57Chapter 2Running Commands from the ShellVariableDescriptionPS1Sets the value (Msn web hosting)
57Chapter 2Running Commands from the ShellVariableDescriptionPS1Sets the value of your shell prompt. There are many items that youcan read into your prompt (date, time, username, host name, andso on). Sometimes a command requires additional prompts, whichyou can set with the variables PS2, PS3, and so on. PWDThis is the directory that is assigned as your current directory. Thisvalue changes each time you change directories using the cdcommand. RANDOMAccessing this variable causes a random number to be generated. The number is between 0 and 99999. SECONDSThe number of seconds since the time the shell was started. SHLVLThe number of shell levels associated with the current shell session. When you log in to the shell, the SHLVLis 1. Each time you start anew bashcommand (by, for example, using suto become a newuser, or by simply typing bash), this number is incremented. TMOUTCan be set to a number representing the number of seconds the shellcan be idle without receiving input. After the number of seconds isreached, the shell exits. This is a security feature that makes it lesslikely for unattended shells to be accessed by unauthorized people. (This must be set in the login shell for it to actually cause the shellto log out the user.) UIDThe user ID number assigned to your username. The user IDnumber is stored in the /etc/passwordfile. Set Your Own Environment VariablesEnvironment variables can provide a handy way of storing bits of information thatyou use often from the shell. You can create any variables that you want (avoidingthose that are already in use) so that you can read in the values of those variablesas you use the shell. (The bash manpage lists variables already in use.) To set an environment variable temporarily, you can simply type a variable nameand assign it to a value. Here s an example: $ AB=/usr/dog/contagious/ringbearer/grind ; export ABThis example causes a long directory path to be assigned to the ABvariable. Theexport ABcommand says to export the value to the shell so that it can be propa- gated to other shells you may open. With ABset, you go to the directory by typingthe following: $cd $AB07_
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