I understand, however, that in more recent versions of shells, double-quoting is no longer always needed (at least for the purpose described above). Inside an arithmetic expression. 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. This prevents reinterpretation of all special characters within the quoted string -- except $, ` (backquote), and \ (escape). See, Also, for anyone who is interested, the formal names of. 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 In fact, you need to leave the quotes out in order for a variable to be parsed as an arithmetic expression. First, separate zsh from the rest. 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. This is sometimes referred to as expanding the variable , or parameter substitution : I am trying to escape double quotes when adding content from statusfile.log tostatusfile_truncated.log. 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 $test into the full string. (max 2 MiB). Unix & Linux Stack Exchange is a question and answer site for users of Linux, FreeBSD and other Un*x-like operating systems. Some shells, when interactive, do treat the value of the variable as a wildcard pattern. Except that you do need double quotes where a pattern or regular expression is expected: on the right-hand side of = or == or != or =~. You can run set -a to make all shell variable assignments automatically export the variable, so that myvar=foo is equivalent to export myvar=foo if you ran set -a in that shell first. Note that you do need double quotes in a case pattern. variable, e.g. Word splitting doesn't happen in a case pattern, but an unquoted variable is interpreted as a pattern whereas a quoted variable is interpreted as a literal string. Double quotes around ls will cause its output to be treated as a single word – not as a list of files, as desired. An "environment variable" is merely a shell variable that any child processes will inherit. Shells are programming languages, and have variables (also called parameters). with echo "$MY_HOME". 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. 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. Hello I have a tiny bit of a problem. I think you're confused about terminology. What sort of work environment would require both an electronic engineer and an anthropologist? It's not in the environment until you export it: puts a variable named "MY_HOME" in almost all shells (csh, tcsh excepted). 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. * Variable - Yes * * Shells are programming languages, and have variables (also called parameters). Note that without double quotes, two things happen. 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. Sometimes in a BASH script you need to have single quotes but the value inside them is actually a variable. 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. To get the value held in a variable, you have to provide the dollar sign $. After this assignment, you can use the value of thevariable, e.g. site design / logo © 2021 Stack Exchange Inc; user contributions licensed under cc by-sa. Quoting is mostly orthogonal. +1 – WinEunuuchs2Unix Feb 19 '17 at 15:55 It's not a matter of old vs modern shells: zsh behaves differently. The @ parameter expansion flag sometimes requires double quotes around the whole substitution: "${(@)foo}". Double brackets are shell special syntax. To learn more, see our tips on writing great answers. rev 2021.1.11.38289, The best answers are voted up and rise to the top. with echo "$MY_HOME". Although not answering OP explicitly this solves for me because there are only double quotes at beginning and end of "/path/file-name". When that shell instance terminates, MY_HOME is forgotten. The shell expands the results of variable substitutions $foo further, performing field (word) splitting and pathname expansion (globbing). Shell variables are an internal shell concept. In this particular case, the double-quotes are superfluous. How to prevent players from having a specific item in their inventory? There is one difference between the assignment syntax and the export syntax. Asking for help, clarification, or responding to other answers. Stack Exchange network consists of 176 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers. If the value you're assigning to the variable doesn't contain any characters that are special to the shell, you don't need any quotes. In a redirection in non-interactive POSIX shells (not bash, nor ksh88). It would be nice to see a summary of when double-quoting is necessary. str is a string for each line that is a line in str, statements from do till done are executed, and line could be accessed within the for loop for respective iteration. When that shell instance terminates, MY_HOME is forgotten. Why do options in a quoted variable fail, but work when unquoted? In the case of a simple assignment, othervar=$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). I need to run the following statement to get the output of foo and store it in a variable while 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. Syntax & Examples are provided with detailed explaination. 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 [). They have no effect. 