One of the most important skills in any language is to know how a basic if then statement works. You set a variable, you say if that variable matches a given pattern (or doesn’t) then do something or else do something different. If/then/else. Other than that, it’s just syntax, and the annoyance of figuring out the operators and syntax.
So let’s go through the below script (which checks a Mac to see if grayscale is enabled in Universal Access), line-by-line:
- Line 1: we’re first going to create a variable called grayscale. We’ll fill it with a 0 or 1, the output of a simple “defaults read com.apple.universalaccess grayscale” command
- If that grayscale variable matches 0 execute whatever is after the then
- One line just for then followed by the command in the next line (which will optionally allow for an else)
- Use echo to display the text on the screen that indicates Grayscale is disabled to stdout (which could be redirected if you want to do something after we’re done processing)
- One line just for the else (optional)
- The echo to display it’s enabled
- fi ends the if/then so you can move on to the rest of the script (even if you have nothing after the if/then you’ll need the fi or the script won’t run)
grayscale=`defaults read com.apple.universalaccess grayscale`
if [[ $grayscale = 0 ]]
then
echo "Grayscale is disabled"
else
echo "User doesn't have a token"
fi
In the above if, we used = as an expression. That’s the most common one you’ll likely use, but there are a lot, including the following:
- [ -a FILE ] True if FILE exists
- [ -b FILE ] True if FILE exists and is a block-special file
- [ -c FILE ] True if FILE exists and is a character-special file
- [ -d FILE ] True if FILE exists and is a directory
- [ -e FILE ] True if FILE exists
- [ -f FILE ] True if FILE exists and is a regular file
- [ -g FILE ] True if FILE exists and its SGID bit is set
- [ -h FILE ] True if FILE exists and is a symbolic link
- [ -k FILE ] True if FILE exists and its sticky bit is set
- [ -p FILE ] True if FILE exists and is a named pipe
- [ -r FILE ] True if FILE exists and is readable
- [ -s FILE ] True if FILE exists and has a size greater than zero
- [ -t FD ] True if file descriptor FD is open and refers to a terminal
- [ -u FILE ] True if FILE exists and its SUID (set user ID) bit is set
- [ -w FILE ] True if FILE exists and is writable
- [ -x FILE ] True if FILE exists and is executable
- [ -O FILE ] True if FILE exists and is owned by the effective user ID
- [ -G FILE ] True if FILE exists and is owned by the effective group ID
- [ -L FILE ] True if FILE exists and is a symbolic link
- [ -N FILE ] True if FILE exists and has been modified since it was last read
- [ -S FILE ] True if FILE exists and is a socket
- [ FILE1 -nt FILE2 ] True if FILE1 has been changed more recently than FILE2, or if FILE1 exists and FILE2 does not
- [ FILE1 -ot FILE2 ] True if FILE1 is older than FILE2, or is FILE2 exists and FILE1 does not
- [ FILE1 -ef FILE2 ] True if FILE1 and FILE2 refer to the same device and inode numbers
- [ -o OPTIONNAME ] True if shell option “OPTIONNAME” is enabled
- [ -z STRING ] True of the length if “STRING” is zero
- [ -n STRING ] or [ STRING ] True if the length of “STRING” is non-zero
- [ STRING1 == STRING2 ] True if the strings are equal. “=” may be used instead of “==” for strict POSIX compliance
- [ STRING1 != STRING2 ] True if the strings are not equal
- [ STRING1 < STRING2 ] True if “STRING1” sorts before “STRING2”
- [ STRING1 > STRING2 ] True if “STRING1” sorts after “STRING2”
- [ ARG1 OP ARG2 ] “OP” is one of -eq, -ne, -lt, -le, -gt or -ge. These arithmetic binary operators return true if “ARG1” is equal to, not equal to, less than, less than or equal to, greater than, or greater than or equal to “ARG2”, respectively. “ARG1” and “ARG2” are integers
Attribution: The above list of expressions was originally borrowed from http://tldp.org/LDP/Bash-Beginners-Guide/html/sect_07_01.html.