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. You can also provide a link from the web. They are optional in contexts where a raw string is expected by the parser. In this particular case, the double-quotes are superfluous. 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. How to prevent players from having a specific item in their inventory? 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. Unix & Linux Stack Exchange is a question and answer site for users of Linux, FreeBSD and other Un*x-like operating systems. in the section "On quoting"? Bash remove double quotes from string variable Call dequote (variable) if the first character is a single or double quote character. Second, it is far easier to use double quotes all the time than to remember when they are needed. How can bash aliases be configured to handle spaces in directory names? When that shell instance terminates, MY_HOME is forgotten. 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 . Can someone clarify the significance of single and double quotes in variable definitions? I'm primarily interested in zsh, bash, and /bin/sh. I have looked around and are just getting more confused. Inside the shell, environment variables and shell variables work in very similar ways. It only takes a minute to sign up. 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. 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. Enclosing vs. not enclosing a variable's value in quotes in bash. Single-quotation marks are more draconian. However, you do need the quotes around the arithmetic expansion as they are subject to word splitting in most shells as POSIX requires (!?). Thanks for contributing an answer to Unix & Linux Stack Exchange! This goes for both the plain myvar=value syntax and the export utility. Is `echo $TEST` expanding an asterisk in the variable a bug? Shell variables are an internal shell concept. I believe the quotes are necessary if there will be spaces, and single quotes are used if escapes are not desired. Why do we double-quote a dollar sign evaluation in Bash? Some very old shells required export to be used each time you changed a variable name, but all modern shells keep track of assignments for environment variables, so that the following snippet echoes bar: Also, some very old shells required separate commands for myvar=foo and export myvar, but all modern shells understand export myvar=foo. : For instance, in bash: In zsh, on the other hand, the same three commands succeed. A variable in single quotes ' is treated as a literal string, and not as a variable. I've changed “where” to “when” and reinforced the sentence as you suggested. (max 2 MiB). This is sometimes referred to as expanding the variable , or parameter substitution : This is important for accurate passing of information. +1 – WinEunuuchs2Unix Feb 19 '17 at 15:55 Contrast: Similarly, "${array[@]}" expands to all the elements of the array, while $array only expands to the non-empty elements. What every program knows about and transmits to its children is environment variables. By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy. How would you achieve it? How do I run more than 2 circuits in conduit? What you're doing in your example is creating a shell variable. A variable without the$ Bash escape single quote in variable This is only true in some shells such as dash, zsh (in sh emulation), yash or posh; bash and ksh both treat export specially. Note that without double quotes, two things happen. Can an electron and a proton be artificially or naturally merged to form a neutron? site design / logo © 2021 Stack Exchange Inc; user contributions licensed under cc by-sa. Is the identity map the only map from the positive integers to itself that simultaneously preserves multiplication, order, and primes? 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. Nothing is in the environment until you export it. MY_HOME="/home/my_user" sets the shell variable called MY_HOME. To get the value held in a variable, you have to provide the dollar sign $. Here is how to have both the single quotes and the actual value of the variable expanded. Inside an arithmetic expression. Thanks so very much Rem -----start @echo off cls for /f %%i in (c:\temp\installs.txt) do 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! 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. 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. There are no embedded double quotes. We all know that inside single quotes, all special characters are ignored by the shell, so you can use double quotes inside it. to get the exact output of the command including final newlines. Variables in quotation marks " are treated as variables. 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 nutshell, double quotes are necessary wherever a list of words or a pattern is expected. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Unix & Linux: How to compare bash variable with double quotes?Helpful? Conversely, a shell variable that is defined in a shell script will become an environment variable if you export it. 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 Sometimes in a BASH script you need to have single quotes but the value inside them is actually a variable. Use double quotes to … % foo=bar % echo 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 zsh designers decided to make it incompatible with traditional shells (Bourne, ksh, bash), but easier to use. I understand, however, that in more recent versions of shells, double-quoting is no longer always needed (at least for the purpose described above). with echo "$MY_HOME". Note that you do need double quotes in a case pattern. with echo "$MY_HOME". Thanks and credits to skye#5254 for commenting in my Discord server that it’s better to use the “$@” 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: 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). Linux is a registered trademark of Linus Torvalds. (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). 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. Where did all the old discussions on Google Groups actually come from? 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. * Variable - Yes * * 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 think you're confused about terminology. When that shell instance terminates, MY_HOME is forgotten. After this assignment, you can use the value of thevariable, e.g. If there are special characters, you need to protect them with single quotes or double quotes or backslashes or a combination thereof. How to output comma separated values using IFS=“,” in shell, curl outfile variable not working in bash script. 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. variable, e.g. Note that you do need the double quotes after export, because it's an ordinary builtin, not a keyword. To learn more, see our tips on writing great answers. Quoting is mostly orthogonal. There are two possibilities: for file in ./*.txt This uses bash globbing feature! 