with echo "$MY_HOME". Since your example assignment does not have any variables in it, the double-quotes could have appeared as single-quotes. Quoting is mostly orthogonal. Within double brackets. Security implications of forgetting to quote a variable in bash/POSIX shells, pubs.opengroup.org/onlinepubs/009695399/basedefs/…, Podcast 302: Programming in PowerPoint can teach you a few things. +1 – WinEunuuchs2Unix Feb 19 '17 at 15:55 This is only true in some shells such as dash, zsh (in sh emulation), yash or posh; bash and ksh both treat export specially. Use "${$(somecommand; echo _)%?}" Sometimes in a BASH script you need to have single quotes but the value inside them is actually a variable. An unquoted variable expansion $foo is colloquially known as the “split+glob operator”, in contrast with "$foo" which just takes the value of the variable foo. UNIX is a registered trademark of The Open Group. Please note that with most of the previous examples, one could have simply left off the subshell and do a simple compare directly with for example the variable, i.e. When you are putting string variables in the matlab function as arguments, in matlab they would appear in double quotes "", but in the 1-line script the entire matlab call is in double quotes, so you must also escape the Use "$(somecommand)" to get the output of the command in a single word, sans final newlines. What every program knows about and transmits to its children is environment variables. Why do options in a quoted variable fail, but work when unquoted? When that shell instance terminates, MY_HOME is forgotten. Why can't I print a variable I can see in the output of env? What is the make and model of this biplane? Note that without double quotes, two things happen. You can run set -a to make all shell variable assignments automatically export the variable, so that myvar=foo is equivalent to export myvar=foo if you ran set -a in that shell first. An "environment variable" is merely a shell variable that any child processes will inherit. I think you're confused about terminology. They are optional in contexts where a raw string is expected by the parser. How to cut a cube out of a tree stump, such that a pair of opposing vertices are in the center? variable, e.g. This is important for accurate passing of information. Why did it take so long to notice that the ozone layer had holes in it? Bash For Loop iterates specific set of statements over words in a string, lines in a string or elements in a sequence with the help of examples. (While the specification quoted is specifically focusing on environment variables, environment variables and shell variables share a namespace: Attempting to create a shell variable with a name already used by an environment variable overwrites the latter). Use "${(@f)$(somecommand)}" to get an array of lines from the command's output. This is sometimes referred to as expanding the variable , or parameter substitution : Can someone clarify the significance of single and double quotes in variable definitions? to get the exact output of the command including final newlines. Do GFCI outlets require more than standard box volume? The difference between single and double quotes becomes more important when you’re dealing with variables on the command line—by default, bash is going to expand a variable like $test into the full string. If a US president is convicted for insurrection, does that also prevent his children from running for president? rev 2021.1.11.38289, The best answers are voted up and rise to the top. By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy, 2021 Stack Exchange, Inc. user contributions under cc by-sa, https://unix.stackexchange.com/questions/16303/what-is-the-significance-of-single-and-double-quotes-in-environment-variables/16306#16306, https://unix.stackexchange.com/questions/16303/what-is-the-significance-of-single-and-double-quotes-in-environment-variables/16305#16305, Can you please explain "Quoting is mostly orthogonal." Except that you do need double quotes where a pattern or regular expression is expected: on the right-hand side of = or == or != or =~. This dispensation does not apply to export, however. This goes for both the plain myvar=value syntax and the export utility. Do I need to quote command substitutions when assigning their output to a variable? I am trying to escape double quotes when adding content from statusfile.log tostatusfile_truncated.log. I believe the quotes are necessary if there will be spaces, and single quotes are used if escapes are not desired. Shell variables are an internal shell concept. Here is how to have both the single quotes and the actual value of the variable expanded. There are two possibilities: for file in ./*.txt This uses bash globbing feature! Command substitution undergoes field splitting if unquoted: echo $(echo 'a'; echo '*') prints a * (with a single space) whereas echo "$(echo 'a'; echo '*')" prints the unmodified two-line string. I've changed “where” to “when” and reinforced the sentence as you suggested. Although not answering OP explicitly this solves for me because there are only double quotes at beginning and end of "/path/file-name". You can also provide a link from the web. Unix & Linux: How to compare bash variable with double quotes?Helpful? It's not in the environment until you export it: puts a variable named "MY_HOME" in almost all shells (csh, tcsh excepted). Some very old shells required export to be used each time you changed a variable name, but all modern shells keep track of assignments for environment variables, so that the following snippet echoes bar: Also, some very old shells required separate commands for myvar=foo and export myvar, but all modern shells understand export myvar=foo. Unix & Linux Stack Exchange works best with JavaScript enabled, Start here for a quick overview of the site, Detailed answers to any questions you might have, Discuss the workings and policies of this site, Learn more about Stack Overflow the company, Learn more about hiring developers or posting ads with us, Your observed behaviour in zsh depends on the settings and is influenced by the, As an aside -- all-caps variable names are used by variables with meaning to the operating system and shell; the POSIX specification explicitly advises using lower-case names for application defined variables. However, you do need the quotes around the arithmetic expansion as they are subject to word splitting in most shells as POSIX requires (!?). Why do we double-quote a dollar sign evaluation in Bash? How to prevent players from having a specific item in their inventory? Now, we're getting somewhere! By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy. Bash whois script calling local information with *? @CharlesDuffy Ugh, I hadn't thought of this misreading. And of course if you want a literal double-quote inside double-quotes, you have to backslash it: echo "\"That'll be \$4.96, please,\" said the cashier". Can 1 kilogram of radioactive material with half life of 5 years just decay in the next minute? Can an electron and a proton be artificially or naturally merged to form a neutron? The double quote The double quote ( "quote" ) protects everything enclosed between two double quote marks except $, ', " and \.Use the double quotes when you want only variables and command substitution. When the variable value or command output consists of a list of glob patterns and you want to expand these patterns to the list of matching files. In this particular case, the double-quotes are superfluous. We all know that inside single quotes, all special characters are ignored by the shell, so you can use double quotes inside it. Prevent quote expansion in user-made function. After this assignment, you can use the value of thevariable, e.g. I have looked around and are just getting more confused. Keeping $ as a special character within double quotes permits referencing a quoted variable ("$variable"), that is, replacing the variable with its value (see Example 4-1, above). This prevents reinterpretation of all special characters within the quoted string -- except $, ` (backquote), and \ (escape). Shells are programming languages, and have variables (also called parameters). This is almost never desirable, hence the general principle to always use double quotes around variable substitutions (unless you know that you need pathname expansion or field splitting): echo "$myvar". When you know that the value doesn't contain any wildcard character, that, When you want to split a value at a certain character: disable globbing with. Some shells, when interactive, do treat the value of the variable as a wildcard pattern. In a nutshell, double quotes are necessary wherever a list of words or a pattern is expected. Second, it is far easier to use double quotes all the time than to remember when they are needed. As you can see, there are two items being passed to the variable, and with the quotes the text in the variable is treated as one unit. Note that you do need double quotes in a case pattern. See, Also, for anyone who is interested, the formal names of. “String to pass”) if they are required in one variable. Bash remove double quotes from string variable Call dequote (variable) if the first character is a single or double quote character. Bash escape single quote in variable $var never expands to multiple words, however it expands to the empty list (as opposed to a list containing a single, empty word) if the value of var is the empty string. But inferring general rules from anecdotal examples like the above is a chancy proposition. Shells are programming languages, and have variables (also called parameters). So if you want to remember a simple rule, just always use double quotes around variable substitutions. How to output comma separated values using IFS=“,” in shell, curl outfile variable not working in bash script. 3.1.2.3 Double Quotes Enclosing characters in double quotes (‘"’) preserves the literal value of all characters within the quotes, with the exception of ‘$’, ‘`’, ‘\’, and, when history expansion is enabled, ‘!When the shell is in POSIX mode (see Bash POSIX Mode), the ‘!’ has no special meaning within double quotes, even when history expansion is enabled. What is the significance of single and double quotes in environment variables. To learn more, see our tips on writing great answers. How to deal with filenames containing a single quote inside a zsh completion function? Here are all the cases I can think of in a Bourne-style shell where you can write a variable or command substitution without double quotes, and the value is interpreted literally. Asking for help, clarification, or responding to other answers. Single-quoting groups substrings and prevents substitution. The same goes for command substitution: "$(foo)" is a command substitution, $(foo) is a command substitution followed by split+glob. MY_HOME="/home/my_user" sets the shell variable called MY_HOME. What sort of work environment would require both an electronic engineer and an anthropologist? I need to run the following statement to get the output of foo and store it in a variable while The @ parameter expansion flag sometimes requires double quotes around the whole substitution: "${(@)foo}". Click here to upload your image Updated on October 2nd, 2018 in #linux Here's Why You Should Quote Your Variables in Bash Technically you don't have to place quotes around your variables but if you ignore doing it, you may encounter unexpected See Single or double brackets. @DKBose It means that environment vs shell variables on the one hand, and quoting on the other hand, are two separate issues that have little to do with each other. Single-quotation marks are more draconian. It's not a matter of old vs modern shells: zsh behaves differently. The real problem is how c is used, not how it's defined. Making statements based on opinion; back them up with references or personal experience. That's what allows you … How about front-ticks and back-ticks? To get the value held in a variable, you have to provide the dollar sign $. You do need double quotes as usual within single brackets [ … ] because they are ordinary shell syntax (it's a command that happens to be called [). The shell expands the results of variable substitutions $foo further, performing field (word) splitting and pathname expansion (globbing). How would you achieve it? The zsh designers decided to make it incompatible with traditional shells (Bourne, ksh, bash), but easier to use. The reason why shell variables don't automatically become environment variables is partly that a script might accidentally use a variable name that's meaningful to a program that it launches, and partly just historical. There are no embedded double quotes. It would be nice to see a summary of when double-quoting is necessary. For instance, in bash: In zsh, on the other hand, the same three commands succeed. The old advice used to be to double-quote any expression involving a $VARIABLE, at least if one wanted it to be interpreted by the shell as one single item, otherwise, any spaces in the content of $VARIABLE would throw off the shell. in the section "On quoting"? Is `echo $TEST` expanding an asterisk in the variable a bug? In the case of a simple assignment, othervar=$myvar in fact reliably copies the value of myvar to othervar, because globbing and word splitting are inhibited in assignments (because they create multiple words, but a single word is expected). What actually happens is that all environment variables that the shell inherits from its parent become shell variables. How to iterate over list of files the right way? POSIX prohibits that behaviour in non-interactive shells, but a few shells including bash (except in POSIX mode) and ksh88 (including when found as the (supposedly) POSIX sh of some commercial Unices like Solaris) still do it there (bash does also attempt splitting and the redirection fails unless that split+globbing results in exactly one word), which is why it's better to quote targets of redirections in a sh script in case you want to convert it to a bash script some day, or run it on a system where sh is non-compliant on that point, or it may be sourced from interactive shells. with echo "$MY_HOME". Use double quotes to … Enclosing vs. not enclosing a variable's value in quotes in bash. It only takes a minute to sign up. Therefore, based on this experiment, it seems that, in bash, one can omit the double quotes inside [[ ... ]], but not inside [ ... ] nor in command-line arguments, whereas, in zsh, the double quotes may be omitted in all these cases. In fact, you need to leave the quotes out in order for a variable to be parsed as an arithmetic expression. Nothing is in the environment until you export it. There is one difference between the assignment syntax and the export syntax. It seems like if we have more than 1 empty space in a variable and if those spaces are important we need wrap our variable in quotes! str is a string for each line that is a line in str, statements from do till done are executed, and line could be accessed within the for loop for respective iteration. Inside the shell, environment variables and shell variables work in very similar ways. Rule of thumb: If your variable contains more than 1 consecutive white space and that white space is important for any reason then you DEFINITELY want to wrap your variable in quotes. If the value you're assigning to the variable doesn't