System User's Guide: Operating System and Devices

C Shell Commands

A simple command is a sequence of words separated by blanks or tabs.

A word is a sequence of characters or numerals, or both, that does not contain blanks without quotation marks. In addition, the following characters and doubled characters also form single words when used as command separators or terminators:

&       |      ;        
&&      ||    <<        > >
<       >     (          )

These special characters can be parts of other words. Preceding them with a \, however, prevents the shell from interpreting them as special characters. Strings enclosed in ' ' or " " (matched pairs of quotation characters) or backquotes can also form parts of words. Blanks, tab characters, and special characters do not form separate words when they are enclosed in these marks. In addition, you can enclose a new-line character within these marks by preceding it with a \.

The first word in the simple command sequence (numbered 0) usually specifies the name of a command. Any remaining words, with a few exceptions, are passed to that command. If the command specifies an executable file that is a compiled program, the shell immediately runs that program. If the file is marked executable but is not a compiled program, the shell assumes that it is a shell script. In this case, the shell starts another instance of itself (a subshell) to read the file and execute the commands included in it.

This section discusses:

• C Shell Built-In Commands
• C Shell Expressions and Operators
• Command Substitution in the C Shell
• Nonbuilt-in C Shell Command Execution
• C Shell Related Information

C Shell Built-In Commands

Built-in commands are run within the shell. If a built-in command occurs as any component of a pipeline, except the last, the command runs in a subshell.

Note: If you enter a command from the C shell prompt, the system searches for a built-in command first. If a built-in command does not exist, the system searches the directories specified by the path shell variable for a system-level command. Some C shell built-in commands and operating system commands have the same name. However, these commands do not necessarily work the same way. Check the appropriate command description for information on how the command works.

If you run a shell script from the shell and the first line of the shell script begins with #!/ShellPathname, the C shell runs the shell specified in the comment to process the script. Otherwise, it runs the default shell (the shell linked to /usr/bin/sh). If run by the default shell, C shell built-in commands might not be recognized. To get the system to run C shell commands, make the first line of the script #!/usr/bin/csh.

Refer to the List of C Shell Built-in Commands for an alphabetic listing of the built-in commands.

C Shell Command Descriptions

The C shell provides the following built-in commands:

time sleep

0.0u 0.0s 0:00 100% 44+4k 0+0io 0pf+0w

C Shell Expressions and Operators

The @ built-in command and the exit, if, and while statements accept expressions that include operators similar to those of C language, with the same precedence. The following operators are available:

In the above list, precedence of the operators decreases down the list - left to right, top to bottom.

Note: The operators + and - are right associative. For example, evaluation of a + b - c is performed as follows:

a + (b - c)

and not:

(a + b) - c

The ==, !=, =~, and !~ operators compare their arguments as strings; all others operate on numbers. The =~ and !~ operators are similar to == and != , except that the rightmost side is a pattern against which the leftmost operand is matched. This reduces the need for use of the switch statement in shell procedures when all that is really needed is pattern matching.

The logical operators or (||) and and (&&) are available as well. They can be used, for example, to check for a range of numbers:

if ($#argv > 2 && $#argv < 7) then

In the preceding example, the number of arguments must be greater than 2 and less than 7.

Strings beginning with zero (0) are considered octal numbers. Null or missing arguments are considered 0. All expressions result in strings representing decimal numbers. Note that two components of an expression can appear in the same word. Except when next to components of expressions that are syntactically significant to the parser (& | < > ( )), expression components should be surrounded by spaces.

Also available in expressions as primitive operands are command executions enclosed in ( ) and file inquiries of the form (-operator Filename), where operator is one of the following:

The specified Filename is command and file name expanded and then tested to see if it has the specified relationship to the real user. If Filename does not exist or is inaccessible, then all inquiries return false(0). If the command runs successfully, the inquiry returns a value of true(1). Otherwise, if the command fails, the inquiry returns a value of false(0). If more detailed status information is required, run the command outside an expression and then examine the status shell variable.

Command Substitution in the C Shell

In command substitution, the shell executes a specified command and replaces that command with its output. To perform command substitution in the C shell, enclose the command or command string in backquotes (` `). The shell normally breaks the output from the command into separate words at blanks, tabs, and new-line characters. It then replaces the original command with this output.

In the following example, the backquotes (` `) around the date command indicate that the output of the command will be substituted:

echo The current date and time is: `date`

The output from this command might look like:

The current date and time is: Wed Apr 8 13:52:14 CDT 1992

The C shell performs command substitution selectively on the arguments of built-in shell commands. This means that it does not expand those parts of expressions that are not evaluated. For commands that are not built-in, the shell substitutes the command name separately from the argument list. The substitution occurs in a child of the main shell, only after the shell performs input or output redirection.

If a command string is surrounded by " ", the shell treats only new-line characters as word separators, thus preserving blanks and tabs within the word. In all cases, the single final new-line character does not force a new word.

Nonbuilt-in C Shell Command Execution

When the C shell determines that a command is not a built-in shell command, it attempts to run the command with the execv system call. Each word in the path shell variable names a directory from which the shell attempts to run the command. If given neither the -c nor -t flag, the shell hashes the names in these directories into an internal table. The shell tries to call the exec system call on a directory only if there is a possibility that the command resides there. If you turn off this mechanism with the unhash command or give the shell the -c or -t flag, the shell concatenates with the given command name to form a path name of a file. The shell also does this in any case for each directory component of the path variable that does not begin with a /. The shell then attempts to run the command.

Parenthesized commands always run in a subshell. For example:

(cd ; pwd) ; pwd

displays the home directory without changing the current directory location. However, the command:

cd ; pwd 

changes the current directory location to the home directory. Parenthesized commands are most often used to prevent the chdir command from affecting the current shell.

If the file has execute permission, but is not an executable binary to the system, then the shell assumes it is a file containing shell commands and runs a new shell to read it.

If there is an alias for the shell, then the words of the alias are prefixed to the argument list to form the shell command. The first word of the alias should be the full path name of the shell.