while [ "a$file1" != "a" ] What is this doing?

chrism01 · 05-10-2017, 08:35 PM

... and I highly recommend [[ ]] over [ ] as explained here http://tldp.org/LDP/abs/html/testcon...ml#DBLBRACKETS. Double brackets are basically more 'robust'/sane.
I was on a ksh (applies equally to bash) course @ HP many years ago and they went into this; tl;dr always use [[ ]] .

MadeInGermany · 05-13-2017, 02:09 AM

Still the [ ] (test command) is a challenge in case the tested variable contains a test operator, for exanple

Code:

file1='!'

or

Code:

file1='-z'

and

Code:

if [ "$file1" = "foo" ]

Workarounds
1. Prepend an extra string, I use it outside the quotes to emphasize it's extra

Code:

if [ x"$file1" = x"foo" ]

2. Or switch the order, put the constant first

Code:

if [ "foo" = "$file1" ]

3. Or use the [[ ]] compound that is not a command and is easier to parse, and has less pitfalls for beginners

Code:

if [[ "$file1" = "foo" ]]

ondoho · 05-13-2017, 05:03 AM

Quote:

Originally Posted by r3sistance

This seems like a horrible way of checking a variable is not empty/null, the '! -z' method given by suicidalggroll is definitely a better test.

but is it really so horrible?
an almost identical construct is still being used in grub configuration (which uses some sort of ashell?) and i think i remember there was an explanation to the effect that it's more robust that way, more compatible and less likely to break with edge cases (variable not defined instead of empty)...
that said, it does look like bad coding (and so does /boot/grub/grub.cfg...).

Quote:

Originally Posted by chrism01

... and I highly recommend [[ ]] over [ ] as explained here http://tldp.org/LDP/abs/html/testcon...ml#DBLBRACKETS. Double brackets are basically more 'robust'/sane.

this fails on e.g. dash (and i guess other lightweight shells), it says: [[: not found