Matt Parkins Matt Parkins - 22 days ago 6
Linux Question

Find and basename not playing nicely

I want to echo out the filename portion of a find on the linux commandline. I've tried to use the following:

find www/*.html -type f -exec sh -c "echo $(basename {})" \;


and

find www/*.html -type f -exec sh -c "echo `basename {}`" \;


and a whole host of other combinations of escaping and quoting various parts of the text. The result is that the path isn't stripped:

www/channel.html
www/definition.html
www/empty.html
www/index.html
www/privacypolicy.html


Why not?

Update: While I have a working solution below, I'm still interested in why "basename" doesn't do what it should do.

Answer

The trouble with your original attempt:

find www/*.html -type f -exec sh -c "echo $(basename {})" \;

is that the $(basename {}) code is executed once, before the find command is executed. The output of the single basename is {} since that is the basename of {} as a filename. So, the command that is executed by find is:

sh -c "echo {}" 

for each file found, but find actually substitutes the original (unmodified) file name each time because the {} characters appear in the string to be executed.

If you wanted it to work, you could use single quotes instead of double quotes:

find www/*.html -type f -exec sh -c 'echo $(basename {})' \;

However, making echo repeat to standard output what basename would have written to standard output anyway is a little pointless:

find www/*.html -type f -exec sh -c 'basename {}' \;

and we can reduce that still further, of course, to:

find www/*.html -type f -exec basename {} \;

Could you also explain the difference between single quotes and double quotes here?

This is routine shell behaviour. Let's take a slightly different command (but only slightly — the names of the files could be anywhere under the www directory, not just one level down), and look at the single-quote (SQ) and double-quote (DQ) versions of the command:

find www -name '*.html' -type f -exec sh -c "echo $(basename {})" \;   # DQ
find www -name '*.html' -type f -exec sh -c 'echo $(basename {})' \;   # SQ

The single quotes pass the material enclosed direct to the command. Thus, in the SQ command line, the shell that launches find removes the enclosing quotes and the find command sees its $9 argument as:

echo $(basename {})

because the shell removes the quotes. By comparison, the material in the double quotes is processed by the shell. Thus, in the DQ command line, the shell (that launches find — not the one launched by find) sees the $(basename {}) part of the string and executes it, getting back {}, so the string it passes to find as its $9 argument is:

echo {}

Now, when find does its -exec action, in both cases it replaces the {} by the filename that it just found (for sake of argument, www/pics/index.html). Thus, you get two different commands being executed:

sh -c 'echo $(basename www/pics/index.html)'    # SQ
sh -c "echo www/pics/index.html"                # DQ

There's a (slight) notational cheat going on there — those are the equivalent commands that you'd type at the shell. The $2 of the shell that is launched actually has no quotes in it in either case — the launched shell does not see any quotes.

As you can see, the DQ command simply echoes the file name; the SQ command runs the basename command and captures its output, and then echoes the captured output. A little bit of reductionist thinking shows that the DQ command could be written as -print instead of using -exec, and the SQ command could be written as -exec basename {} \;.

If you're using GNU find, it supports the -printf action which can be followed by Format Directives such that running basename is unnecessary. However, that is only available in GNU find; the rest of the discussion here applies to any version of find you're likely to encounter.