#Introduction
This a parser for a csh-like syntax. It is to be used for the ush shell project.
There are five files:
. README this file . Makefile . main.c driver program that reads and prints commands . parse.c define parsing function and other support functions . parse.h
To use in your program, you must include parse.h and call the function parse(), which return a Pipe data structure. (See main.c) The Pipe contains all the information in a line of input to a shell.
ush - the micro (mu) shell command interpreter.
ush
ush is a command interpreter with a syntax similar to UNIX C shell, csh(1). However, it is for instructional purposes only, therefore it is much simpler.
When first stared, ush normally performs commands from the file ̃/.ushrc, provided that it is readable. Commands in this file are processed just the same as if they were taken from standard input.
After startup processing, an interactive ush shell begins reading commands from the terminal, prompting with hostname%. The shell then repeatedly performs the following actions: a line of command input is read and broken into words; this sequence of words is parsed (as described under Usage); and the shell executes each command in the current line.
The shell splits input lines into words separated by whitespace (spaces or tabs), with the following exceptions:
- The special characters &, |, ;, <, and > and the multi-character sequences >>, |&, >& and >>& are always separate words, whether or not they are surrounded by whitespace.
- Special characters preceded by a backslash character () are not interpreted by the shell. Tw o back- slashes together sends the backslash character to the shell; otherwise the backslash is stripped from the input.
- Characters enclosed in double quotes (") or single quotes (’) form a single word. Special characters inside of strings do not form separate words.
- A simple command is a sequence of words, the first of which specifies the command to be executed. A pipeline is a sequence of one or more simple commands separated by | or |&. With |, the standard output of the preceding command is redirected to the standard input of the command that follows. With |&, both the standard error and the standard output are redirected through the pipeline.
- A list is a sequence of one or more pipelines separated by ; or &. Pipelines joined into sequences with ; will be executed sequentially. Pipelines ending with & are executed asynchronously. In which case, the shell does not wait for the pipeline to finish; instead, it displays the job number (see Job Control) and associated process ID, and begins processing the subsequent pipelines (prompting if necessary).
The following separators indicate that the subsequent word is the name of a file to which the command’s standard input, standard output, or standard error is redirected.
- < Redirect the standard input.
-
, >& Redirect the standard output to a file. If file does not exist, it is created. If it does exist, it is overwritten and its previous contents are lost. The & form redirects both standard output and standard error to the file.
-
, >>& Append the standard output. Like >, but places output at the end of the file rather than overwriting. The & form appends both standard output and standard error to the file.
If the command is an ush shell built-in, the shell executes it directly. Otherwise, the shell searches for a file by that name with execute access. If the command-name contains a /, the shell takes it as a pathname and searches for it. If a pathname begins with a /, then the path is absolute; otherwise, the path is relative to the current working directory. If the command-name does not contain a /, the shell attempts to resolve it to a pathname, searching each directory in the PATH variable for the command.
When a file, with its pathname resolved, is found that has proper execute permission, the shell forks a new process to run the command. The shell also passes along any arguments to the command. Using one of the flavor of exec system call, such as execv(2V), the newly forked process attempts to overlay the desired program. If successful, the shell is silent.
If the file does not have execute permissions, or if the pathname matches a directory, a ‘‘permission denied’’ message is displayed. If the pathname cannot be resolved a ‘‘command not found’’ message is displayed. If either of these errors occurs with any component of a pipeline the entire pipeline is aborted, even though some of the pipeline may already be executing.
Environment variables may be accessed via the setenv built-in commands. When a program is exec’d the environment variables are passed as parameters to execv or equivalent.
The shell ignores QUIT signals. Background jobs are immune to signals generated from the keyboard, including hangups (HUP). Other signals have the values that ush inherited from its environment. Ush catches the TERM signal.
The shell associates a numbered job with each command sequence, to keep track of those commands that are running in the background or have been stopped with TSTP signals (typically CTRL-Z). When a command is started in the background using &, the shell displays a line with the job number in brackets and the process number; e.g., [1] 2345
To see the current list of jobs, use the jobs built-in command. The job most recently stopped (or put into the background if none are stopped) is referred to as the current job.
To manipulate jobs, refer to the built-in commands bg, fg, kill.
A reference number to a job begins with a ‘%’. To refer to job number j use %j, as in: bg %j.
While running interactively, the shell tracks the status of each job and reports whenever it finishes or becomes blocked. It displays a message to this effect as it issues a prompt, so as to avoid disturbing the appearance of your input.
Built-in commands are executed within ush. If a built-in command occurs as any component of a pipeline except the last, it is executed in a subshell.
- bg %job Puts the specified job into the background.
- cd [dir] Change the working directory of the shell to dir, provided it is a directory and the shell has the appropriate permissions. Without an argument, it changes the working directory to the original (home) directory.
- fg %job Brings the specified job into the foreground.
- echo word ... Writes each word to the shell’s standard output, separated by spaces and terminated with a newline.
- jobs Lists the active jobs.
- kill %job Send the TERM (terminate) signal to the indicated job
- logout Exits the shell.
- nice [[+/-]number] [command] Sets the scheduling priority for the shell to number, or, without number, to 4. With command, runs command at the appropriate priority. The greater the number, the less cpu the process gets. If no sign before the number, assume it is positive.
- pwd Prints the current working directory.
- setenv [VAR [word]] without arguments, prints the names and values of all environment variables. Given VAR, sets the environment variable VAR to word or, without word, to the null string.
- unsetenv VAR Removes environment variable whose name matches VAR.
- where command Reports all known instances of command, including builtins and executables in path.
̃/.ushrc Read at the beginning of execution by each shell.
csh(1), dup(2), execv(2), fork(2), killpg(2), pipe(2), sigvec(2), vfork(2), wait(2), environ(7), Introduction to the C Shell