UNIX操作系统教学课件英文版第十一章.ppt

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * $ sh -n check_syntax [Return] . . . Run it. check_syntax: syntax error at line 10 else unexpected $_……………Return to prompt. 11.6 DEBUGGING SHELL PROGRAMS $ check_syntax one two three [Return] . . check_syntax: checking the program syntax Number of the command line arguments: 3 $ check_syntax [Return] check_syntax: checking the program syntax No command line arguments $_ ................... 11.6 DEBUGGING SHELL PROGRAMS * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * 11.4.1 Arithmetic Operations: The expr Command You can use the expr command to evaluate expressions. This command provides arithmetic operations capability and evaluates either numeric or nonnumeric character strings. The expr command takes the arguments as expressions, evaluates them, and display the results on the standard output device. 11.4.1 Arithmetic Operations: The expr Command Arithmetic Operators The following command sequences introduce the arithmetic operator and demonstrate the use of the expr command to evaluate constants. $ expr 1 + 2 [Return] . . . The + sign is the addition operator. 3 $ expr 15 - 6 [Return] . . The - sign is the subtraction operator. 9 11.4.1 Arithmetic Operations: The expr Command $ expr 6 - 15 [Return] -9 $ expr 2.4 - 1[ Return] . . This is a bad argument (2.4); integers only. expr: nonnumeric argument . UNIX error- message. $ expr 1+1 [Return] .... This is a bad argument (1+1). 1+1 ............ It displays string 1+1. $_ $ expr 10 / 2....... The / sign is the division operator. ? 5 $ expr 10 \* 2 [Return] The * is the multiplication operator. 20 $ expr 10 \% 3 [Return] The % is the remainder operator. 1 $ $ x=10 [Return] ..... $ x=`expr $x + 1` [Return] $ echo $x [Return] .... 11 $_............ 11.4.1 Arithmetic Operations: The expr Command Relational Operators = equal to !=