Linux Bible. Christopher Negus

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СКАЧАТЬ for the commands you enter. To see your current path, enter the following:

       $ echo $PATH /usr/local/bin:/usr/bin:/bin:/usr/local/sbin:/usr/sbin:/sbin:↵ /home/chris/bin

      The results show a common default path for a regular Linux user. Directories in the path list are separated by colons. Most user commands that come with Linux are stored in the /bin, /usr/bin, or /usr/local/bin directory. The /sbin and /usr/sbin directories contain administrative commands (some Linux systems don't put those directories in regular users' paths). The last directory shown is the bin directory in the user's home directory (/home/chris/bin).

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      If you want to add your own commands or shell scripts, place them in the bin directory in your home directory (such as /home/chris/bin for the user named chris). This directory is automatically added to your path in some Linux systems, although you may need to create that directory or add it to your PATH on other Linux systems. So, as long as you add the command to your bin with execute permission, you can begin using it by simply typing the command name at your shell prompt. To make commands available to all users, add them to /usr/local/bin.

      Unlike some other operating systems, Linux does not, by default, check the current directory for an executable before searching the path. It immediately begins searching the path, and executables in the current directory are run only if they are in the PATH variable or you give their absolute (such as /home/chris/scriptx.sh) or relative (for example, ./scriptx.sh) location.

The path directory order is important. Directories are checked from left to right. So, in this example, if there is a command called foo located in both the /usr/bin and /bin directories, the one in /usr/bin is executed. To have the other foo command run, you either type the full path to the command or change your PATH variable. (Changing your PATH and adding directories to it are described later in this chapter.)

      Not all of the commands you run are located in directories in your PATH variable. Some commands are built into the shell. Other commands can be overridden by creating aliases that define any commands and options that you want the command to run. There are also ways of defining a function that consists of a stored series of commands. Here is the order in which the shell checks for the commands you type:

      1 Aliases. These are names set by the alias command that represent a particular command and a set of options. Type alias to see what aliases are set. Often, aliases enable you to define a short name for a long, complicated command. (I describe how to create your own aliases later in this chapter.)

      2 Shell reserved word. These are words reserved by the shell for special use. Many of these are words that you would use in programming-type functions, such as do, while, case, and else. (I cover some of these reserved words in Chapter 7, “Writing Simple Shell Scripts.”)

      3 Function. This is a set of commands that is executed together within the current shell.

      4 Built-in command. This is a command built into the shell. As a result, there is no representation of the command in the filesystem. Some of the most common commands that you will use are shell built-in commands, such as cd (to change directories), echo (to output text to the screen), exit (to exit from a shell), fg (to bring a command running in the background to the foreground), history (to see a list of commands that were previously run), pwd (to list the present working directory), set (to set shell options), and type (to show the location of a command).

      5 Filesystem command. This command is stored in and executed from the computer's filesystem. (These are the commands that are indicated by the value of the PATH variable.)

      To determine the location of a particular command, you can use the type command. (If you are using a shell other than bash, use the which command instead.) For example, to find out where the bash shell command is located, enter the following:

       $ type bash bash is /bin/bash

Try these few words with the type command to see other locations of commands: which, case, and return. If a command resides in several locations, you can add the -a option to have all of the known locations of the command printed. For example, the command type -a ls should show an aliased and filesystem location for the ls command.

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      Sometimes, you run a command and receive an error message that the command was not found or that permission to run the command was denied. If the command was not found, check that you spelled the command correctly and that it is located in your PATH variable. If permission to run the command was denied, the command may be in the PATH variable but may not be executable. Also remember that case is important, so typing CAT or Cat will not find the cat command.

      If a command is not in your PATH variable, you can use the locate command to try to find it. Using locate, you can search any part of the system that is accessible to you. (Some files are only accessible to the root user.) For example, if you wanted to find the location of the chage command, you could enter the following:

       $ locate chage /usr/bin/chage /usr/sbin/lchage /usr/share/man/fr/man1/chage.1.gz /usr/share/man/it/man1/chage.1.gz /usr/share/man/ja/man1/chage.1.gz /usr/share/man/man1/chage.1.gz /usr/share/man/man1/lchage.1.gz /usr/share/man/pl/man1/chage.1.gz /usr/share/man/ru/man1/chage.1.gz /usr/share/man/sv/man1/chage.1.gz /usr/share/man/tr/man1/chage.1.gz

      Notice that locate not only found the chage command, it also found the lchage command and a variety of man pages associated with chage for different languages. The locate command looks all over your filesystem, not just in directories that contain commands. (If locate does not find files recently added to your system, run updatedb as root to update the locate database.)

      In the coming chapters, you learn to use additional commands. For now, I want you to become more familiar with how the shell itself works. So next I discuss features for recalling commands, completing commands, using variables, and creating aliases.

Recalling Commands Using Command History

      Being able to repeat a command you ran earlier in a shell session can be convenient. Recalling a long and complex command line that you mistyped can save you some trouble. Fortunately, some shell features enable you to recall previous command lines, edit those lines, or complete a partially typed command line.

The shell history is a list of the commands that you have entered before. Using the history command in a bash shell, you can view your previous commands. Then using various shell features, you can recall individual command lines from that list and change them however you please.

      The rest of this section describes how to do command-line editing, how to complete parts of command lines, and how to recall and work with the history list.

      Command-line editing

      If you type something wrong on a command line, СКАЧАТЬ