has array variables with a different assignment syntax `set -A myfiles "someprefix"*.txt` (see [assignation variable under different ksh environment](https://stackoverflow.com/questions/22862038/assignation-variable-under-different-ksh-environment) if you need ksh88/bash portability). Bourne/POSIX-style shells have a single one array, the array of positional parameters `"$@"` 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. Some commands (like `sh`, `set` or `sort`) also accept options that start with `+`. 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 `-` or `+`.
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 referring to the same file that's guaranteed not to start with `-` nor `+`.
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 https://unix.stackexchange.com/questions/110750/what-is-the-difference-between-du-sh-and-du-sh/110756#110756 for further discussion.
## How do I store a command in a variable?
“Command” can mean three things: a command name (the name as an executable, with or without full path, or the name of a function, builtin or alias), a command name with arguments, or a piece of shell code. There are accordingly different ways of storing them in a variable.
If you have a command name, just store it and use the variable with double quotes as usual.
command_path="$1"
…
"$command_path" --option --message="hello world"
If you have a command with arguments, the problem is the same as with a list of file names above: this is a list of strings, not a string. You can't just stuff the arguments into a single string with spaces in between, because if you do that you can't tell the difference between spaces that are part of arguments and spaces that separate arguments. If your shell has arrays, you can use them.
cmd=(/path/to/executable --option --message="hello world" --)
cmd=("${cmd[@]}" "$file1" "$file2")
"${cmd[@]}"
What if you're using a shell without arrays? You can still use the positional parameters, if you don't mind modifying them.
set -- /path/to/executable --option --message="hello world" --
set -- "$@" "$file1" "$file2"
"$@"
What if you need to store a complex shell command, e.g. with redirections, pipes, etc.? Or if you don't want to modify the positional parameters? Then you can build a string containing the command, and use the `eval` builtin.
code='/path/to/executable --option --message="hello world" -- /path/to/file1 | grep "interesting stuff"'
eval "$code"
Note the nested quotes in the definition of `code`: the single quotes `'…'` delimit a string literal, so that the value of the variable `code` is the string `/path/to/executable --option --message="hello world" -- /path/to/file1`. The `eval` builtin tells the shell to parse the string passed as an argument as if it appeared in the script, so at that point the quotes and pipe are parsed, etc.
Using `eval` is tricky. Think carefully about what gets parsed when. In particular, you can't just stuff a file name into the code: you need to quote it, just like you would if it was in a source code file. There's no direct way to do that. Something like `code="$code $filename"` breaks if the file name contains any shell special character (spaces, `$`, `;`, `|`, `<`, `>`, etc.). `code="$code \"$filename\""` still breaks on `` "$\` ``. Even `code="$code '$filename'"` breaks if the file name contains a `'`. There are two solutions.
* Add a layer of quotes around the file name. The easiest way to do that is to add single quotes around it, and replace single quotes by `'\''`.
quoted_filename=$(printf %s. "$filename" | sed "s/'/'\\\\''/g")
code="$code '${quoted_filename%.}'"
* Keep the variable expansion inside the code, so that it's looked up when the code is evaluated, not when the code fragment is built. This is simpler but only works if the variable is still around with the same value at the time the code is executed, not e.g. if the code is built in a loop.
code="$code \"\$filename\""
Finally, do you really need a variable containing code? The most natural way to give a name to a code block is to define a function.
## 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 `$IFS` (without `-r`, backslash also escapes those). For example, if the input is a line containing three words, then `read first second third` sets `first` to the first word of input, `second` to the second word and `third` to the third word. If there are more words, the last variable contains everything that's left after setting the preceding ones. Leading and trailing whitespace are trimmed.
Setting `IFS` to the empty string avoids any trimming. See https://unix.stackexchange.com/questions/18886/why-is-while-ifs-read-used-so-often-instead-of-ifs-while-read/18936#18936 for a longer explanation.
## What's wrong with `xargs`?
The input format of `xargs` is whitespace-separated strings which can optionally be single- or double-quoted. No standard tool outputs this format.
`xargs -L1` or `xargs -l` is not to split the input on *lines*, but to run one command per line of input (that line still split to make up the arguments, and continued on the next line if ending in blanks).
`xargs -I PLACEHOLDER` does use one line of input to substitute the `PLACEHOLDER` but quotes and backslashes are still processed and leading blanks trimmed.
You can use `xargs -r0` where applicable (and where available: GNU (Linux, Cygwin), BusyBox, BSDs, OSX, but it isn't in POSIX). That's safe, because null bytes can't appear in most data, in particular in file names and external command arguments. To produce a null-separated list of file names, use `find … -print0` (or you can use `find … -exec …` as explained below).
### How do I process files found by `find`?
find … -exec some_command a_parameter another_parameter {} +
`some_command` needs to be an external command, it can't be a shell function or alias. If you need to invoke a shell to process the files, call `sh` explicitly.
find … -exec sh -c '
for x do
… # process the file "$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](https://unix.stackexchange.com/questions/ask).