Unix - Shell File Test Operators Example


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We have a few operators that can be used to test various properties associated with a Unix file.

Assume a variable file holds an existing file name "test" the size of which is 100 bytes and has read, write and execute permission on −

Operator Description Example
-b file Checks if file is a block special file; if yes, then the condition becomes true. [ -b $file ] is false.
-c file Checks if file is a character special file; if yes, then the condition becomes true. [ -c $file ] is false.
-d file Checks if file is a directory; if yes, then the condition becomes true. [ -d $file ] is not true.
-f file Checks if file is an ordinary file as opposed to a directory or special file; if yes, then the condition becomes true. [ -f $file ] is true.
-g file Checks if file has its set group ID (SGID) bit set; if yes, then the condition becomes true. [ -g $file ] is false.
-k file Checks if file has its sticky bit set; if yes, then the condition becomes true. [ -k $file ] is false.
-p file Checks if file is a named pipe; if yes, then the condition becomes true. [ -p $file ] is false.
-t file Checks if file descriptor is open and associated with a terminal; if yes, then the condition becomes true. [ -t $file ] is false.
-u file Checks if file has its Set User ID (SUID) bit set; if yes, then the condition becomes true. [ -u $file ] is false.
-r file Checks if file is readable; if yes, then the condition becomes true. [ -r $file ] is true.
-w file Checks if file is writable; if yes, then the condition becomes true. [ -w $file ] is true.
-x file Checks if file is executable; if yes, then the condition becomes true. [ -x $file ] is true.
-s file Checks if file has size greater than 0; if yes, then condition becomes true. [ -s $file ] is true.
-e file Checks if file exists; is true even if file is a directory but exists. [ -e $file ] is true.

Example

The following example uses all the file test operators −

Assume a variable file holds an existing file name "/var/www/tutorialspoint/unix/test.sh" the size of which is 100 bytes and has read, write and execute permission −

#!/bin/sh

file="/var/www/tutorialspoint/unix/test.sh"

if [ -r $file ]
then
   echo "File has read access"
els e
   echo "File does not have read access"
fi

if [ -w $file ]
then
   echo "File has write permission"
else
   echo "File does not have write permission"
fi

if [ -x $file ]
then
   echo "File has execute permission"
else
   echo "File does not have execute permission"
fi

if [ -f $file ]
then
   echo "File is an ordinary file"
else
   echo "This is sepcial file"
fi

if [ -d $file ]
then
   echo "File is a directory"
else
   echo "This is not a directory"
fi

if [ -s $file ]
then
   echo "File size is zero"
else
   echo "File size is not zero"
fi

if [ -e $file ]
then
   echo "File exists"
else
   echo "File does not exist"
fi

The above script will produce the following result −

File has read access
File has write permission
File has execute permission

File is an ordinary file
This is not a directory
File size is zero
File exists

The following points need to be considered while using file test operators −

  • There must be spaces between the operators and the expressions. For example, 2+2 is not correct; it should be written as 2 + 2.

  • if...then...else...fi statement is a decision-making statement which has been explained in the next chapter.


unix-basic-operators.htm

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