Fun Bash Script: test statement

Fun Bash Script: test statement

1st articles

Test

Test is the meaning of testing. It is often used as a condition in flow control statements. The following is an introduction.

Test integer

The test of the integer is the comparison of the size relationship. Unlike other languages, <,> is not used in Bash to compare the values greater than or equal to the number.

Assume there are two integer variables, a and B. Then compare whether a is greater than B to writeTest $ a-gt $ B , Returns the true value 1 if the condition is met. In actual test on the terminal, you can write the test statement execution result in this way.

test $a -gt $ $b && echo “Yes”

If the condition is true, print Yes. If the condition is not true, do not print.

All link comparison options are:

Option	Description	Full name
Eq	Equal	Equal
Gt	Greater	Greater
Lt	Less	Less
Ne	Not equal	Not equal
Ge	Greater than or equal	Greater or equal
Le	Less than or equal	Less or equal

Bash scripts, or various commands in Shell terminals, have rich options. Therefore, I suggest memorizing the full English name and understanding its meaning when memorizing it, this will reduce the difficulty of memory. Test string

A string test is nothing more than a test that checks whether two strings are equal and whether a string is null.

Assume that str1 and str2 are variables that hold two strings (directly test the two strings instead of the string variables without adding $, which is understandable ). Usage:

Usage	Description
Test $ str1 = $ str2	Test whether the values are equal. If the values are equal, 1 is returned.
Test $ str1! = $ Str2	Whether the test is not equal. If not, 1 is returned.
Test $ str1 \ <$ str2	If the Lexicographic Order of str1 is after str2, 1 is returned.
Test $ str1 \> $ str2	If the Lexicographic Order of str1 is before str2, 1 is returned.
Test $ str1	If not empty, 1 is returned.
Test-n $ str1	If not empty, 1 is returned.
Test-z $ str1	If it is an empty string, 1 is returned.

The two comparisons of lexicographic orders are actually greater>And Yu no.<. The two symbols in bash have redirection meaning, so the backslash is used here.\Escape.

Option-n is short for nonzero. Option-z is short for zero. This is a good memory.

Test File

A large number of options are required for testing files. Here I only write one common option.Man test

For a single file

Option	Description
D	Is it a directory?
F	Is it a common file?
X	Execution permission?
R	Read Permission?
W	Write and read permissions?
E	Exist?
S	Whether the file size is greater than 0
C	Whether it is a character Device File
B	Whether it is a block Device File

If the preceding conditions are true, 1 is returned. For example:

test -f hello.ctest -d /home...

For two files

Usage	Description
Test file1-nt file2	Test whether the modification time of file1 is later than that of file2 new)
Test file1-ot file2	Test whether the modification time of file1 is earlier than that of file2 old)
Test file1-ef file2	Test whether the two devices are the same and have the same inode number.

Similarly, if the condition is true, true value 1 is returned. Note that file1 and file2 are strings of file names.

# Test a. c-nt B. c # Or a = a. cb = B. ctest $ a-nt $ B

No more details. Logical operation

Logical operations are and or not.

-A logic and-o logic or! Non-logical

Usage example:

Test $ a-lt $ B-a $ a-gt $ ctest $ a-lt $ B-o $ a-gt $ ctest! -D sleep. sh & echo Yes # If sleep. sh is not a directory, print Yes

The full version is as follows:

test $a -lt $b -a test $a -gt $ctest $a -lt $b -o test $a -gt $c 

Simplified Version test

After reading this part, you may be surprised, why don't you tell me earlier. Haha.

In fact, all the above test commands can be replaced with square brackets. For example

Test-f hello. c

Can be changed

[-F hello. c]

It's very convenient. Note that there is a square brackets and an expression before and after each SpaceInterval. Do not discard it. Actually [space-f hello. c space]

Other notes are the logical expressions in square brackets, such

Test $ a-lt $ B-a $ a-gt $ c

Can be converted

[$ A-lt $ B-a $ a-lt $ c]

Or separate them as two statements.

[$ A-lt $ B-a] & [$ a-lt $ c]

This is nothing special, not exclusive to square brackets. When the current statement is true, run the following statement. All Bash statements can be connected with & |.