bash if statement

Question

I found this bash script (which works). This script plays an important role. The whole code quality of the script is good. I'm just wondering if there's any sense at the following construction?

if [ "${G_FILE}" != "" ] && [ -f "${G_FILE}" ] && [ -r "${G_FILE}" ]; then
....

The manual says:

[ -f FILE ] True if FILE exists and is a regular file.
[ -r FILE ] True if FILE exists and is readable.
[ STRING1 != STRING2 ]  True if the strings are not equal.

My thoughts:

[ -n "${G_FILE}" ] or [ "${G_FILE}" ] should be instead of [ "${G_FILE}" != "" ].
Only [ -r "${G_FILE}" ] will do the same.

Am I right?

ormaaj · Accepted Answer · 2012-12-25 09:01:31Z

3

First, [ only makes sense over [[ to begin with if you require strict POSIX conformance. -f and -r tests should always be false if empty provided their option arguments are properly quoted, so the first test is useless. -r will be true for e.g. named pipes, sockets, etc. -f restricts things to regular files only.

Typically, you don't need to check that a file is actually readable. That check can just be bundled in with a test for whether the thing that's using the file was successful most of the time. A failed redirect is false, and it is better to write functions in a way that return an appropriate status upon failure than to test for a readable file first.

answered Dec 25, 2012 at 9:01

ormaaj

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1 Comment

idobr Over a year ago

Thanks a lot for the good explanation. [] and [[]] is important too, there are lots of materials and discussions. I'm on my way to read them. For someone, how is interested too

Ignacio Vazquez-Abrams · Accepted Answer · 2012-12-25 08:58:14Z

2

Unless you specifically require it to be a regular file, using only -r is enough.

answered Dec 25, 2012 at 8:58

Ignacio Vazquez-Abrams

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3 Comments

Noctua Over a year ago

Indeed, without the -f, directories would pass, too.

Ignacio Vazquez-Abrams Over a year ago

This will prevent you from using sockets, FIFOs, devices, etc. as sources. If you want to specifically prevent directories then you should use ! -d instead of -f.

idobr Over a year ago

In this case a regular file is exactly what I need: read some parameters from a file. I could left it without editing, but I was interested about the sense of this expression for the future.

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