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Wizard Boot Camp, Part One: Linux, Shells & Commands

Here's the start of a series on little-known topics that wizards should know. Knowledge of the Harry Potter series not required.

Over the past five years, this column has covered a lot of not-so-obvious features of Linux — mostly techniques that you’d use from the shell or another non-graphical environment. The start of your columnist’s sixth year seemed like a good time to try something a little different: a series of columns on features of Linux and shells that many books about Linux basically ignore, but that wizards should know. Studying these obscure features isn’t just for trivia buffs. Understanding how each feature works can help you grok “the big picture” of some pieces that make your favorite operating system the power platform that it is.

This month, let’s start with a quick overview of Linux and shell concepts that you should know, then dig into some shell features that might be new to you.

Linux Overview in Ten Paragraphs

Let’s review some of the fundamentals to start from the same place. (Please note: this isn’t an exact, complete description. Some details are simplified or omitted.)

A Linux system manages a set of processes, letting them share system resources (the processors, disks, networks, and so on). Basically, a process is a running program, a program waiting to run, or a suspended program. A process has a name (often the name of the program file that it’s executing) and a list of arguments (for instance, some filenames). Each process has a unique process ID, or PID, that’s assigned when the process starts. The ps utility lists processes.

A process runs under the user ID, or UID, of one of the accounts on the system. In most cases, a process runs with the UID of the account it was started under. So, if user joe starts a text editor, that process inherits the abilities that joe has: for instance, it can read and write his files. The superuser, root, has UID 0. A process running under UID 0 is privileged: it can change its UID (actually, its effective UID) to any other UID on the system. The su and sudo programs, which run under UID 0, allow any user to run processes as another user.

A parent process can start other processes called child processes. A child process inherits many of its parent’s attributes — as you’ll see next. A child process can also start its own child processes. The process named init, with PID 1, is the parent (or “grandparent”, or great-grandparent, and so on) of all processes. After a parent process starts a child process, it typically waits for the child to finish. However, if the parent doesn’t wait, it can “disown” the child, and that “orphaned” process becomes a child of init.

A process has attributes including: a current directory (the starting place for relative pathnames); a list of environment variables (for example, EDITOR=emacs or EDITOR=vi); and a list of open files for input/output.

The open files include three as standard input and output: the standard input, abbreviated stdin, file descriptor number 0; standard output, stdout, with descriptor 1; and standard error, stderr, with file descriptor 2.

A child process inherits attributes from its parent. For instance, the child starts in the same current directory as its parent, and has the same open files and environment variables. Once the child begins running, it can manage these attributes, and any changes made in the child process don’t affect its parent.

When a process starts, its standard I/O files are often connected to a terminal device, such as an X Window System terminal or the system console. So, for instance, if a process prints a message to its stdout, the text appears on the terminal. Because a child process inherits its parent’s open files, it too writes to and reads from the same terminal (by default) as its parent.

When a process exits, it returns a numeric exit status to its parent. By convention, a zero status signals success and a non-zero status means there was some kind of error.

If a process was started under an X Window System, it can also open windows there. Then, although that process also has standard I/O available, it generally won’t use them.

Shells in Five Easy Steps

One particularly interesting kind of program is the shell. A shells is a program designed especially to run other programs (usually, non-shell programs– though a shell can start a child shell). Here’s how a shell works:

1.The shell outputs a prompt and waits for input (unless the shell is reading a shell script file).

2.It reads the command-line, parses the command-line to find the command names, handles operators like | and > (which tell the shell to redirect the child’s standard I/O), and interprets any arguments (and special characters in them, such as wildcards).

3.The shell then starts one or more child processes to run the specified program (s), unless the command you specified is built-in to the shell. (For example, the built-in command cd changes the shell’s current directory. It doesn’t start a child process because the command needs to affect the current process.)

4.It waits for the program to finish (unless you suspend the program after it’s started running by typing Control-Z, or if you started the program in the background by typing & at the end of the command-line).

5.The shell then repeats step 1, unless the script file ends or you used the built-in command exit, in which case the shell process terminates.

There are several different shells. This series of columns cover bash.

How Shells Get Commands

A shell is a command interpreter (as you saw in the previous section) that reads commands and runs them. Commands can come from several places:

*A shell can run a command that you pass as a single argument when the shell is invoked. You might do this from a non-shell program that needs to run a command-line entered by a user. Most shells have a –c option for this purpose. For example, to cd to the user’s subdirectory named lib, list its contents, and rename a file, another program could execute:

$ sh -c "cd ~/lib && ls; mv afile afile.old"

The shell expands ~ into the absolute pathname of the user’s home directory; the result is passed to cd. The && operator executes ls only if cd succeeded. The ; operator executes mv after ls finishes.