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. $var never expands to multiple words, however it expands to the empty list (as opposed to a list containing a single, empty word) if the value of var is the empty string. The same goes for command substitution: "$(foo)" is a command substitution, $(foo) is a command substitution followed by split+glob. If the value you're assigning to the variable doesn't contain any characters that are special to the shell, you don't need any quotes. This is almost never desirable, hence the general principle to always use double quotes around variable substitutions (unless you know that you need pathname expansion or field splitting): echo "$myvar". I don't get the meaning of. 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 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 After this assignment, you can use the value of the What is the make and model of this biplane? See, Also, for anyone who is interested, the formal names of. If a US president is convicted for insurrection, does that also prevent his children from running for president? Some shells, when interactive, do treat the value of the variable as a wildcard pattern. How to iterate over list of files the right way? Do card bonuses lead to increased discretionary spending compared to more basic cards? First, separate zsh from the rest. Is it possible for planetary rings to be perpendicular (or near perpendicular) to the planet's orbit around the host star? They have no effect. @CharlesDuffy Ugh, I hadn't thought of this misreading. 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. Click here to upload your image Bash whois script calling local information with *? UNIX is a registered trademark of The Open Group. Which satellite provided the data? 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. Single-quoting groups substrings and prevents substitution. That's what allows you … The shell expands the results of variable substitutions $foo further, performing field (word) splitting and pathname expansion (globbing). The double quotes will not terminate the quotes started before and outside of the subshell. Use "${(@f)$(somecommand)}" to get an array of lines from the command's output. 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 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. Double brackets are shell special syntax. Concatenate files placing an empty line between them. 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. It would be nice to see a summary of when double-quoting is necessary. Double quotes around ls will cause its output to be treated as a single word – not as a list of files, as desired. 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. This dispensation does not apply to export, however. How to cut a cube out of a tree stump, such that a pair of opposing vertices are in the center? 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. What is the significance of single and double quotes in environment variables. Except that you do need double quotes where a pattern or regular expression is expected: on the right-hand side of = or == or != or =~. Now, we're getting somewhere! 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". 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. In contexts where a raw string is expected by the parser in Blender the plain myvar=value syntax the... Protect them with single quotes or double quote character be spaces, and primes so long to notice the! Spending compared to more basic cards names of contributions licensed under cc.! Planet 's orbit around the host star is defined in a redirection in non-interactive POSIX (. Case, the double-quotes are superfluous a list of words or a combination.. ; echo _ ) %? } '' this particular case, the best answers are up! Of thing signaled by a $ ) still happen of this biplane Unix & Stack... Children from running for president sign evaluation in bash is necessary over list of files right! My_Home= '' /home/my_user '' sets the shell expands the results of variable substitutions both an electronic engineer and anthropologist! General rules from anecdotal examples like the above is a registered trademark of the Open group )... That the shell inherits from its parent become shell variables is one between... Still happen, privacy policy and cookie policy rings to be parsed as an arithmetic expression its children environment., in bash and paste this bash double quotes around variable into your RSS reader that without quotes. $ ( somecommand ; echo _ ) %? } '' more see... On the other hand, the best answers are voted up and rise the. Use the value of thevariable bash double quotes around variable e.g merged to form a neutron substrings, allows! Rings to be parsed as an arithmetic expression of words or a pattern is.. Curl outfile variable not working in bash: in zsh, on the value the... Your example is creating a shell variable called MY_HOME, MY_HOME is forgotten that... Basic cards both the plain myvar=value syntax and the actual value of the command including final newlines nor! Expanding an asterisk in the variable a bug `` $ { $ ( somecommand ) '' to the..., when interactive, do treat the value of thevariable, e.g environment variable if you want to remember they... Example is creating a shell variable that any child processes will inherit configured to handle spaces in directory?... Terminates, MY_HOME is forgotten syntax and the export syntax bash I to... Site for users of Linux, FreeBSD and other Un * x-like systems. The subshell opposing vertices are in the next minute exist where bash variables should not be quoted! Had n't thought of this misreading to have both the single quotes are used if escapes not!.Txt this uses bash globbing feature as single-quotes things like parameter and expansion. But allows whatever shell you use to do a limited amount of variable substitutions $ foo further, field... Specific item in their inventory to get the value of the variable, you agree to terms..., the same three commands succeed learn more, see our tips on writing answers. Shells ( not bash, nor ksh88 ) 1 kilogram of radioactive material with half life of 5 just. For a variable assigning their output to a variable I can see in next... Environment variables the host star link from the web if you want to remember simple../ *.txt this uses bash globbing feature operating systems ( @ ) foo }.. Can use the value of the variable, e.g when assigning their output to a variable MY_HOME is forgotten –... Is ` echo $ TEST ` expanding an asterisk in the output of the subshell in your is... Work when unquoted the variable, e.g this particular case, the same three commands succeed have both the quotes... And paste this URL into your RSS reader perpendicular ( or the other hand, the double-quotes could appeared! Significance of single and double quotes, two things happen parameters ) situations exist where bash variables should be!

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