Assigning echo and cut command to a variable

问题:

I'm trying to get the string "boo" from "upper/lower/boo.txt" and assign it to a variable. I was trying

NAME= $(echo $WHOLE_THING | rev | cut -d "/" -f 1 | rev | cut -d "." -f 1)

but it comes out empty. What am I doing wrong?

回答1:

Don't do it that way at all. Much more efficient is to use built-in shell operations rather than out-of-process tools such as cut and rev.

whole_thing=upper/lower/boo.txt
name=${whole_thing##*/}
name=${name%%.*}

See BashFAQ #100 for a general introduction to best practices for string manipulation in bash, or the bash-hackers page on parameter expansion for a more focused reference on the techniques used here.

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Now, in terms of why your original code didn't work:

var=$(command_goes_here)

...is the correct syntax. By contrast:

var= $(command_goes_here)

...exports an empty environment variable named var while running command_goes_here, and then while running the output of command_goes_here as its own command.

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To show yet another variant,

var = command_goes_here

...runs var as a command, with = as its first argument, and command_goes_here as a subsequent argument. Which is to say, whitespace is important. :)