bash - Check if command error contains a substring

Question

I have a lot of bash commands.Some of them fail for different reasons. I want to check if some of my errors contain a substring.

Here's an example:

#!/bin/bash

if [[ $(cp nosuchfile /foobar) =~ "No such file" ]]; then
    echo "File does not exist. Please check your files and try again."
else
    echo "No match"
fi

When I run it, the error is printed to screen and I get "No match":

$ ./myscript
cp: cannot stat 'nosuchfile': No such file or directory
No match

Instead, I wanted the error to be captured and match my condition:

$ ./myscript
File does not exist. Please check your files and try again.

How do I correctly match against the error message?

P.S. I've found some solution, what do you think about this?

out=`cp file1 file2 2>&1`
if [[ $out =~ "No such file" ]]; then
    echo "File does not exist. Please check your files and try again."
elif [[ $out =~ "omitting directory" ]]; then
    echo "You have specified a directory instead of a file"
fi

Answer 1

I'd do it like this

# Make sure we always get error messages in the same language
# regardless of what the user has specified.
export LC_ALL=C

case $(cp file1 file2 2>&1) in 
    #or use backticks; double quoting the case argument is not necessary
    #but you can do it if you wish
    #(it won't get split or glob-expanded in either case)
    *"No such file"*)
        echo >&2 "File does not exist. Please check your files and try again." 
        ;;
    *"omitting directory"*)
        echo >&2 "You have specified a directory instead of a file"
        ;;
esac

This'll work with any POSIX shell too, which might come in handy if you ever decide to convert your bash scripts to POSIX shell (dash is quite a bit faster than bash).

You need the first 2>&1 redirection because executables normally output information not primarily meant for further machine processing to stderr. You should use the >&2 redirections with the echos because what you're ouputting there fits into that category.