We often need to take different actions depending on user input, status of a command, status of a file, and so on – In order to perform conditional dependent actions, we need conditional statements – For conditional statements, we first need to be able to test conditions
3/14/2011 7:09:46 PM
unix-lect12-shell-cond-loop.ppt
2
Testing Conditions
The two general forms of testing are: test or [ ] – The latter method is easier to read • Note a space before and after the bracket is required
– A condition can be reversed with a ! before the condition (this is the same as not condition) [ ! ] – A ‘:’ command in place of condition always returns true 3/14/2011 7:09:47 PM
unix-lect12-shell-cond-loop.ppt
3
Testing File Attributes
To test if a file/directory is readable [ -r prog.txt ] [ -r $1.c ]
To test if a file/directory is writable [ -w specialfile.txt ] Note that a directory is writable only if the permission has both w and x bits
To test if a file/directory is executable [ -x prog4.sh ]
To test if a file exists and it is a regular file (e.g., not a directory) [ -f temp.text ]
To test if it is a directory [ -d cop3353 ]
Testing for the negation - use ! (eg. not writeable) [ ! -w nochange.txt ]
3/14/2011 7:09:47 PM
unix-lect12-shell-cond-loop.ppt
4
List of File Testing in “man bash” Not required for exams except the ones given on the previous slide
3/14/2011 7:09:48 PM
unix-lect12-shell-cond-loop.ppt
5
List of File Testing in “man bash” – cont.
Not required on the exam (but you may need some for homework assignment #4) For example, Problem 6 in homework assignment #4 asks you to make a copy of a file if the source file has been updated or the target file does not exist - This can be done using if [ targetfile –ot sourcefile ] then # copy a file fi
3/14/2011 7:09:49 PM
unix-lect12-shell-cond-loop.ppt
6
Numeric Tests
The following operators can be used for numeric tests: { -eq, -ne, -gt, -ge, -lt, -le }
Examples [ [ [ [
$1 $1 $# $#
–lt –gt -eq -lt
3/14/2011 7:09:49 PM
$2 ] 0 ] 2 ] 3 ]
unix-lect12-shell-cond-loop.ppt
7
Testing Strings
[ -z string ] – True if the string is empty
[ -n string ] or [ string ] – True if the string is non-zero
[ string1 = string2 ] or [ string1 == string2 ] – True if the two strings are the same
[ string1 != string2 ] – True if the two strings are different
3/14/2011 7:09:50 PM
unix-lect12-shell-cond-loop.ppt
8
Testing Strings – cont.
[ string1 < string2 ] – True if string1 is lexicographically before string2
[ string1 > string2 ] – True if string1 is lexicographically after string2
Similarly – [ string1 >= string2 ] • True if string1 is lexicographically after string2 or the same
