CODEWITHSUNDEEP
linuxbashshellshredhat
meallhour
meallhour

Reputation: 15581

Using double quotes within double quotes

I have a file1.txt with the below contents:

time="2016-04-25T17:43:11Z" level=info msg="SHA1 Fingerprint=9F:AD:D4:FD:22:24:20:A2:1E:0C:7F:D0:19:C5:80:42:66:56:AC:6F"

I want the file to look as below:

9F:AD:D4:FD:22:24:20:A2:1E:0C:7F:D0:19:C5:80:42:66:56:AC:6F

Actually, I need to pass the command as a string. That is why the bash command needs to be encapsulated in a string with double quotes. However, when i include 

 " grep -Po '(?<=Fingerprint=)[^"]*' "

I don't get the desired output. It seems that i need to escape the double quotes correctly.

Upvotes: 2

Views: 190

Answers (1)

Charles Duffy
Charles Duffy

Reputation: 295443

To answer the literal question, you can use a backslash to escape literal double quotes in your command. However, for the reasons given in BashFAQ #50, this is exceedingly poor practice:

# Avoid this absent a very good reason
grep_cmd_str="grep -Po '(?<=Fingerprint=)[^\"]*'" 
eval "$grep_cmd_str" <file1.txt # eval is necessary, with all the issues that implies

Better practice when you need to store a simple command (with no redirections or other shell constructs)[1] in a variable is to use an array[2], not a scalar variable, to hold its arguments:

# Use this principally if you need to dynamically build up an argument list
grep_args=( grep -Po '(?<=Fingerprint=)[^"]*' )
"${grep_args[@]}" <file1.txt

If you don't have any constraints that require you to use either of the above, consider a function (which does allow redirections and shell constructs so long as they're hardcoded):

# Use this whenever possible, in preference to the above
grep_fp() { grep -Po '(?<=Fingerprint=)[^"]*' "$@"; }
grep_fp <file1.txt

[1] - Not evaluating shell constructs, in this context, is a security feature: it protects you against malicious filenames or similar content in values which have been substituted into the value you're using as a command.

[2] - Note that arrays are not available in POSIX sh, which your question is also tagged for. That said, similar functionality is available by overriding "$@" (presumably within a limited scope, such as a function, such that its original value is preserved).

Upvotes: 5

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