Agile Otter Blog

This is one of those things that just doesn't stick in my head, so I'm dropping this note to remind myself.  I often have trouble recalling cryptic variable names in bash, make, and perl. For my own sake, I thought I'd make a quick list of favorites and refer some sources.   Maybe you'll find this useful too.  Variable value, but exit if no such variable: ${var:?"error message"} Use default if   variable isn't set:  ${var:="default"} Get a substring : ${var: start: length}   Length: ${#var}  Uppercase:  ${var@U} Lowercase : ${var@L}   The first one is quite helpful when you expect a particular set of parameters and don't want to write if/then logic about each of them.   #/bin/env bash directory=${1:? You must provide a directory to search} pattern=${2:? You must provide a search term for filenames} find ${directory} -name ${pattern}   See more at the gnu shell expansion page. 

There is a clever feature in bash, in that you can do a lot of manipulations while accessing a variable. Note that I always use "clever" in a pejorative sense, but while this is clever (argh) it is also helpful to me. I find that I, like all users, will tolerate a little cleverness if it gets my work done. In this case, it's the substitution features that get my attention. For variables: ${VAR:-zzz} returns $VAR, or "zzz" if $VAR is empty ${VAR//x/y} returns $VAR after replacing all "x" with "y" ${VAR:=zzz} returns $VAR or "zzz" if VAR is unset, and assigns VAR (ick) And of course the coolest version is the $() operator (AKA: command substitution), which used to be done with ugly backticks. This runs a command and returns the output of the command into a variable: $(echo $fname| cut -d. -f1) $(echo $(basename $fname) | cut -d -f1) Which leads to this kind of "clever" coding: dest=${DESTDIR:-/tmp}/$(echo $(basename ${sourc...