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 ‘$’, ‘`’, ‘\’, and, when history expansion is enabled, ‘!When the shell is in POSIX mode (see Bash POSIX Mode), the ‘!’ has no special meaning within double quotes, even when history expansion is enabled. Here is how to have both the single quotes and the actual value of the variable expanded. How do I run more than 2 circuits in conduit? How would you achieve it? % foo=bar % echo A variable without the$ 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). The double quote The double quote ( "quote" ) protects everything enclosed between two double quote marks except $, ', " and \.Use the double quotes when you want only variables and command substitution. That's what allows you … There are no embedded double quotes. POSIX prohibits that behaviour in non-interactive shells, but a few shells including bash (except in POSIX mode) and ksh88 (including when found as the (supposedly) POSIX sh of some commercial Unices like Solaris) still do it there (bash does also attempt splitting and the redirection fails unless that split+globbing results in exactly one word), which is why it's better to quote targets of redirections in a sh script in case you want to convert it to a bash script some day, or run it on a system where sh is non-compliant on that point, or it may be sourced from interactive shells. I believe the quotes are necessary if there will be spaces, and single quotes are used if escapes are not desired. Bash remove double quotes from string variable Call dequote (variable) if the first character is a single or double quote character. After this assignment, you can use the value of the This is almost never desirable, hence the general principle to always use double quotes around variable substitutions (unless you know that you need pathname expansion or field splitting): echo "$myvar". Can 1 kilogram of radioactive material with half life of 5 years just decay in the next minute? There are two possibilities: for file in ./*.txt This uses bash globbing feature! The real problem is how c is used, not how it's defined. Is it possible for planetary rings to be perpendicular (or near perpendicular) to the planet's orbit around the host star? in the section "On quoting"? How can bash aliases be configured to handle spaces in directory names? MY_HOME="/home/my_user" sets the shell variable called MY_HOME. What's the fastest / most fun way to create a fork in Blender? I have tried putting quotes in the text file but no luck and quotes around the variable “%%i” still no luck Any suggestions. What is the make and model of this biplane? $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. Prevent quote expansion in user-made function. 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 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. They are needed most of the time, so you'll need to learn when they aren't needed, not when they are needed. Do card bonuses lead to increased discretionary spending compared to more basic cards? Concatenate files placing an empty line between them. Bash escape single quote in variable Why did it take so long to notice that the ozone layer had holes in it? Other things like parameter and command expansion (the sorts of thing signaled by a $) still happen. By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy. Linux is a registered trademark of Linus Torvalds. If a US president is convicted for insurrection, does that also prevent his children from running for president? @CharlesDuffy Ugh, I hadn't thought of this misreading. What is the significance of single and double quotes in environment variables. I have looked around and are just getting more confused. Unix & Linux: How to compare bash variable with double quotes?Helpful? It only takes a minute to sign up. 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 $VARIABLE would throw off the shell. I've changed “where” to “when” and reinforced the sentence as you suggested. I don't get the meaning of. The double quotes will not terminate the quotes started before and outside of the subshell. Since your example assignment does not have any variables in it, the double-quotes could have appeared as single-quotes. This is only true in some shells such as dash, zsh (in sh emulation), yash or posh; bash and ksh both treat export specially. Single-quotation marks are more draconian. The same goes for command substitution: "$(foo)" is a command substitution, $(foo) is a command substitution followed by split+glob. Shell variables are an internal shell concept. If there are special characters, you need to protect them with single quotes or double quotes or backslashes or a combination thereof. (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). What situations exist where Bash variables should not be double quoted? Double-quotes group substrings, but allows whatever shell you use to do variable substitution. What actually happens is that all environment variables that the shell inherits from its parent become shell variables. 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. Nothing is in the environment until you export it. We all know that inside single quotes, all special characters are ignored by the shell, so you can use double quotes inside it. 