bash diff fails to found existing files when providing a path variable containing tilde (~)

Question

I have a very surprising problem while trying to execute a diff command inside a bash script.

Here is a working code illustrating the point:

#!/bin/bash
cd
mkdir foo bar
head -c 1024 /dev/urandom >foo/qux
head -c 1024 /dev/urandom >bar/qux

# works properly as expected
diff ~/{foo,bar}/qux

folder="~"

# this fails with the ~ inside a variable
diff $folder/{foo,bar}/qux

# cleaning the mess
rm -rf foo bar

So my question is:

Why ? :-)

score 2 · Answer 1 · answered Jul 07 '17 at 21:36

2

Don't quote the ~ when assigning it to a variable. The ~ is only expanded by bash when you don't quote it.

answered Jul 07 '17 at 21:36

Nicola Worthington

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hahahaha :) Thank you very much – Romain Vincent Jul 07 '17 at 21:40

score 1 · Accepted Answer · edited Aug 18 '18 at 12:08

1

~ is a feature of shell expansion.
Double quotes limit the expansion to only three features:

Command substitution: $(some command here) or `some command here`
Variable substitution: $VAR or ${VAR}
Arithmetic: $((2+2))

so when put inside double quotes, the ~ is not expanded

edited Aug 18 '18 at 12:08

Romain Vincent

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answered Jul 07 '17 at 22:04

hedgar2017

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This doesn't necessarily explain why `$folder` (unquoted) doesn't expand the tilde. – chepner Jul 08 '17 at 00:50
What do you mean? Please, give an example. – hedgar2017 Jul 08 '17 at 10:16

score 1 · Answer 3 · answered Jul 08 '17 at 00:54

Tilde expansion only applies to unquoted tildes. The tilde must be expanded at the time you perform the assignment to folder, because tilde expansion is not applied to parameter expansions, only word splitting and pathname expansion.

folder=~  # ~ is expanded, and the result is assigned to folder

bash diff fails to found existing files when providing a path variable containing tilde (~)

Why ? :-)

3 Answers3