If else and greater than, less than, equal to logical expressions in the Linux shell

In Linux shell programming, in most cases, test commands can be used to test the condition, here is a simple introduction, easy to use friends

For example, comparing strings, judging whether the file exists and whether it is readable, etc., usually uses "[]" to represent the condition test.
Note: The space here is important. The space to ensure the square brackets. I have wasted a lot of precious time because the space is missing or the position is wrong.
If ....; Then
....
Elif ...; Then
....
Else
....
Fi
[-F "somefile"]: Determine if it is a file
[-X "/bin/ls"]: Determine if/bin/ls exists and has executable permissions
[-N ' $var]: Determine if the $var variable has a value
["$a" = "$b"]: Determine if $ A and $b are equal
-r file user readable as True
-W file user can write as true
-X file user can execute as true
-F file is true for regular files
-d file files are directory-True
-C File file is true for character special files
-B file files are true for block special files
-S file files non-0 o'clock True
-T file is true when the specified device is terminal (default = 1)

Shell scripts with conditional selection are generally competent for simple shell scripts that do not contain variables. However, when you perform some decision-making tasks, you need to include the if/then criteria to judge. Shell scripting supports such operations, including comparison operations, determining whether a file exists, and so on.
The basic if Condition command options are:-eq-compare two parameters for equality (for example, if [2–eq 5])
-ne-comparison of two parameters is not equal
-lt-parameter 1 is less than parameter 2
-le-parameter 1 is less than or equal to parameter 2
-gt-parameter 1 is greater than parameter 2
-ge-parameter 1 is greater than or equal to parameter 2
-f-Check if a file exists (for example, if [-F "filename"])
-D Check if directory exists
Almost all judgments can be implemented with these comparison operators. The common-f command option in a script checks to see if it exists before executing a file.

Let 's take two examples to make it easier for everyone to understand.

1. Determine if the file exists

The code is as follows:

#! / bin / sh
# Determine if the file exists
YACCESS = `date -d yesterday +% Y% m% d`
FILE = "access_ $ YACCESS.log.tgz"
cd / data / nginx / logs
if [-f "$ FILE"]; then
echo "OK"
else
echo "error $ FILE"> error.log
fi

The code is as follows:

#! / bin / sh
# Clear related files and record logs by time period
DIR = / data / img_cache
DAY = `date +"% Y-% m-% d% H:% M "`
NUM = `ls $ DIR | wc -l`
DIRNAME = `ls $ DIR | grep leveldb | head -n 1 | awk‘ {print $ NF} ’`
if [[$ NUM -gt 3]]; then
   rm -rf $ DIR / $ DIRNAME
   echo "--------- $ DAY ---- ($ DIR) -----------------------" >> / tmp / img_cache.log
   echo "$ DIRNAME Deleted successful" >> /tmp/img_cache.log
fi 

Reference Link: http://www.jb51.net/article/34332.htm

If else and greater than, less than, equal to logical expressions in the Linux shell