The Test in the UNIX Shell - Files and strings
(Page 2 of 4 )
We check traits for the files quite often. We want to find out if a file exists, if we have the right to delete it or even if there is a valid, executable file. These tasks become simple with the syntaxes I am going to show you. The only question is what the syntax of the expression is. After this, you can replace that with either of the syntaxes of the test presented on the previous page. For the file it can be:
Expression | Test checks for what trait? |
-d file | True if the file exists and it is a directory (d) |
-e file | True if the file exists (e) |
-f file | True if the file exists and is a regular file (f) |
-L file | True if the file exists and it is a symbolic link (L) |
-r file | True if the file exists and you can read (r) it |
-s file | True if the file exists and its size (s) is greater than zero |
-w file | True if the file exists and you can write (w) to it |
-x file | True if the file exists and you can execute (x) it |
-O file | True if the file exists and it its owner (O) is the actual user |
-G file | True if the file exits and its owner is in your group (g) |
File1 -nt file2 | True if File1 is newer than (nt) file2 (from the point of view of the access time) |
File1 -ot file2 | True if File1 is older than (ot) file2 |
File1 -ef file2 | True if the two files are equal files (ef). For this, the i-node and device numbers must be the same. |
A couple of examples:
#check if we can run the first argument of a shell script
if test -x $1
then
echo " We can and will start the program"
bash $1
fi
#if alfa.txt exists and we can delete it, remove it
if [ -e alfa.txt ] && [ -w alfa.txt ]
then
rm alfa.txt
else
echo "The file does not exists or no right to delete it"
fi
Then again, there we have the expressions related to strings.
Expression | Test checks for what trait? |
-z string | True if the string has a length equal to zero (z) |
-n string | True if the length of the string is not (n) zero |
string1 = string2 | True if the strings are the same |
string1 != string2 | True if the strings differ |
The usage of this is just as simple. Let us consider the following script snippets, where the $1 is the first argument of the script:
if test -z alfa
then
echo The variables alfa has a length equal with zero
fi
if [ $1 = "Yeti" ]
then
echo The first argument is Yeti.
fi
Next: Logical expressions and numbers >>
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