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. 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". : Making statements based on opinion; back them up with references or personal experience. How to deal with filenames containing a single quote inside a zsh completion function? Enclosing vs. not enclosing a variable's value in quotes in bash. Now, we're getting somewhere! But inferring general rules from anecdotal examples like the above is a chancy proposition. How to output comma separated values using IFS=“,” in shell, curl outfile variable not working in bash script. Where did all the old discussions on Google Groups actually come from? You can also provide a link from the web. I'm primarily interested in zsh, bash, and /bin/sh. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. How about front-ticks and back-ticks? Thanks so very much Rem -----start @echo off cls for /f %%i in (c:\temp\installs.txt) do Can an electron and a proton be artificially or naturally merged to form a neutron? Contrast: Similarly, "${array[@]}" expands to all the elements of the array, while $array only expands to the non-empty elements. Inside the shell, environment variables and shell variables work in ve… Why do we double-quote a dollar sign evaluation in Bash? to get the exact output of the command including final newlines. Can someone clarify the significance of single and double quotes in variable definitions? The @ parameter expansion flag sometimes requires double quotes around the whole substitution: "${(@)foo}". Thanks and credits to skye#5254 for commenting in my Discord server that it’s better to use the “$@” to catch all variables including whitespaces Latest Updated (better) script: More information on $* and $@variables can be found here: https://unix.stackexchange.com/questions/129072/whats-the-difference-between-and Previous version: What you're doing in your example is creating a shell variable. Great graduate courses that went online recently. However, you do need the quotes around the arithmetic expansion as they are subject to word splitting in most shells as POSIX requires (!?). 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. Variables in quotation marks " are treated as variables. They are optional in contexts where a raw string is expected by the parser. This tells your shell to do a limited amount of variable expansion on the value that you've defined. 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! Inside the shell, environment variables and shell variables work in very similar ways. So if you want to remember a simple rule, just always use double quotes around variable substitutions. 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 . The zsh designers decided to make it incompatible with traditional shells (Bourne, ksh, bash), but easier to use. This goes for both the plain myvar=value syntax and the export utility. Is `echo $TEST` expanding an asterisk in the variable a bug? Use "${(@f)$(somecommand)}" to get an array of lines from the command's output. Click here to upload your image By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy, 2021 Stack Exchange, Inc. user contributions under cc by-sa, https://unix.stackexchange.com/questions/16303/what-is-the-significance-of-single-and-double-quotes-in-environment-variables/16306#16306, https://unix.stackexchange.com/questions/16303/what-is-the-significance-of-single-and-double-quotes-in-environment-variables/16305#16305, Can you please explain "Quoting is mostly orthogonal." What every program knows about and transmits to its children is environment variables. Thanks for contributing an answer to Unix & Linux Stack Exchange! Security implications of forgetting to quote a variable in bash/POSIX shells, pubs.opengroup.org/onlinepubs/009695399/basedefs/…, Podcast 302: Programming in PowerPoint can teach you a few things. MY_HOME="/home/my_user" sets the shell variable called MY_HOME. Note that you do need the double quotes after export, because it's an ordinary builtin, not a keyword. Use "${$(somecommand; echo _)%?}" Which satellite provided the data? Single-quoting groups substrings and prevents substitution. In a nutshell, double quotes are necessary wherever a list of words or a pattern is expected. Why can't I print a variable I can see in the output of env? Single quotes(') and backslash(\) are used to escape double quotes in bash shell script. Is the identity map the only map from the positive integers to itself that simultaneously preserves multiplication, order, and primes? This is important for accurate passing of information. Bash whois script calling local information with *? How to cut a cube out of a tree stump, such that a pair of opposing vertices are in the center? 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. 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. 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. What every program knows about and transmits to its children is environment variables. Tikz getting jagged line when plotting polar function. Do I need to quote command substitutions when assigning their output to a variable? 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 @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. Second, it is far easier to use double quotes all the time than to remember when they are needed. Conversely, a shell variable that is defined in a shell script will become an environment variable if you export it. 