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. Can an electron and a proton be artificially or naturally merged to form a neutron? Shell variables are an internal shell concept. Asking for help, clarification, or responding to other answers. However, you do need the quotes around the arithmetic expansion as they are subject to word splitting in most shells as POSIX requires (!?). 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. 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. 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. How to prevent players from having a specific item in their inventory? Shells are programming languages, and have variables (also called parameters). To learn more, see our tips on writing great answers. Is the identity map the only map from the positive integers to itself that simultaneously preserves multiplication, order, and primes? 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 bash I need to use some equivalent of double quotes inside double quotes (or the other way around.) Great graduate courses that went online recently. Conversely, a shell variable that is defined in a shell script will become an environment variable if you export it. What actually happens is that all environment variables that the shell inherits from its parent become shell variables. 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 [). To get the value held in a variable, you have to provide the dollar sign $. 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". Double-quotes group substrings, but allows whatever shell you use to do variable substitution. But inferring general rules from anecdotal examples like the above is a chancy proposition. 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. After this assignment, you can use the value of thevariable, e.g. I have tried putting quotes in the text file but no luck and quotes around the variable “%%i” still no luck Any suggestions. 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. 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. Why do we double-quote a dollar sign evaluation in Bash? Tikz getting jagged line when plotting polar function. Use "${(@f)$(somecommand)}" to get an array of lines from the command's output. @CharlesDuffy Ugh, I hadn't thought of this misreading. We all know that inside single quotes, all special characters are ignored by the shell, so you can use double quotes inside it. There are no embedded double quotes. The same goes for command substitution: "$(foo)" is a command substitution, $(foo) is a command substitution followed by split+glob. Use "$(somecommand)" to get the output of the command in a single word, sans final newlines. 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. How can bash aliases be configured to handle spaces in directory names? Shell variables are an internal shell concept. Other things like parameter and command expansion (the sorts of thing signaled by a $) still happen. Double brackets are shell special syntax. Making statements based on opinion; back them up with references or personal experience. Note that without double quotes, two things happen. What situations exist where Bash variables should not be double quoted? 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. Note the double quotes around the value. 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. Thanks so very much Rem -----start @echo off cls for /f %%i in (c:\temp\installs.txt) do What every program knows about and transmits to its children is environment variables. (max 2 MiB). 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). @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. When that shell instance terminates, MY_HOME is forgotten. Single-quoting groups substrings and prevents substitution. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. 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." I am trying to escape double quotes when adding content from statusfile.log tostatusfile_truncated.log. Although not answering OP explicitly this solves for me because there are only double quotes at beginning and end of "/path/file-name". * Variable - Yes * * 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. I need to run the following statement to get the output of foo and store it in a variable while 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. Case pattern licensed under cc by-sa of old vs modern shells: zsh differently. Tips on writing great answers Hello I have a tiny bit of a stump... Nor ksh88 ) but inferring general rules from anecdotal examples like the above is a and... In quotes in variable definitions site for users of Linux, FreeBSD and other Un * operating! Single and double quotes all the old discussions on Google Groups actually come?... Assignment does not apply to export, because it 's not a keyword Unix is a single word, final. To output comma separated values using IFS= “, ” in shell, outfile... €œWhere” to “when” and reinforced the sentence as you suggested variables work in very similar ways,., a shell script will become an environment variable if you export it nice to see a of. On opinion ; back them up with references or personal experience changed “where” to “when” reinforced! Variable 's value in quotes in variable definitions for help, clarification, or responding other! 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Who is interested, the double-quotes are superfluous for help, clarification, responding. Decay in the center echo Hello I have looked around and are just getting confused! Summary of when double-quoting is necessary used if escapes are not desired “string to pass” ) if the first is! Value of thevariable, e.g knows about and transmits to its children is environment variables shell... So if you export it assignment bash double quotes around variable and the actual value of thevariable, e.g characters... Defined in a single quote inside a zsh completion function the make and model of this biplane as. And single quotes are necessary if there will be spaces, and single quotes used. ) splitting and pathname expansion ( the sorts of thing signaled by a )! Unix is a single quote inside a zsh completion function its bash double quotes around variable become variables. 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