Expansion of variables inside single quotes in a command in Bash

Question

I want to run a command from a bash script which has single quotes and some other commands inside the single quotes and a variable   e g  repo forall -c       variable   In this format    is escaped and the variable is not expanded   I tried the following variations but they were rejected   repo forall -c        variable     repo forall -c         variable       repo forall -c       variable     repo forall -c          variable       If I substitute the value in place of the variable the command is executed just fine   Please tell me where am I going wrong

User · Accepted Answer

Inside single quotes everything is preserved literally, without exception.

That means you have to close the quotes, insert something, and then re-enter again.

'before'"$variable"'after'
'before'"'"'after'
'before'\''after'

Word concatenation is simply done by juxtaposition. As you can verify, each of the above lines is a single word to the shell. Quotes (single or double quotes, depending on the situation) don't isolate words. They are only used to disable interpretation of various special characters, like whitespace, $, ;... For a good tutorial on quoting see Mark Reed's answer. Also relevant: Which characters need to be escaped in bash?

Do not concatenate strings interpreted by a shell

You should absolutely avoid building shell commands by concatenating variables. This is a bad idea similar to concatenation of SQL fragments (SQL injection!).

Usually it is possible to have placeholders in the command, and to supply the command together with variables so that the callee can receive them from the invocation arguments list.

For example, the following is very unsafe. DON'T DO THIS

script="echo \"Argument 1 is: $myvar\""
/bin/sh -c "$script"

If the contents of $myvar is untrusted, here is an exploit:

myvar='foo"; echo "you were hacked'

Instead of the above invocation, use positional arguments. The following invocation is better -- it's not exploitable:

script='echo "arg 1 is: $1"'
/bin/sh -c "$script" -- "$myvar"

Note the use of single ticks in the assignment to script, which means that it's taken literally, without variable expansion or any other form of interpretation.

User · Answer

Below is what worked for me -  QUOTE     hive -e  alter table TBL NAME set location  QUOTE TBL HDFS DIR PATH QUOTE

User · Answer

Variables can contain single quotes   myvar        variable    repo forall -c  myvar

User · Answer

just use printf  instead of   repo forall -c       variable    use printf to replace the variable token with the expanded variable   For example   template        s   repo forall -c   printf    template      variable

User · Answer

EDIT   As per the comments in question    I ve been looking into this since then  I was lucky enough that I had repo laying around  Still it s not clear to me whether you need to enclose your commands between single quotes by force  I looked into the repo syntax and I don t think you need to  You could used double quotes around your command  and then use whatever single and double quotes you need inside provided you escape double ones

User · Answer

Does this work for you   eval repo forall -c       variable

User · Answer

The repo command can t care what kind of quotes it gets   If you need parameter expansion  use double quotes   If that means you wind up having to backslash a lot of stuff  use single quotes for most of it  and then break out of them and go into doubles for the part where you need the expansion to happen  repo forall -c  literal stuff goes here    quot stuff with  parameters here quot   more literal stuff   Explanation follows  if you re interested  When you run a command from the shell  what that command receives as arguments is an array of null-terminated strings  Those strings may contain absolutely any non-null character  But when the shell is building that array of strings from a command line  it interprets some characters specially  this is designed to make commands easier  indeed  possible  to type   For instance  spaces normally indicate the boundary between strings in the array  for that reason  the individual arguments are sometimes called  quot words quot    But an argument may nonetheless have spaces in it  you just need some way to tell the shell that s what you want  You can use a backslash in front of any character  including space  or another backslash  to tell the shell to treat that character literally   But while you can do something like this  echo     That  ll  be    4 96   please   quot   said  the  cashier    it can get tiresome   So the shell offers an alternative  quotation marks   These come in two main varieties  Double-quotation marks are called  quot grouping quotes quot    They prevent wildcards and aliases from being expanded  but mostly they re for including spaces in a word   Other things like parameter and command expansion  the sorts of thing signaled by a    still happen   And of course if you want a literal double-quote inside double-quotes  you have to backslash it  echo  quot   quot That ll be   4 96  please   quot  said the cashier quot  Single-quotation marks are more draconian   Everything between them is taken completely literally  including backslashes   There is absolutely no way to get a literal single quote inside single quotes  Fortunately  quotation marks in the shell are not word delimiters  by themselves  they don t terminate a word   You can go in and out of quotes  including between different types of quotes  within the same word to get the desired result  echo   quot That    ll be  4 96  please  quot  said the cashier  So that s easier - a lot fewer backslashes  although the close-single-quote  backslashed-literal-single-quote  open-single-quote sequence takes some getting used to  Modern shells have added another quoting style not specified by the POSIX standard  in which the leading single quotation mark is prefixed with a dollar sign  Strings so quoted follow similar conventions to string literals in the ANSI standard version of the C programming language  and are therefore sometimes called  quot ANSI strings quot  and the        pair  quot ANSI quotes quot   Within such strings  the above advice about backslashes being taken literally no longer applies  Instead  they become special again - not only can you include a literal single quotation mark or backslash by prepending a backslash to it  but the shell also expands the ANSI C character escapes  like  n for a newline   t for tab  and  xHH for the character with hexadecimal code HH   Otherwise  however  they behave as single-quoted strings  no parameter or command substitution takes place  echo    quot That  ll be  4 96  please  quot  said the cashier   The important thing to note is that the single string received as the argument to the echo command is exactly the same in all of these examples  After the shell is done parsing a command line  there is no way for the command being run to tell what was quoted how  Even if it wanted to

[bash] Expansion of variables inside single quotes in a command in Bash

Do not concatenate strings interpreted by a shell

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