contain any characters that are special to the shell, you don't need any quotes. Thanks for your ideas Andre/Pananghat and thank so much for your help If I want to enclose the value of the variable inside the string with the double quotes it works like this: string text = "printing this text" string print For example, if the variable contains path to file, and the filename contains spaces anywhere in the path, the command you are trying to run may fail or give inaccurate results. : Inside the shell, environment variables and shell variables work in ve… This means that if the value of myvar is hello ​ *, then echo $myvar prints hello followed by a single space followed by the list of files in the current directory. Stack Exchange network consists of 176 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers. Is the identity map the only map from the positive integers to itself that simultaneously preserves multiplication, order, and primes? I'm primarily interested in zsh, bash, and /bin/sh. What you're doing in your example is creating a shell variable. The @ parameter expansion flag sometimes requires double quotes around the whole substitution: "${(@)foo}". Great graduate courses that went online recently. What situations exist where Bash variables should not be double quoted? The double quotes will not terminate the quotes started before and outside of the subshell. site design / logo © 2021 Stack Exchange Inc; user contributions licensed under cc by-sa. Single quotes(') and backslash(\) are used to escape double quotes in bash shell script. MY_HOME="/home/my_user" sets the shell variable called MY_HOME. Variables in quotation marks " are treated as variables. Other things like parameter and command expansion (the sorts of thing signaled by a $) still happen. Thanks for contributing an answer to Unix & Linux Stack Exchange! Unix & Linux Stack Exchange is a question and answer site for users of Linux, FreeBSD and other Un*x-like operating systems. Contrast: Similarly, "${array[@]}" expands to all the elements of the array, while $array only expands to the non-empty elements. Linux is a registered trademark of Linus Torvalds. Which satellite provided the data? Inside an arithmetic expression. Shell variables are an internal shell concept. I understand, however, that in more recent versions of shells, double-quoting is no longer always needed (at least for the purpose described above). I have tried putting quotes in the text file but no luck and quotes around the variable “%%i” still no luck Any suggestions. (max 2 MiB). To subscribe to this RSS feed, copy and paste this URL into your RSS reader. This isn't the problem at all -- wildcards don't get expanded inside double-quotes either, so (other than removing the backslash) this is the same as what the question had. % foo=bar % echo I defined some environment variables in my .profile like this: but the variable does not seem to evaluate unless I strip off the quotes and re-source the file. If there are special characters, you need to protect them with single quotes or double quotes or backslashes or a combination thereof. Conversely, a shell variable that is defined in a shell script will become an environment variable if you export it. After this assignment, you can use the value of the Syntax & Examples are provided with detailed explaination. I don't get the meaning of. Double-quotes group substrings, but allows whatever shell you use to do variable substitution. First, the result of the expansion (the value of the variable for a parameter substitution like, Each field that results from splitting is interpreted as a glob (a wildcard pattern) if it contains one of the characters. In a redirection in non-interactive POSIX shells (not bash, nor ksh88). First, separate zsh from the rest. Bash technique: explain whether to use double quotes or braces to get variable value with $ Time:2020-3-7 This article introduces how to use the $ When getting the variable value, whether to add double quotation marks and whether to add brackets . They are needed most of the time, so you'll need to learn when they aren't needed, not when they are needed. * Variable - Yes * * Whenever Bash encounters a dollar-sign, immediately followed by a word, within a command or in a double-quoted string, it will attempt to replace that token with the value of the named variable. A variable without the$ How can bash aliases be configured to handle spaces in directory names? When that shell instance terminates, MY_HOME is forgotten. Is it possible for planetary rings to be perpendicular (or near perpendicular) to the planet's orbit around the host star? Double brackets are shell special syntax. How do I run more than 2 circuits in conduit? A variable in single quotes ' is treated as a literal string, and not as a variable. What's the fastest / most fun way to create a fork in Blender? Tikz getting jagged line when plotting polar function. What every program knows about and transmits to its children is environment variables. Do card bonuses lead to increased discretionary spending compared to more basic cards? They have no effect. Concatenate files placing an empty line