Escape sequences passed as args to gawk not interpreted

Question

I want to be able to pass an argument on the command line to gawk that is evaluated for escape sequences.

The issue:

$ gawk 'BEGIN { print ARGV[1]; }' '\t'
\t

Instead, I would like to get an actual tab character.

From the gawk docs:

The escape sequences in the preceding list are always processed first, for both string constants and regexp constants. This happens very early, as soon as awk reads your program.

How can I interpret character escapes in the command line args?

The end goal is myscript.awk --sep '\t', where separator is a format string, so passing a literal tab isn't an option. I'm also familiar with the easy way I could perform this in bash, but I'm interested in a way to do this in [g]awk.

Answer 1

How can I print the unescaped version of command line args?

print ARGV[1]

The problem is that you don't want the unescaped command line argument. You want to interpret it. You're passing \t (the two-character string backslash, lowercase T), and you want that to be translated to a backslash. You'll need to do this manually. Just translating \t to a tab is easy — gsub(/\\t/, "\t") — but if you want to support octal escapes as well, and remove backslash before a non-recognized character, that's cumbersome in awk.

split ARGV[1], a, "\\";
s = a[1]; delete a[1];
for (x in a) {
    if (skip_next) {
        skip_next = 0;
    } else if (x == "") {
        s = s "\\";
        skip_next = 1;
    } else if (x ~ /^[0-7][0-7][0-7]/) {
        s = s sprintf("%c", 64*substr(x,1,1) + 8*substr(x,2,1) + substr(x,3,1));
        sub(/^.../, x);
    } else if (x ~ /^[0-7][0-7]/) {
        s = s sprintf("%c", 0 + 8*substr(x,1,1) + substr(x,2,1));
        sub(/^../, x);
    } else if (x ~ /^[0-7]/) {
        s = s sprintf("%c", 0 + substr(x,1,1));
        sub(/^./, x);
    } else {
        sub(/^a/, "\a", x) ||
        sub(/^b/, "\b", x) ||
        sub(/^n/, "\n", x) ||
        sub(/^r/, "\r", x) ||
        sub(/^t/, "\t", x) ||
        sub(/^v/, "\v", x);
    }
    s = s x;
}

(Warning: untested code!) Instead of this complex code, you could invoke printf in a subshell. Even that isn't so easy to do when the string could be multiline.

s = ARGV[1]
gsub(/'/, "'\\''", s)
cmd = "printf %b '" s "'."
s = ""
while ((cmd | getline line) > 0) s = s line "\n"
sub(/..$/, "", s)

Note that when you write "\t" in an awk script, that's a string containing the tab character. It's the way the awk syntax is: backslash has a special meaning in a string literal. Note: in a string literal, not in a string. If a string contains a backslash, that's just another character. The source code snippet "\t", consisting of four characters, is an expression whose value is the one-character string containing a tab, in the same way that the source code snippet 2+2, consisting of three characters, is an expression whose value is the number 4.

It would be better for your awk script to take the separator argument as a literal string. That would make it easier to use: your interface requires the caller to escape backslashes in the argument. If you want the separator to be a tab, pass an actual tab character.

Answer 2

First of all, you're not actually passing a tab to your awk. Remember that the shell evaluates the arguments before passing them to awk and '\t' in quotes is evaluated as a literal \ followed by a \t:

$ set -x
$ gawk 'BEGIN { print ARGV[1]; }' '\t'
+ gawk 'BEGIN { print ARGV[1]; }' '\t'
\t

As you can see above, you are not passing a tab to gawk so you can hardly expect it to print one. Compare with the version below which does pass a tab:

$ gawk 'BEGIN { print ARGV[1]; }' "$(printf '\t')"
++ printf '\t'
+ gawk 'BEGIN { print ARGV[1]; }' ' '  ## note the tab
                         ## This line contains a printed tab

Alternatively, you could pass the tab as a variable:

gawk -v t='\t' 'BEGIN {print t}'

Here, the '\t' is being expanded by awk, not the shell, so the tab is interpreted correctly.

Answer 3

The solution is to use getline.

Inside a file:

BEGIN { 
    sep = ARGV[1]
    gsub(/'/, "'\\''", sep);
    gsub(/%/, "%%", sep);
    "printf -- '" sep "'" | getline sep; 
    printf sep;
}