*If you pass an argument that isn’t associated with an option, the shell opens that argument as a file and reads command-lines from it. This is how shell scripts are read.

This method and the previous one make the shell non-interactive: the shell doesn’t prompt a user for input, it simply reads one or more commands and executes them. After the last child process exits, the shell process itself exits.

*A shell invoked without any arguments or only with certain options runs in interactive mode. After reading initialization files like .bashrc, the shell outputs a prompt string and waits to read input from stdin.

The latter is the most complex (and interesting) way that commands are input. Let’s look into it some more.

Linux newbies think of a shell reading everything they type on the keyboard until the Return (Enter) key is pressed. That’s the simplest case. Shells like bash with built-in command line editing can edit multiple-line commands like while and for loops. You might be wondering how multi-line statements like these are entered from a prompt.

As you’re entering a command-line, the shell reads the line you’ve typed (actually, the text from its standard input) when you press Enter. The shell checks the line to be sure it’s a complete statement. If it’s not complete, bash prints its secondary prompt, >, and keeps reading input until it finds the end of the statement.

What’s an incomplete statement? Here are some examples:

*A line with unmatched single quotes (), double quotes (), and backquotes (`); unquoted opening parentheses ( and curly braces {; and so on.

For instance, to make a shell script output a multi-line message, you don’t need to use one echo command for each line. Instead, pass a multi-line argument (containing newline characters) to echo, surrounded by quote marks.

$ echo 'This is the first line
and this is the second line.'

Here’s how that example looks when it’s typed at the primary ($) and secondary (>) shell prompts:

$ echo 'This is the first line
> and this is the second line.'
This is the first line
and this is the second line.

*A loop before you type done. Here’s a for loop that finds all TIFF images (all filenames ending with .tif) in the current directory), strips off the .tif, and uses the ImageMagick convert utility to create a JPEG image:

$ for file in *.tif
> do
>   echo "doing $file..."
>   base=${file%.tif}
>   convert "$file" "$base.jpg"
> done

*An unfinished pipeline. Typing a pipe symbol at the end of a line tells the shell that there’s another command to be read. Writing pipelines this way can make a shell script easier to read. You can intersperse comments, too:

# List the 20 largest files:
ls -S | head -20 |
# open and search for errors:
xargs grep ERROR |
# write result to logging host:
ssh $loghost "cat > errors"

Problematic Ways that Shells Get Commands

The previous section discussed shells reading commands interactively from a terminal — actually, from standard input. A shell can also read commands from its standard input non-interactively, by redirection from another process. Here are two examples:

$ bash < script-file
$ command-generator | bash

script-file is a shell script file, and command-generator is a program that outputs shell command lines for bash to read and execute.

The problem here is that the standard input of bash is redirected. So, if any of the commands in script-file, or in the output of command-generator, try to read from the standard input, they’ll take input from script-file or command-generator, which is almost certainly not the right place! (Because child processes inherit the parent’s standard I/O channels, the programs that bash runs get standard input from the same place as bash does.)

This is a big “gotcha” in redirecting a shell’s standard input. You may be able to work around this with the shell operator m<&n (which we’ll cover in November), but it’s likely to be painful at best.

The Shell Expands Wildcards

A wildcard is a character like * or ? that stands for other characters in a filename or pathname. Some operating systems require each program to interpret wildcards. Linux doesn’t.

Linux shells interpret wildcards. They replace arguments containing wildcards with a list of one or more matching names. (However, shells don’t replace quoted wildcards. For example, in the command find*foo/.-name”bar*”, the shell replaces *foo/. with a list of all subdirectory names ending with foo, like myfoo/., hisfoo/., and so on. But the shell passes bar* to find literally, so that find can interpret that argument in each of the subdirectories it visits.)

If an unquoted wildcarded argument doesn’t match any pathnames, what happens depends on the shell you’re using and its settings. By default, bash simply passes the unexpanded wildcard to the command you’re running and lets the command complain if it wants to. Most shells can also be set to print an error and not run the command; that’s the default for tcsh.

Shell Quoting

It’s common knowledge that quoting special characters — that is, preceding a special character with a backslash (like afile\*) or surrounding its argument with quotes (like “afile*” or ‘afile*’) — disables those characters’ special meanings. The shell strips off the quoting character (s) and passes the special character, as-is, as part of the argument. Hence, cp afile*somedir copies all filenames starting with afile into somedir, and cp afile\*somedir copies the file named afile* to somedir.

A pair of single quotes () is “stronger” than a pair of double quotes (). Within double quotes, a few special characters keep their meanings, including $ for variables (like cp”$var”somedir), and backquotes, as in message=”The files are: `ls`.”. Knowing this helps you decide what kind of quotes to use.

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