How to escape single quotes within single quoted strings

Question

Let\'s say, you have a Bash alias like:

alias rxvt=\'urxvt\'

which works fine.

However:

Answer 1

Most of these answers hit on the specific case you're asking about. There is a general approach that a friend and I have developed that allows for arbitrary quoting in case you need to quote bash commands through multiple layers of shell expansion, e.g., through ssh, su -c, bash -c, etc. There is one core primitive you need, here in native bash:

quote_args() {
    local sq="'"
    local dq='"'
    local space=""
    local arg
    for arg; do
        echo -n "$space'${arg//$sq/$sq$dq$sq$dq$sq}'"
        space=" "
    done
}

This does exactly what it says: it shell-quotes each argument individually (after bash expansion, of course):

$ quote_args foo bar
'foo' 'bar'
$ quote_args arg1 'arg2 arg2a' arg3
'arg1' 'arg2 arg2a' 'arg3'
$ quote_args dq'"'
'dq"'
$ quote_args dq'"' sq"'"
'dq"' 'sq'"'"''
$ quote_args "*"
'*'
$ quote_args /b*
'/bin' '/boot'

It does the obvious thing for one layer of expansion:

$ bash -c "$(quote_args echo a'"'b"'"c arg2)"
a"b'c arg2

(Note that the double quotes around $(quote_args ...) are necessary to make the result into a single argument to bash -c.) And it can be used more generally to quote properly through multiple layers of expansion:

$ bash -c "$(quote_args bash -c "$(quote_args echo a'"'b"'"c arg2)")"
a"b'c arg2

The above example:

1. shell-quotes each argument to the inner quote_args individually and then combines the resulting output into a single argument with the inner double quotes.
2. shell-quotes bash, -c, and the already once-quoted result from step 1, and then combines the result into a single argument with the outer double quotes.
3. sends that mess as the argument to the outer bash -c.

That's the idea in a nutshell. You can do some pretty complicated stuff with this, but you have to be careful about order of evaluation and about which substrings are quoted. For instance, the following do the wrong things (for some definition of "wrong"):

$ (cd /tmp; bash -c "$(quote_args cd /; pwd 1>&2)")
/tmp
$ (cd /tmp; bash -c "$(quote_args cd /; [ -e *sbin ] && echo success 1>&2 || echo failure 1>&2)")
failure

In the first example, bash immediately expands quote_args cd /; pwd 1>&2 into two separate commands, quote_args cd / and pwd 1>&2, so the CWD is still /tmp when the pwd command is executed. The second example illustrates a similar problem for globbing. Indeed, the same basic problem occurs with all bash expansions. The problem here is that a command substitution isn't a function call: it's literally evaluating one bash script and using its output as part of another bash script.

If you try to simply escape the shell operators, you'll fail because the resulting string passed to bash -c is just a sequence of individually-quoted strings that aren't then interpreted as operators, which is easy to see if you echo the string that would have been passed to bash:

$ (cd /tmp; echo "$(quote_args cd /\; pwd 1\>\&2)")
'cd' '/;' 'pwd' '1>&2'
$ (cd /tmp; echo "$(quote_args cd /\; \[ -e \*sbin \] \&\& echo success 1\>\&2 \|\| echo failure 1\>\&2)")
'cd' '/;' '[' '-e' '*sbin' ']' '&&' 'echo' 'success' '1>&2' '||' 'echo' 'failure' '1>&2'

The problem here is that you're over-quoting. What you need is for the operators to be unquoted as input to the enclosing bash -c, which means they need to be outside the $(quote_args ...) command substitution.

Consequently, what you need to do in the most general sense is to shell-quote each word of the command not intended to be expanded at the time of command substitution separately, and not apply any extra quoting to the shell operators:

$ (cd /tmp; echo "$(quote_args cd /); $(quote_args pwd) 1>&2")
'cd' '/'; 'pwd' 1>&2
$ (cd /tmp; bash -c "$(quote_args cd /); $(quote_args pwd) 1>&2")
/
$ (cd /tmp; echo "$(quote_args cd /); [ -e *$(quote_args sbin) ] && $(quote_args echo success) 1>&2 || $(quote_args echo failure) 1>&2")
'cd' '/'; [ -e *'sbin' ] && 'echo' 'success' 1>&2 || 'echo' 'failure' 1>&2
$ (cd /tmp; bash -c "$(quote_args cd /); [ -e *$(quote_args sbin) ] && $(quote_args echo success) 1>&2 || $(quote_args echo failure) 1>&2")
success

