In a bash script, what is the difference between wrapping text in single quotes and wrapping it in double quotes, and which shell behaviours does each one turn off?

## What quoting is really doing Quoting in bash is not a way of "making a string" the way it is in most programming languages. Bash reads a line, then runs a series of expansions over it, and finally hands the resulting words to a command as separate arguments. Quoting marks characters so that some of those stages leave them alone. That is why the interesting question is never "does this make a string" but "which stage does this quote switch off". Two of those stages cause almost all real bugs: **word splitting**, where an unquoted expansion is chopped into several words at spaces, tabs and newlines, and **pathname expansion** (globbing), where characters like `*`, `?` and `[` are replaced by matching filenames. Both quote forms switch both of those off. The difference between them is what still runs *inside* the quotes. ## Double quotes: expansion yes, splitting and globbing no Inside `"..."` the shell still performs: - parameter expansion — `$name`, `${name}`, `${name:-default}` - command substitution — `$(date)` and the older backtick form - arithmetic expansion — `$((count + 1))` What it no longer does is split the result on `IFS` or expand `*` against the filesystem: ```bash file='My Report.txt' ls -l $file # two arguments: 'My' and 'Report.txt' -> two errors ls -l "$file" # one argument: 'My Report.txt' ``` The backslash keeps a limited special meaning inside double quotes. It escapes only `$`, a backtick, `"`, another backslash, and a newline. Before anything else it is just a literal backslash — `echo "a\tb"` prints `a\tb`, not a tab, because `\t` means nothing to the shell. Producing a real tab is `printf`'s job, or the `$'...'` form. A few characters that people expect to be neutralised are not: `"` obviously ends the quote, and history expansion with `!` still fires inside double quotes in an interactive shell (in a script, history expansion is off, so this is not a scripting concern). ## Single quotes: completely literal Inside `'...'` no character is special. There is no expansion and there is no escape character: ```bash name=World echo "Hello, $name" # Hello, World echo 'Hello, $name' # Hello, $name echo 'a\tb' # a\tb - backslash is literal too ``` The consequence people get caught by is that a single quote cannot be escaped inside single quotes. `'it\'s'` does not work; the quote simply ends at the second `'`. The standard trick is to close the quote, emit an escaped quote, and reopen: `'it'\''s'`, or just use double quotes when the text contains an apostrophe but no `$` or backtick. Single quotes are the right choice whenever you are writing text that another program will interpret: an `awk` or `sed` program, a `grep` regular expression, a `find` pattern that `find` itself should expand, a JSON body, a password with `$` in it. In all of those cases you want bash to hand the characters over untouched. ```bash grep '^\$[0-9]' prices.txt # bash passes ^\$[0-9] straight to grep find . -name '*.log' # find does the matching, not bash ``` If you had written `find . -name *.log`, bash would expand the glob first against the current directory, and `find` would receive whatever filenames happened to match — a classic source of "it worked on my machine". ## Quote removal After all expansions are done, bash strips the quote characters themselves so the command never sees them. That is why `ls -l "$file"` passes `My Report.txt` and not `"My Report.txt"` — the quotes are shell syntax, not part of the data. It also means you cannot add quotes by putting them in a variable: the contents of a variable are data, and data is not re-scanned for quoting. ## The practical rule Double-quote every expansion by default. If you find yourself wanting an expansion unquoted so it splits into several words, that is almost always a sign you want an array instead. Use single quotes for literal text that must reach another program unchanged. ShellCheck flags the unquoted case as `SC2086`, which is by a wide margin the most common finding in real shell code.