Bash bad substitution with glob expansion for environment variables

Question

How can I match environment variables which include the case-insensitive segment "proxy" that is not a prefix? I'm on bash:

root@PDPINTDEV9:~# echo ${SHELL}
/bin/bash

I want to unset a bunch of proxy variables simultaneously. They all have "proxy" or "PROXY" in the name, such as http_proxy or NO_PROXY. I would like to use glob expansion, which this answer & comment says is what bash uses.

Also based on that answer, I see that I can find environment vars which start with "PROXY":

root@PDPINTDEV9:~# echo "${!PROXY*}"
PROXY_IP PROXY_PORT

But that doesn't make sense with what I've read about glob expansion. Based on those, "${!PROXY*}" should match anything that doesn't start with proxy... I think.

Furthermore, I can't get anything that does make sense with glob syntax to actually work:

root@PDPINTDEV9:~# echo ${*proxy}
-bash: ${*proxy}: bad substitution
root@PDPINTDEV9:~# echo "${!*[pP][rR][oO][xX][yY]}"
-bash: ${!*[pP][rR][oO][xX][yY]}: bad substitution

SOLVED below: Turns out you can't. Crazy, but thanks everyone.

BTW -- no need to edit "SOLVED" into your question; accepting an answer changes colors/formatting in the list to mark a question answered (and relying on that rather than editing a summary into the question lets the site's mechanics around voting &c. to select the first answer to display do their thing properly). — Charles Duffy, Oct 27 '17 at 21:30

randomir · Accepted Answer · 2017-10-27T21:20:03.033

2

Variable name expansion, as a special case of shell parameter expansion, does not support globbing. But it has two flavors:

${!PREFIX*}
${!PREFIX@}

In both, the * and @ characters are hard-coded.

The first form will expand to variable names prefixed with PREFIX and joined by the first character of the IFS (which is a space, by default):

$ printf "%s\n" "${!BASH*}"
BASH BASHOPTS BASHPID BASH_ALIASES BASH_ARGC BASH_ARGV BASH_CMDS BASH_COMMAND ...

The second form will expand to variable names (prefixed with PREFIX), but as separate words:

$ printf "%s\n" "${!BASH@}"
BASH
BASHOPTS
BASHPID
BASH_ALIASES
BASH_ARGC
...

Both of these forms are case-sensitive, so to get the variable names in a case-insensitive manner, you can use set, in combination with some cut and grep:

$ (set -o posix; set) | cut -d= -f1 | grep -i ^proxy
PROXY_IP
proxy_port

edited Oct 27 '17 at 21:20

answered Oct 27 '17 at 21:11

randomir

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Note that extracting variable names via this `set`-based approach runs aground if any of the values contain embedded newlines. I happen to have a several git-related variables with that property in my own environment. Subsequently filtering the results *probably* cleans that up again, but one can't be completely confident. – John Bollinger Oct 27 '17 at 21:48
@JohnBollinger, good point. Something to be aware. In that case, filtering should probably be a little finer, perhaps with `awk -v FPAT='[^=]+=' '{print $1}'`. – randomir Oct 27 '17 at 22:09
Unlike `env`, `set` appears to use `$'...'` quoting to display a variable that would otherwise cause problems. – chepner Oct 28 '17 at 00:30

score 1 · Answer 2 · answered Oct 27 '17 at 21:07

But that doesn't make sense with what I've read about glob expansion. Based on those, "${!PROXY*}" should match anything that doesn't start with proxy... I think.

No and no.

In the first place, the ! character is not significant to pathname expansion, except when it appears at the beginning of a character class in a pattern, in which case the sense of the class is inverted. For example, fo[!o] is a pattern that matches any three-character string whose first two characters are "fo" and whose third is not another 'o'. But there is no character class in your expression.

But more importantly, pathname expansion isn't relevant to your expression ${!PROXY*} at all. There is no globbing there. The '!' and '*' are fixed parts of the syntax for one of the forms of parameter expansion. That particular expansion produces, by definition, the names of all shell variables whose names start with "PROXY", separated by the first character of the value of the IFS variable. Where it appears outside of double quotes, it is equivalent to ${!PROXY@}, which is less susceptible to globbing-related confusion.

Furthermore, I can't get anything that does make sense with glob syntax to actually work: [...]

No, because, again, there is no globbing going on. You need exactly ${! followed by the name prefix of interest, followed by *} or @} to form the particular kind of parameter expansion you're asking about.

How can I match environment variables which include the case-insensitive segment "proxy"?

You need to explicitly express the case variations of interest to you. For example:

${!PROXY*} ${!proxy*} ${!Proxy*}

So there is no way to match `http_proxy` and `no_proxy` with one variable name expansion? — jeremysprofile, Oct 27 '17 at 21:16
That is correct, @jeremysprofile. Variable name expansion identifies variable names with a given *prefix*. However, you could probably write a shell function that collects shell variable names and filters them to get the ones you want. — John Bollinger, Oct 27 '17 at 21:19

Bash bad substitution with glob expansion for environment variables

2 Answers2