Shell eval usage

1. Eval command-line

Command-new Roman; "> line is a common command line typed on the terminal.HoweverEvalThe result is shell.Scan it twice before executing the command line.For example:

New Roman; "> pipe =" |"

New Roman; "> eval ls $ pipe WC-l

New Roman; "> shell replaces the pipe value when scanning command lines for 1st times. | then, Eval enables it to scan command lines again. Then, shell uses | as the pipe symbol.

If the variable contains any shellYou can use the eval command to directly view the characters (not the replacement result) in the command line.. Command line terminator (;| &), I/New Roman; "> ORedirection character (<>) And quotation marks belong to the shellSymbols of special significance must appear in the command line directly.

2. Eval echo/$ #Get the last parameter

For example, cat last

New Roman; "> eval echo/$ #

New Roman; ">./last one two three four

New Roman; "> four

After the first scan, shell removes the backslash. When shell scans the row again, it replaces the value of $4 and runs the echo command.

3.How to UseEvalCommand to create a pointer to the variable ":

New Roman; "> x= 100

New Roman; "> ptrx = x

New Roman; "> eval echo/$ ptrx points to ptrx. Here we can understand the example in B.

New Roman; "> 100 print 100

New Roman; "> eval $ ptrx = 50: save 50 to the new Roman;"> variable pointed to by ptrx.

New Roman; "> echo $ x

New Roman; "> 50 print 50