Linux shell programming : Test

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Test

In shell, test is called as [:

$ ls /bin
[		chmod		date		domainname	expr		ksh		ln		mv		pwd		sh		sync		unlink
bash		cp		dd		echo		hostname	launchctl	ls		pax		rm		sleep		tcsh		wait4path
cat		csh		df		ed		kill		link		mkdir		ps		rmdir		stty		test		zsh

'[' is actually a program, just like 'ls' and other programs, so it must be surrounded by spaces. So, the following code:

if [ $foo = "bar" ]

will be translated into:

if SPACE [ SPACE "$foo" SPACE = SPACE "bar" SPACE ]

Note that '[' is a program and need a space, the following won't work:

if [$foo = "bar" ]

That's because it is interpreted as the following:

if test$foo = "bar" ]

Test with if

test is most often used with the if statement:

if [ ... ]; then
  echo "do something"
elif [ something_else ]; then
  echo "something else"
else
  echo "none of the above"
fi

If we want to perform a different action based on whether the file exists or not simply use the if/then construct:

# t.sh
#!/bin/bash
FILE=/etc/resolv.conf
if [ -f $FILE ]; then
  echo "$FILE exist"
else
  echo "$FILE does not exist"
fi

$ ./t.sh
/etc/resolv.conf exist

Actually, the test command takes one of the following syntax forms:

test EXPRESSION
[ EXPRESSION ]
[[ EXPRESSION ]]

So, the following script behaves exactly the same way:

#!/bin/bash
FILE=/etc/resolv.conf
if test -f $FILE; then
  echo "$FILE exist"
else
  echo "$FILE does not exist"
fi

We can also use the test command without the if statement. The command after the && operator will only be executed if the exit status of the test command is true:

$ FILE=/etc/resolv.conf

$ test -f $FILE && echo "$FILE exists"
/etc/resolve.conf exists

$ [ -f $FILE ] && echo "$FILE exists"
/etc/resolve.conf exists

$ [[ -f $FILE ]] && echo "$FILE exists"
/etc/resolve.conf exists

We can use the operators -d to test whether a file is a directory or not:

# d.sh
#!/bin/bash
FILE=/etc/apache2
if [ -d $FILE ]; then
  echo "$FILE is a directory"
fi

$ ./d.sh
/etc/apache2 is a directory

$ FILE=/etc/apache2

$ test -d $FILE && echo "$FILE is a directory"
/etc/apache2 is a directory

$ [ -d $FILE ] && echo "$FILE is a directory"
/etc/apache2 is a directory

We can check if a file does not exist:

# t.sh
#!/bin/bash
FILE=/etc/resolv.conf
if [ ! -f $FILE ]; then
  echo "$FILE does not exist"
else
  echo "$FILE exist"
fi

$ ./t.sh
/etc/resolv.conf exist

Test operators

-b FILE - True if the FILE exists and is a block special file.
-c FILE - True if the FILE exists and is a special character file.
-d FILE - True if the FILE exists and is a directory.
-e FILE - True if the FILE exists and is a file, regardless of type (node, directory, socket, etc.).
-f FILE - True if the FILE exists and is a regular file (not a directory or device).
-G FILE - True if the FILE exists and has the same group as the user running the command.
-h FILE - True if the FILE exists and is a symbolic link.
-g FILE - True if the FILE exists and has set-group-id (sgid) flag set.
-k FILE - True if the FILE exists and has a sticky bit flag set.
-L FILE - True if the FILE exists and is a symbolic link.
-O FILE - True if the FILE exists and is owned by the user running the command.
-p FILE - True if the FILE exists and is a pipe.
-r FILE - True if the FILE exists and is readable.
-S FILE - True if the FILE exists and is socket.
-s FILE - True if the FILE exists and has nonzero size.
-u FILE - True if the exists and set-user-id (suid) flag is set.
-w FILE - True if the FILE exists and is writable.
-x FILE - True if the FILE exists and is executable.