Introduction to if else and logical expressions greater than, less than, or equal to in linux shell

For example, to compare strings, determine whether a file exists, and whether the file is readable, "[]" is usually used to represent a conditional test.

Note: spaces are important. Make sure that the square brackets have spaces. I used to waste a lot of valuable time due to lack of spaces or incorrect positions.

If...; then
....
Elif...; then
....
Else
....
Fi
[-F "somefile"]: determines whether it is a file.
[-X "/bin/ls"]: determines whether/bin/ls exists and has the executable permission.
[-N "$ var"]: determines whether the $ var variable has a value.
["$ A" = "$ B"]: determines whether $ a and $ B are equal.
-R file user readable to true
-W file users can write to true
-X file user executable is true
-F file: the regular file is true.
-D file: the directory is true.
-C file: the special character file is true.
-B file: The Block special file is true.
-S file is true if the file size is not 0
-T file: this parameter is true when the specified device is a terminal in the file descriptor (1 by default ).

Shell scripts with conditional selection are generally competent for simple shell scripts for tasks without variables. However, when executing some decision-making tasks, it is necessary to include the if/then condition judgment. Shell script programming supports such operations, including comparison operations and determining whether a file exists.
The basic if condition Command Options include-eq-to compare whether two parameters are equal (for example, if [2-eq 5]).
-Ne-compare whether two parameters are not equal
-Lt-parameter 1: whether it is smaller than parameter 2
-Le-parameter 1: whether it is less than or equal to parameter 2
-Gt-whether parameter 1 is greater than parameter 2
-Whether ge-parameter 1 is greater than or equal to parameter 2
-F-check whether a file exists (for example, if [-f "filename"])
-D-check whether the directory exists
Almost all judgments can be implemented using these comparison operators. In the script, the common-f Command Option checks whether a file exists before executing it.

Here are two examples for your understanding.

1. Determine whether a file exists

Copy codeThe Code is as follows :#! /Bin/sh
# Determining whether a file exists
# Link: www.jb51.net
# Date: 2013/2/27

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
Mail-s "$ FILE backup fail" test123@jb51.net <error. log
Fi

Copy codeThe Code is as follows :#! /Bin/sh
# Clear related files and record logs by time period
# Link: www.jb51.net
# Date: 2013/2/27
#
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