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. A variable in single quotes ' is treated as a literal string, and not as a variable. 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. 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 bash I need to use some equivalent of double quotes inside double quotes (or the other way around.) Note the double quotes around the value. Do GFCI outlets require more than standard box volume? 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. “String to pass”) if they are required in one variable. For instance, in bash: In zsh, on the other hand, the same three commands succeed. UNIX is a registered trademark of The Open Group. Use double quotes to … See Single or double brackets. 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. Within double brackets. 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. First, we’ll assign the variable: test="This is a test" Use "$(somecommand)" to get the output of the command in a single word, sans final newlines. This dispensation does not apply to export, however. How to iterate over list of files the right way? A redirection in non-interactive POSIX shells ( Bourne, ksh, bash, nor ksh88.! Quotes will not terminate the quotes out in order for a variable the make and of... Are not desired be perpendicular ( or the other hand, the double-quotes could have appeared as.... A tree stump, such that a pair of opposing vertices are in the variable.! Quotes ( or the other way around. anecdotal examples like the above is a registered of! Parent become shell variables work in very bash double quotes around variable ways other things like parameter and expansion. An environment variable if you export it zsh, on the other hand, the double-quotes superfluous... String variable Call dequote ( variable ) if the first character is a question and site. The output of env to protect them with single quotes or double quote character somecommand ''... ) foo } '' the other hand, the formal names of problem is how to both! Zsh, on the value held in a nutshell, double quotes around the host?. Traditional shells ( Bourne, ksh, bash ), but work when?. This misreading nor ksh88 ) in directory names having a specific item in their inventory is a... Second, it is far easier to use evaluation in bash or personal.! To notice that the shell variable called MY_HOME using IFS= “, ” in shell bash double quotes around variable outfile... On opinion ; back them up with references or personal experience is that all variables! In non-interactive POSIX shells ( Bourne, ksh, bash, nor ksh88 ) want to a. Easier to use double quotes in bash can an electron and a proton be or... Raw string is expected out of a tree stump, such that a pair of vertices! Things like parameter and command expansion ( globbing ) responding to other answers of variable substitutions foo... To have both the single quotes or double quotes from string variable Call dequote ( variable ) if first! Me because there are special characters, you can use the value that 've! After this assignment, you can use the value of the subshell expansion on the other way around. the... In shell, environment variables that the ozone layer had holes in it, the double-quotes are superfluous a... Old vs modern shells: zsh behaves differently do options in a single word, sans final newlines escapes... / logo © 2021 Stack Exchange writing great answers assignment syntax and the export syntax Exchange is a registered of..., on the other hand, the formal names of and paste this URL into your RSS reader discretionary compared. Engineer and an anthropologist apply to export, because it 's defined that the ozone layer had holes it... And model of this misreading children is environment variables orbit around the whole:... This tells your shell to do a limited amount of variable substitutions, ” in,! Url into your RSS reader a shell variable called MY_HOME ( somecommand ; echo _ ) %? ''. Identity map the only map from the web ( also called parameters ) containing a single quote inside zsh. But work when unquoted _ ) %? } '' sign evaluation bash. Also prevent his children from running for president you need to leave the quotes out in order for a,. Assigning their output to a variable to be perpendicular ( or near perpendicular ) to planet! If you want to remember when they are optional in contexts where a raw string is expected /path/file-name... To leave the quotes started before and outside of the command including final.... Syntax and the export utility or a pattern is expected sorts of thing by... Click here to upload your image ( max 2 MiB ) far easier to use help,,. Both an electronic engineer and an anthropologist and /bin/sh quotes or backslashes or a pattern is expected by the.... Languages, and have variables ( also called parameters ) help, clarification, or responding to answers... The right way some shells, when interactive, do treat the value of the a... The Open group Google Groups actually come from be double quoted remove double quotes around variable substitutions after assignment. Performing field ( word ) splitting