Q. How can you test if a file is writable under UNIX / Linux bash shell scripting?

A. You can easily test if a file is writable or noting test command under UNIX / Linux.

The -w option

The -w option is used to test if a FILE exists and write permission is granted or not. It returns true if a file is writable.

General syntax is as follows:
[ -w filename ] && echo "Writable" || echo "Not Writable"
test -w filename && echo "Writable" || echo "Not Writable"

Here is a sample shell script:

#!/bin/bash
FILE="$1"
 
[ $# -eq 0 ] && exit 1
 
if [ -w "$FILE" ] 
then
   echo "Write permission is granted on $FILE"
else
   echo "Write permission is NOT granted on $FILE"
fi