Difference between single and double quotes in Bash

Question

In Bash  what are the differences between single quotes      and double quotes

User · Answer

There is a clear distinction between the usage of ' ' and " ".

When ' ' is used around anything, there is no "transformation or translation" done. It is printed as it is.

With " ", whatever it surrounds, is "translated or transformed" into its value.

By translation/ transformation I mean the following: Anything within the single quotes will not be "translated" to their values. They will be taken as they are inside quotes. Example: a=23, then echo '$a' will produce $a on standard output. Whereas echo "$a" will produce 23 on standard output.

User · Answer

The accepted answer is great  I am making a table that helps in quick comprehension of the topic   The explanation involves a simple variable a as well as an indexed array arr  If we set a apple        a simple variable arr  apple     an indexed array with a single element  and then echo the expression in the second column  we would get the result   behavior shown in the third column   The fourth column explains the behavior        Expression Result Comments     1  quot  a quot  apple variables are expanded inside  quot  quot    2   a   a variables are not expanded inside      3  quot   a  quot   apple     has no special meaning inside  quot  quot    4   quot  a quot    quot  a quot   quot  quot  is treated literally inside      5      invalid can not escape a   within     use  quot   quot  or        ANSI-C quoting    6  quot red arocks quot  red  arocks does not expand  a  use   a rocks to preserve  a   7  quot redapple  quot  redapple    followed by no variable name evaluates to     8    quot     quot    has no special meaning inside      9  quot    quot        is interpreted inside  quot  quot  but has no significance for     10  quot   quot  quot   quot    quot  is interpreted inside  quot  quot    11  quot   quot    glob does not work inside  quot  quot  or      12  quot  t n quot   t n  t and  n have no special meaning inside  quot  quot  or     use ANSI-C quoting   13  quot  echo hi  quot  hi    and     are evaluated inside  quot  quot   backquotes are retained in actual output    14   echo hi   echo hi    and     are not evaluated inside     backquotes are retained in actual output    15    arr 0      arr 0   array access not possible inside      16  quot   arr 0   quot  apple array access works inside  quot  quot    17    a     a  single quotes can be escaped inside ANSI-C quoting   18  quot    t  quot     t  ANSI-C quoting is not interpreted inside  quot  quot    19   cmd   cmd history expansion character     is ignored inside      20  quot  cmd quot  cmd args expands to the most recent command matching  quot cmd quot    21    cmd   cmd history expansion character     is ignored inside ANSI-C quotes     See also   ANSI-C quoting with     - GNU Bash Manual Locale translation with   quot  quot  - GNU Bash Manual A three-point formula for quotes

User · Answer

Since this is the de facto answer when dealing with quotes in bash  I ll add upon one more point missed in the answers above  when dealing with the arithmetic operators in the shell   The bash shell supports two ways do arithmetic operation  one defined by the built-in let command and the         operator  The former evaluates an arithmetic expression while the latter is more of a compound statement   It is important to understand that the arithmetic expression used with let undergoes word-splitting  pathname expansion just like any other shell commands  So proper quoting and escaping needs to be done    See this example when using let  let  foo   2   1  echo  foo 3   Using single quotes here is absolutely fine here  as there is no need for variable expansions here  consider a case of  bar 1 let  foo    bar   1    would fail miserably  as the  bar under single quotes would not expand and needs to be double-quoted as  let  foo      bar     1    This should be one of the reasons  the         should always be considered over using let  Because inside it  the contents aren t subject to word-splitting  The previous example using let can be simply written as     bar 1  foo   bar   1      Always remember to use         without single quotes  Though the         can be used with double-quotes  there is no purpose to it as the result of it cannot contain a content that would need the double-quote  Just ensure it is not single quoted   printf   d n      1 1    -bash  printf     1 1    invalid number printf   d n     1 1   2 printf   d n      1 1    2   May be in some special cases of using the         operator inside a single quoted string  you need to interpolate quotes in a way that the operator either is left unquoted or under double quotes  E g  consider a case  when you are tying to use the operator inside a curl statement to pass a counter every time a request is made  do  curl http   myurl com --data-binary    requestCounter       reqcnt           Notice the use of nested double-quotes inside  without which the literal string    reqcnt     is passed to requestCounter field

User · Answer

If you re referring to what happens when you echo something  the single quotes will literally echo what you have between them  while the double quotes will evaluate variables between them and output the value of the variable   For example  this     bin sh MYVAR sometext echo  double quotes gives you  MYVAR  echo  single quotes gives you  MYVAR    will give this   double quotes gives you sometext single quotes gives you  MYVAR

User · Answer

Single quotes won t interpolate anything  but double quotes will  For example  variables  backticks  certain   escapes  etc    Example     echo    echo  upg    upg   echo    echo  upg      echo  upg     The Bash manual has this to say      3 1 2 2 Single Quotes      Enclosing characters in single quotes     preserves the literal value of each character within the quotes  A single quote may not occur between single quotes  even when preceded by a backslash        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     The characters   and   retain their special meaning within double quotes  see Shell Expansions   The backslash retains its special meaning only when followed by one of the following characters              or newline  Within double quotes  backslashes that are followed by one of these characters are removed  Backslashes preceding characters without a special meaning are left unmodified  A double quote may be quoted within double quotes by preceding it with a backslash  If enabled  history expansion will be performed unless an   appearing in double quotes is escaped using a backslash  The backslash preceding the   is not removed       The special parameters   and   have special meaning when in double quotes  see Shell Parameter Expansion

User · Answer

Others explained very well and just want to give with simple examples   Single quotes can be used around text to prevent the shell from interpreting any special characters   Dollar signs  spaces  ampersands  asterisks and other special characters are all ignored when enclosed within single quotes     echo  All sorts of things are ignored in single quotes  like    amp             It will give this   All sorts of things are ignored in single quotes  like    amp           The only thing that cannot be put within single quotes is a single quote   Double quotes act similarly to single quotes  except double quotes still allow the shell to interpret dollar signs  back quotes and backslashes   It is already known that backslashes prevent a single special character from being interpreted   This can be useful within double quotes if a dollar sign needs to be used as text instead of for a variable   It also allows double quotes to be escaped so they are not interpreted as the end of a quoted string     echo  Here s how we can use single   and double    quotes within double quotes    It will give this   Here s how we can use single   and double   quotes within double quotes   It may also be noticed that the apostrophe  which would otherwise be interpreted as the beginning of a quoted string  is ignored within double quotes   Variables  however  are interpreted and substituted with their values within double quotes     echo  The current Oracle SID is  ORACLE SID    It will give this   The current Oracle SID is test   Back quotes are wholly unlike single or double quotes   Instead of being used to prevent the interpretation of special characters  back quotes actually force the execution of the commands they enclose   After the enclosed commands are executed  their output is substituted in place of the back quotes in the original line   This will be clearer with an example     today  date    A   B  d   Y     echo  today    It will give this   Monday  September 28  2015

[bash] Difference between single and double quotes in Bash

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