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'string2

What 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'string2

The same principles hold for double quotes of course:

$ x="string1"'"'"string2"
$ echo $x
string1"string2

You’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"string2

However, 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'string2

Solution

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:
  • Quoting
• Stack Overflow articles:
  • Difference between single and double quotes in Bash
  • How to escape a double quote inside double quotes

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1. 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 write x="string1'string2" and stop all the messing about, but that would not be much of an example!↩

2. 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!↩