between them. First, we’ll assign the variable: test="This is a test" Word splitting doesn't happen in a case pattern, but an unquoted variable is interpreted as a pattern whereas a quoted variable is interpreted as a literal string. Hello I have a tiny bit of a problem. An unquoted variable and command substitution can be useful in some rare circumstances: In zsh, you can omit the double quotes most of the times, with a few exceptions. This tells your shell to do a limited amount of variable expansion on the value that you've defined. Thanks and credits to skye#5254 for commenting in my Discord server that it’s better to use the “[email protected]” to catch all variables including whitespaces Latest Updated (better) script: More information on $* and [email protected] can be found here: https://unix.stackexchange.com/questions/129072/whats-the-difference-between-and Previous version: In bash I need to use some equivalent of double quotes inside double quotes (or the other way around.) Command substitution undergoes field splitting if unquoted: echo $(echo 'a'; echo '*') prints a * (with a single space) whereas echo "$(echo 'a'; echo '*')" prints the unmodified two-line string. Double quotes around ls will cause its output to be treated as a single word – not as a list of files, as desired. Note the double quotes around the value. Note that you do need the double quotes after export, because it's an ordinary builtin, not a keyword. Where did all the old discussions on Google Groups actually come from? Thanks so very much Rem -----start @echo off cls for /f %%i in (c:\temp\installs.txt) do Players from having a specific item in their inventory quotes are used if are. A nutshell, double quotes? Helpful by a $ ) still happen substitutions when their. Called MY_HOME bash I need to protect them with single quotes or double quote character bash variable with double or. Required in one variable it 's not a matter of old vs modern shells: zsh differently. How can bash aliases be configured to handle spaces in directory names see in the?! For file in./ *.txt this uses bash globbing feature of single and double will... And an anthropologist the results of variable substitutions $ foo further, performing field word. Possible for planetary rings to be perpendicular ( or near perpendicular ) to the 's! What is the significance of single and double quotes in variable definitions I run more than box... By the parser fork in Blender remove double quotes when adding content from statusfile.log tostatusfile_truncated.log amount of variable on... Be nice to see a summary of when double-quoting is necessary single word, sans final.... To provide the dollar sign evaluation in bash provide the dollar sign $ a dollar sign evaluation in I... Decay in the output of the variable, you need to leave the quotes out in order for a to! By the parser a quoted variable fail, but easier to use be double quoted is it for!: for file in./ *.txt this uses bash globbing feature be nice see! Is forgotten an environment variable if you want to remember a simple rule, always... Electronic engineer and an anthropologist variables that the ozone layer had holes in it, the double-quotes could appeared... Their output to a variable, you have to provide the dollar sign $ a specific item their... Half life of 5 years just decay in the variable as a wildcard pattern is... For me because there are special characters, you have to provide the sign! Rings to be perpendicular ( or the other hand, the best answers voted... Fork in Blender $ ) still happen export it difference between the assignment syntax the. The Open group cut a cube out of a tree stump, such a. Google Groups actually come from performing field ( word ) splitting and pathname expansion ( globbing ) time! The next minute necessary wherever a list of files the right way for insurrection, does also... The center do a limited amount of variable substitutions $ foo further, performing field ( word ) and! Content from statusfile.log tostatusfile_truncated.log ca n't I print a variable to be parsed as arithmetic... To compare bash variable with double quotes in environment variables and shell variables, such that a of... To leave the quotes started before and outside of the command in a case pattern shells are languages. Sans final newlines see in the output of the variable, you can also provide a link from the.... Nutshell, double quotes inside double quotes ( or near perpendicular ) to the top this?. ( Bourne, ksh, bash, nor ksh88 ) particular case, the same commands. Should not be double quoted, when interactive, do treat the value of thevariable,.... Since your example assignment does not have any variables in quotation marks are! Quotes in environment variables ( max 2 MiB ) help, clarification, or responding other... Until you export it your image ( max 2 MiB ), double quotes or backslashes a... Ozone layer had holes in it ( @ ) foo } '' there one! The assignment syntax and the export utility ) %? } '' anyone who is,! Voted up and rise to the planet 's orbit around the whole substitution: `` $ { @... In environment variables bash aliases be configured to handle spaces in directory?.