Common judgment formulas for Linux script shell
In shell script programming, predicates are often used. There are two ways to use predicates, one is to use test, and the other is to use [].
Let's take a look at how to use these two methods through two simple examples.
Example 1
# test –e /tmp/a.txt ; echo $?
The result is 0
We can see that the result above is 0. This is because the file a.txt exists in the /tmp directory. Of course, according to what we have learned in other programming languages, this result should be wrong. Because if the /tmp/a.txt file exists, the result should be true, so why is it 0? Doesn't 0 represent false? Of course, a special symbol $? is involved here, which represents the return value of the previous command. For details, please refer to "Detailed Explanation of Command Return Value $? in Linux" .
-e is used to check whether the specified file name exists. In addition to the method introduced in Example 1 above, you can also use && and || to view the result.
Example 2
# test –e /tmp/a.txt && echo 'Yes' || echo 'No'
&& and || are what we know as 'and' and 'or' in other programming languages.
In addition to using test as a keyword for judgment, you can also use []. Below we can improve Example 1 by using [].
Example 3
# [ -e /tmp/a.txt ] ; echo $?
It should be noted here that there should be spaces between the two boundaries [] and the content inside [space judgment space].
In the above example, we only used –e to determine whether the specified file exists. What other flags can be used in the judgment formula, and what do these flags mean? We can refer to the following table (this table is taken from Bird Brother’s Linux Private Recipe)
| Regarding the "file type" judgment of a certain file name, such as test -e filename to determine whether it exists | |
| -e | Does the file name exist? (Common) |
| -f | Does the "file name" exist and is it a file? (Common) |
| -d | Does the "file name" exist and is it a directory? (Common) |
| -b | Does the file name exist and is it a block device? |
| -c | Does the filename exist and is it a character device? |
| -S | Does the "file name" exist and is it a socket file? |
| -p | Does the file name exist and is it a FIFO (pipe) file? |
| -L | Does the "file name" exist and is it a linked file? |
| Regarding file permission detection, such as test -r filename to determine whether it is readable (but root permissions are often exceptions) | |
| -r | Check if the file name exists and has "readable" permission? |
| -w | Check if the file name exists and has writable permissions? |
| -x | Check if the file name exists and has "executable" permissions? |
| -u | Detect whether the file name exists and has the "SUID" attribute? |
| -g | Detect whether the file name exists and has the "SGID" attribute? |
| -k | Detect whether the file name exists and has the "Sticky bit" attribute? |
| -s | Check if the file name exists and is it a "non-blank file"? |
| Comparison between two files, such as: test file1 -nt file2 | |
| -nt | (newer than) Determine whether file1 is newer than file2 |
| -ot | (older than) Determine whether file1 is older than file2 |
| -ef | Determine whether file1 and file2 are the same file, which can be used to determine the hard link. The main significance is to determine whether the two files point to the same inode! |
| About the judgment between two integers, such as test n1 -eq n2 | |
| -eq | Two values are equal |
| -ne | Two values are not equal |
| -gt | n1 is greater than n2 |
| -lt | n1 is less than n2 |
| -ge | n1 is greater than or equal to n2 |
| -le | n1 is less than or equal to n2 |
| Determine the string data | |
| test -z string | Check if the string is 0? If string is an empty string, it is true |
| test -n string |
Check if string is not 0? If string is an empty string, it returns false. Note: -n can also be omitted |
| test str1 = str2 | Determines whether str1 is equal to str2. If they are equal, returns true. |
| test str1 != str2 | Determines whether str1 is not equal to str2. If they are equal, returns false. |
| Multiple conditional judgment, for example: test -r filename -a -x filename | |
| -a | (and) Both conditions are true at the same time! For example, test -r file -a -x file will return true only when file has both r and x permissions. |
| -o | (or) Either of the two conditions is true! For example, test -r file -o -x file will return true if file has r or x permissions. |
| ! | Inverted state, such as test ! -x file, when file does not have x, it returns true |
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