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🔗 Quelle: hackerpublicradio.org
Bash and quoted strings
An issue I just hit in Bash was that I had a quoted string, and I wanted to enclose it in quotes. How to do this?
This is the umpteenth time I have stumbled over this issue, and I realised I had found out how to solve it a while back but the information hadn’t rooted itself into my mind!
I have always been less clear in my mind about quoted strings in Bash than I should be, so, assuming others might have similar confusion I thought I’d try and clarify things in the form of an HPR show.
The problem
The thing I was having difficulties with was an alias definition of a useful pipeline:
nmap -sn 192.168.0.0/24 | awk '/^Nmap scan report/{print ""; print; next}{print}'This uses nmap (see Ken’s show 3052 for a discussion of its use) piped into an awk one-liner that formats the information returned by nmap.
The alias command can be used to store such a command or command sequence as a single simple command. It’s usually added to the ~/.bashrc file so it gets added to every Bash shell you start up (note Bash Tips #22, currently being written, will cover these startup files).
An alias definition looks something like this:
alias la='ls -Al'The alias itself 'la' is defined as the command ls -Al.
So how to make my nmap sequence into an alias given that the commands contain both single and double quotes?
Quoted strings in Bash
Bash is (to my mind) a bit weird with quoted strings.
There are two sorts of quotes in Bash (leaving aside the backquote or backtick – `):
Single quotes, also called hard quotes (
'). The literal value of characters between the quotes is preserved. Single quotes are not allowed, even if preceded by backslash escape characters.Double quotes, also called soft quotes (
"). Certain characters within the quotes have special meanings, such as'$'and'\'. Double quotes are allowed in the string when preceded by a backslash.
There’s a more comprehensive treatment of these quoting types (and others) in the Bash Reference Manual.
Changing quotes and concatenating strings
To make a variable containing a string with embedded quotes you can do this:
$ x='string1'"'"'string2'
$ echo $x
string1'string2What we did here was close 'string1', start a new string enclosed in double quotes "'", then append a second string 'string2'. Bash treats the three strings as one, but they have to be contiguous. There must be no intervening spaces1.
This solution is rather ugly. You could also use Bash string concatenation to do this, though it’s more long-winded:
$ x='string1'
$ x+="'"
$ x+='string2'
$ echo $x
string1'string2The same principles hold for double quotes of course:
$ x="string1"'"'"string2"
$ echo $x
string1"string2You’d probably not want to do this though.
Using backslashes
You can use backslashes to escape double quotes inside a double quoted string in Bash as we have seen.
$ x="string1\"string2"
$ echo $x
string1"string2However, as discussed earlier, it’s not possible to use backslashes to escape single quotes inside a single quoted string in Bash. However, outside a string a backslashed character is escaped. For example, if you have files which have spaces in their names, you can quote the name or use the backslash escape to protect the spaces2:
$ ls -l a\ file\ with\ spaces.awk
-rw-r--r-- 1 hprdemo hprdemo 0 Apr 22 22:25 'a file with spaces.awk'So, knowing this, you can exit a string, concatenate with a backslashed quote then restart a string like this:
$ x='string1'\''string2'
$ echo $x
string1'string2Solution
So now we can see how to achieve the alias definition I wanted earlier:
alias show_network='nmap -sn 192.168.0.0/24 | awk '\''/^Nmap scan report/{print ""; print; next}{print}'\'''
Epilogue
There’s more to be said about this subject, but too much of this stuff is not healthy.
Links
Ken Fallon’s HPR show where he describes
nmap: hpr3052 :: Locating computers on a network- Bash Reference Manual:
- Stack Overflow articles:
This is quite an artificial example to make a point. You wouldn’t do things this way in reality. Using
x='string1'"'string2"would also work ('string1'in single quotes, and"'string2'"in double quotes). Also, you could just writex="string1'string2"and stop all the messing about, but that would not be much of an example!↩The backslash is making the space a literal space, otherwise Bash would see it as an argument delimiter, and would look for the files
'a','file','with'and'spaces.awk'to list details about!↩