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Is set to the exit code of the last-executed command, and $_ is set to the last argument to that command. $! contains the PID of the last command executed in the background, and $- is set to the option flags currently in effect. I’ll discuss these parameters in more detail as they come up in the course of writing scripts. Variables A variable is a parameter denoted by a name; a name is a word containing only letters, numbers, or underscores and beginning with a letter or an underscore. Values can be assigned to variables in the following form: name=VALUE Many variables are set by the shell itself, including three you have already seen: HOME, PWD, and PATH.

These are 0, 1, and 2, respectively. The stream names are also often contracted to stdin, stdout, and stderr. I/O streams can be redirected to (or from) a file or into a pipeline. Redirection: >, >>, and < In Chapter 1, you redirected standard output to a file using the > redirection operator. 13 CHAPTER 2 „ INPUT, OUTPUT, AND THROUGHPUT When redirecting using >, the file is created if it doesn’t exist. If it does exist, the file is truncated to zero length before anything is sent to it. You can create an empty file by redirecting an empty string (that is, nothing) to the file: printf "" > FILENAME or by simply using this: > FILENAME Redirection is performed before any command on the line is executed.

37 CHAPTER 4 „ COMMAND LINE PARSING AND EXPANSION When the file name on the command line is read, it produces the output of the command. Process substitution can be used in place of a pipeline, allowing variables defined within a loop to be visible to the rest of the script. In this snippet, totalsize is not available to the script outside the loop: $ ls -l | > while read perms links owner group size month day time file > do > totalsize=$(( ${totalsize:=0} + ${size:-0} )) > done $ echo ${totalsize-unset} ## print "unset" if variable is not set unset By using process substitution instead, it becomes available: $ while read perms links owner group size month day time file > do > printf "%10d %s\n" "$size" "$file" > totalsize=$(( ${totalsize:=0} + ${size:-0} )) > done < <(ls -l *) $ echo ${totalsize-unset} 12879 Parsing Options The options to a shell script, single characters preceded by a hyphen, can be parsed with the built-in command getopts.