Once you've done this, the entire string is fair game for further quoting to arbitrary levels of evaluation:

$ bash -c "$(quote_args cd /tmp); $(quote_args bash -c "$(quote_args cd /); $(quote_args pwd) 1>&2")"
/
$ bash -c "$(quote_args bash -c "$(quote_args cd /tmp); $(quote_args bash -c "$(quote_args cd /); $(quote_args pwd) 1>&2")")"
/
$ bash -c "$(quote_args bash -c "$(quote_args bash -c "$(quote_args cd /tmp); $(quote_args bash -c "$(quote_args cd /); $(quote_args pwd) 1>&2")")")"
/
$ bash -c "$(quote_args cd /tmp); $(quote_args bash -c "$(quote_args cd /); [ -e *$(quote_args sbin) ] && $(quote_args echo success) 1>&2 || $(quote_args echo failure) 1>&2")"
success
$ bash -c "$(quote_args bash -c "$(quote_args cd /tmp); $(quote_args bash -c "$(quote_args cd /); [ -e *sbin ] && $(quote_args echo success) 1>&2 || $(quote_args echo failure) 1>&2")")"
success
$ bash -c "$(quote_args bash -c "$(quote_args bash -c "$(quote_args cd /tmp); $(quote_args bash -c "$(quote_args cd /); [ -e *$(quote_args sbin) ] && $(quote_args echo success) 1>&2 || $(quote_args echo failure) 1>&2")")")"
success

etc.

These examples may seem overwrought given that words like success, sbin, and pwd don't need to be shell-quoted, but the key point to remember when writing a script taking arbitrary input is that you want to quote everything you're not absolutely sure doesn't need quoting, because you never know when a user will throw in a Robert'; rm -rf /.

To better understand what is going on under the covers, you can play around with two small helper functions:

debug_args() {
    for (( I=1; $I <= $#; I++ )); do
        echo -n "$I:<${!I}> " 1>&2
    done
    echo 1>&2
}

debug_args_and_run() {
    debug_args "$@"
    "$@"
}

that will enumerate each argument to a command before executing it:

$ debug_args_and_run echo a'"'b"'"c arg2
1: 2: 3: 
a"b'c arg2

$ bash -c "$(quote_args debug_args_and_run echo a'"'b"'"c arg2)"
1: 2: 3: 
a"b'c arg2

$ bash -c "$(quote_args debug_args_and_run bash -c "$(quote_args debug_args_and_run echo a'"'b"'"c arg2)")"
1: 2:<-c> 3:<'debug_args_and_run' 'echo' 'a"b'"'"'c' 'arg2'> 
1: 2: 3: 
a"b'c arg2

$ bash -c "$(quote_args debug_args_and_run bash -c "$(quote_args debug_args_and_run bash -c "$(quote_args debug_args_and_run echo a'"'b"'"c arg2)")")"
1: 2:<-c> 3:<'debug_args_and_run' 'bash' '-c' ''"'"'debug_args_and_run'"'"' '"'"'echo'"'"' '"'"'a"b'"'"'"'"'"'"'"'"'c'"'"' '"'"'arg2'"'"''> 
1: 2:<-c> 3:<'debug_args_and_run' 'echo' 'a"b'"'"'c' 'arg2'> 
1: 2: 3: 
a"b'c arg2

$ bash -c "$(quote_args debug_args_and_run bash -c "$(quote_args debug_args_and_run bash -c "$(quote_args debug_args_and_run bash -c "$(quote_args debug_args_and_run echo a'"'b"'"c arg2)")")")"
1: 2:<-c> 3:<'debug_args_and_run' 'bash' '-c' ''"'"'debug_args_and_run'"'"' '"'"'bash'"'"' '"'"'-c'"'"' '"'"''"'"'"'"'"'"'"'"'debug_args_and_run'"'"'"'"'"'"'"'"' '"'"'"'"'"'"'"'"'echo'"'"'"'"'"'"'"'"' '"'"'"'"'"'"'"'"'a"b'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'c'"'"'"'"'"'"'"'"' '"'"'"'"'"'"'"'"'arg2'"'"'"'"'"'"'"'"''"'"''> 
1: 2:<-c> 3:<'debug_args_and_run' 'bash' '-c' ''"'"'debug_args_and_run'"'"' '"'"'echo'"'"' '"'"'a"b'"'"'"'"'"'"'"'"'c'"'"' '"'"'arg2'"'"''> 
1: 2:<-c> 3:<'debug_args_and_run' 'echo' 'a"b'"'"'c' 'arg2'> 
1: 2: 3: 
a"b'c arg2