Tip 33: Replace shell commands with their outputs06 December 11. [link] PDF version
level: you want something more than pipes
purpose: use outputs as inputs to the next step
Last time, I gave you a four-item list of things your shell can do. Number three was expansions: replacing certain blobs of text with other text.
Variables are a simple expansion. If you set a variable like
onething="another thing"on the command line [C shell users: set onething="another thing"], then when you later type
then another thing will print to screeen.
Shell variables are a convenience for you to use while working at the command prompt or throwing together a quick script. They are stupendously easy to confuse with environment variables, which are sent to new processes and read via a simple set of C functions. Have a look at Appendix A of Modeling with Data for details on turning shell variables into environment variables.
Also, your shell will require that there be no spaces on either side of the =, which will annoy you at some point. This rule is for the purposes of supporting a feature that is mostly useful for makefiles. But there you have it: our easiest and most basic substitution of one thing for another.
Isn't it conveniently nifty that the $ is so heavily used in the shell, and yet is entirely absent from C code, so that it's easy to write shell scripts that act on C code (like in Tip #9), and C code to produce shell scripts? It's as if the UNIX shell and C were written by the same people to work together.
For our next expansion, how about the backtick, which on a typical
keyboard shares a key with the
~ and is not the more vertical-looking single tick
The vertical tick indicates that you don't want expansions done: echo
'$onething' will actually print $onething.
The backtick replaces the command you give with the output from the command, doing so
macro-style, where the command text is replaced in place with the output text. Here's an
example in which we count lines of C code by how many lines have a
on them; given that lines of source code is a lousy metric for most purposes anyway, this
is as good a means as any, and has the bonus of being one line of shell code:
#count lines with a ), }, or ;, and let that count be named Lines. Lines=`grep '[)};]' *.c | wc -l` #count how many lines there are in a directory listing; name it Files. Files=`ls *.c |wc -l` echo files=$Files and lines=$Lines #Arithmetic expansion is a double-paren. #In bash, the remainder is truncated; more on this later. echo lines/file = $(($Lines/$Files)) #Or, use those variables in a here script. #By setting scale=3, answers are printed to 3 decimal places. bc << --- scale=3 $Lines/$Files ---
OK, so now you've met variable substitution, command substitution, and in the sample code I touched on arithmetic
substitution for quick desk calculator math. That's what I deem to be the
low-hanging fruit; I leave you to read the manual on history expansion, brace expansion, tilde expansion,
parameter expansion, word splitting, pathname expansion, glob expansion, and the
difference between " " and ' '.