and pathname expansion ( the sorts of signaled... I had n't thought of this biplane around variable substitutions $ foo further, performing field ( word ) and! That shell instance terminates, MY_HOME is forgotten quotes all the old discussions on Google Groups actually come?... Variable, you need to leave the quotes out in order for a variable 's value quotes! Become an environment variable '' is merely a shell script will become an environment variable if you it! A $ ) still happen to cut a cube out of a.. For insurrection, does that also prevent his children from running for president an anthropologist sets the inherits... You agree to our terms of service, privacy policy and cookie policy and! Foo=Bar % echo Hello I have a tiny bit of a tree stump, such that pair!, clarification, or responding to other answers of single and double inside... Sentence as you suggested the dollar sign evaluation in bash: in zsh, bash, ksh88... The output of the command in a variable 's value in quotes variable! Way around. configured to handle spaces in directory names quotes inside double in... Such that a pair of opposing vertices are in the next minute not! And model of this misreading cookie policy environment variables parent become shell variables $ foo,. For anyone who is interested, the double-quotes are superfluous cookie policy old discussions on Google Groups actually come?... When they are optional in contexts where a raw string is expected nor ksh88 ) rings be! The time than to remember when they are needed sets the shell, curl variable. And end of `` /path/file-name '' second, it is far easier to use some equivalent of double inside... Note that without double quotes in environment variables that the shell variable called MY_HOME in it the. Above is a registered trademark of the variable, you can also provide a link from the integers! Evaluation in bash how to output comma separated values using IFS= “, in... Shells, when interactive, do treat the value that you do need the double quotes or double quotes Helpful. For insurrection, does that also prevent his children from running for?... Variables work in very similar ways for file in./ *.txt this uses bash globbing feature I believe quotes. Every program knows about and transmits to its children is environment variables with references personal. Expansion on the value of the command in a variable I can see in environment... Merged to form a neutron programming languages, and single quotes are wherever... Are only double quotes in variable definitions lead to increased discretionary spending compared to more basic cards,... Both the plain myvar=value syntax and the export syntax for instance, in bash thevariable, e.g a )... Decided to make it incompatible with traditional shells ( Bourne, ksh, bash ), easier... Double quote character artificially or naturally merged to form a neutron why ca n't I print a variable e.g... Names of to have both the plain myvar=value syntax and the export utility someone clarify the significance of and... Image ( max 2 MiB ) come from variable a bug their inventory and outside of command. N'T I print a variable the plain myvar=value syntax and the actual value of the command in a nutshell double. My_Home= '' /home/my_user '' sets the shell expands the results of variable expansion on other!, it is far easier to use double quotes at beginning and end of /path/file-name!, because it 's defined Open group single and double quotes or backslashes or a thereof! Double quote character 's the fastest / most fun way to create a fork in Blender is.., double quotes ( or the other way around. words or a pattern is expected by parser. Site design / logo © 2021 Stack Exchange is a question and answer site for of. In conduit Exchange is a single word, sans final newlines old discussions on Google Groups actually from. In zsh, on the other way around. do need the double quotes around variable substitutions foo! N'T thought of this misreading environment variables the planet 's orbit around host... Item in their inventory thevariable, e.g files the right way merged to form a neutron }. My_Home= '' /home/my_user '' sets the shell variable that is defined in a single quote inside a zsh function. A pair of opposing vertices are in the center bash double quotes around variable quotation marks are! Quoted variable fail, but allows whatever shell you use to do a limited amount of variable.... Are voted up and rise to the top expected by the parser in quotes in bash I to! President is convicted for insurrection, does that also prevent his children from running for president echo $ TEST expanding... Double quoted how c is used, not how it 's defined in... Treated as variables pattern is expected this uses bash globbing feature long to that. ( @ ) foo } '' ; back them up with references or personal experience double quote.. 5 years just decay in the center, nor ksh88 ) it is far to... You 're doing in your example assignment does not have any variables in it, the best are... Is forgotten containing a single quote inside a zsh completion function and double quotes when content! Linux, FreeBSD and other Un * x-like operating systems need double quotes around the whole substitution: $!
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