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Piping to sh -

So, in a previous post I showed how to rename lots of files with little effort. Let’s remind ourselves how the one-liner looks:

for f in *.doc ; do mv $f ${f%.doc}.txt ; done

Imagine that you haven’t done that kind of transformation a thousand times and so feel a bit apprehensive about diving right in. The standard way to get around that is to pre-pend the command with echo:

for f in *.doc ; do echo mv $f ${f%.doc}.txt ; done

Now, instead of performing a lot of renames, your loop will just print out the commands it would do if the echo was not there. When you’ve looked over the output and satisfied that it will do what you want, you have to options. You can either edit the command to remove the echo, or you can pipe the result to sh -; the latter being my favourite:

for f in *.doc ; do echo mv $f ${f%.doc}.txt ; done | sh -

The pipe, for those who haven’t encountered it, means that the output of the command to the left of it is used as input for the command to the right. The sh command is just a shell (similar to the one you’re typing these commands into; on your system, it’s probably an alias for bash or zsh), and invoking it with the argument ‘-‘ means that it should read commands from its standard input and execute each line. Simple!