Linuxshell programming-shell judgment

Linuxshell programming ---- judge 1test in shell to judge 1test-f $ file name to determine whether the file exists test-f $ file name. example: use the test-f command to find whether inputtxt exists in the home. if exist is output, otherwise, output no Linux shell programming ---- judge 1 test in shell to Judge 1 test-f $ file name to determine whether the file exists test-f $ file name. example: use the test-f command to find the input.txt file in the/homedrop folder, if exist is output, otherwise, no exist 2 is output, and there are many other commands: 1 test-d $. the directory name determines whether the directory exists. 2 test-r $ the file name determines whether the file has the read permission. 3 test-w $ the file name determines. whether the file has the write permission 4 test-x $ whether the file has the execution permission 2 [[Judge 1 must be separated by spaces in brackets, for example, ['10' <'20'] In this way, it is best to have four spaces in brackets. 2. variables in brackets, it is best to enclose a = "this" B = "this is" in double quotes, for example, ["$ "! = "$ B"] 3 constant in brackets, it is best to enclose them in single quotes, for example, ['4' = '4'] 3 (The following is just an example of [], or test, only [] looks beautiful) 1. single branch judgment structure if []; then statement fi 2 dual branch judgment structure if []; then statement else statement fi 3 multi-branch judgment structure if []; then statement elif []; then statement else statement fi 4 case realize multi-branch judgment structure case $ Variable in condition 1) statement; condition 2) statement; condition 3) statement ;;............................. *) statement; esac