Csh capture error

csh capture error

1. FILE DESCRIPTORS The most common problem encountered in csh programming is that you can't do file-descriptor manipulation. All you are able to do is redirect. Overwrite (y/n)? The problem is that if I redirect output of the shell program to a file, I never see the error message, and end up wondering what. The use of the terminal bell to signal errors or multiple matches can be inhibited by setting the variable nobeep. Normally, all files in the particular.

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Home lpr' to make a command that pr's its arguments to the line printer.

Variable substitution

The shell maintains a set of variables, each of which has as value a list of zero or more words. Some of these variables are set by the shell or referred to by it. For instance, the argv variable holds the shell's argument list, and words of this variable's value are referred to in special ways.

The values of variables may be displayed and changed using the set and unset commands. Of the variables referred to by the shell a number are toggles; the shell does not care what their value is, only whether they are set or not. For instance, the verbose variable is a toggle that causes command input to be echoed. The setting of this variable results from the -v command-line option.

Other operations treat variables numerically. The @ command permits numeric calculations to be performed and the result assigned to a variable. Variable values are, however, always represented as (zero or more) strings. For numeric operations, the nullstring is considered to be zero, and the second and additional words of multiword values are ignored.

After the input line is aliased and parsed, and before each command is executed, variable substitution is performed, keyed by $ characters. This expansion can be prevented by preceding the $ with a \ except within double quotes ("), where substitution always occurs, and within single quotes ('), where it never occurs. Strings quoted by backticks (` `) are interpreted later (see Command Substitution, below), so $ substitution does not occur there until later, if at all. A $ is passed unchanged if followed by a blank, tab, or end-of-line.

Input/output redirections are recognized before variable expansion, and are variable expanded separately. Otherwise, the command name and entire argument list are expanded together. It is thus possible for the first (command) word (to this point) to generate more than one word, the first of which becomes the command name, and the rest of which become arguments.

Unless enclosed in " or given the :q modifier, the results of variable substitution may eventually be command and file name substituted. Within ", a variable whose value consists of multiple words expands to (a portion of) a single word, with the words of the variable's value separated by blanks. When the :q modifier is applied to a substitution, the variable expands to multiple words with each word separated by a blank and quoted to prevent later command or file name substitution.

The following metasequences are provided for introducing variable values into the shell input. Except as noted, it is an error to reference a variable that is not set.


Are replaced by the words of the value of variable name, each separated by a blank. Braces insulate name from following characters that would otherwise be part of it. Shell variables have names consisting of up to 20 letters and digits starting with a letter. The underscore character is considered a letter. If name is not a shell variable, but is set in the environment, then that value is returned (but : modifiers and the other forms given below are not available here).

May be used to select only some of the words from the value of name. The selector is subjected to $ substitution and may consist of a single number or two numbers separated by a -. The first word of a variable's value is numbered 1. If the first number of a range is omitted it defaults to 1. If the last number of a range is omitted it defaults to $#name. The selector* selects all words. It is not an error for a range to be empty if the second argument is omitted or in range.

Gives the number of words in the variable. This is useful for later use in a "$argv[selector]".
$0Substitutes the name of the file from which command input is being read. An error occurs if the name is not known.

Equivalent to "$argv[number]".
$*Equivalent to "$argv[*]".

The modifiers :e, :h, :t, :r, :q, and :x may be applied to the substitutions above as may :gh, :gt, and :gr. If braces {} appear in the command form, then the modifiers must appear in the braces. The current implementation allows only one : modifier on each $ expansion.

The following substitutions may not be modified with : modifiers.


Substitutes the string "1" if name is set, "0" if it's not.
$?0Substitutes 1 if the current input file name is known, 0 if it's not.
$$Substitute the (decimal) process number of the (parent) shell. Do NOT use this mechanism for generating temporary file names; see mktemp instead.
$!Substitute the (decimal) process number of the last background process started by this shell.
$<Substitutes a line from the standard input, with no further interpretation. It can read from the keyboard in a shell script.

Command and file name substitution

The remaining substitutions, command and file name substitution, are applied selectively to the arguments of built-in commands. Here, "selectively" means that portions of expressions that are not evaluated are not subjected to these expansions. For commands that are not internal to the shell, the command name is substituted separately from the argument list. This occurs very late, after input-output redirection is performed, and in a child of the main shell.

Command substitution

Command substitution is shown by a command enclosed in `. The output from such a command is normally broken into separate words at blanks, tabs, and newlines, with null words being discarded; this text then replaces the original string. Within double quotes ("), only newlines force new words; blanks and tabs are preserved.

In any case, the single final newline does not force a new word. Note that it is therefore possible for a command substitution to yield only part of a word, even if the command outputs a complete line.

File name substitution

If a word contains any of the characters *, ?, [, or {, or begins with the character ~, then that word is a candidate for file name substitution, also known as "globbing". This word is then regarded as a pattern, and replaced with an alphabetically sorted list of file names that match the pattern. In a list of words specifying file name substitution it is an error for no pattern to match an existing file name, but it is not required for each pattern to match. Only the metacharacters *, ?, and [ imply pattern matching, the characters ~ and { being more akin to abbreviations.

In matching file names, the character . at the beginning of a file name or immediately following a /, and the character / must be matched explicitly. The character * matches any string of characters, including the null string. The character ? matches any single character.

The sequence "[...]" matches any one of the characters enclosed. Within "[...]", a pair of characters separated by - matches any character lexically between the two (inclusive). Within "[...]", the name of a character class enclosed in [: and :] stands for the list of all characters belonging to that class. Supported character classes:

  • alnum
  • alpha
  • blank
  • cntrl
  • digit
  • graph
  • lower
  • print
  • space
  • upper
  • punct
  • xdigit

These match characters using the macros specified in ctype. A character class may not be used as an endpoint of a range.

The character ~ at the beginning of a file name refers to home directories. Standing alone, i.e., ~, it expands to the invoker's home directory as reflected in the value of the variable home. When followed by a name consisting of letters, digits, and - characters, the shell searches for a user with that name and substitutes their home directory; thus "~ken" might expand to "/usr/ken" and "~ken/chmach" to "/usr/ken/chmach". If the character ~ is followed by a character other than a letter or /, or does not appear at the beginning of a word, it is left undisturbed.

The metanotation "a{b,c,d}e" is a shorthand for "abe ace ade". Left to right order is preserved, with results of matches being sorted separately at a low level to preserve this order. This construct can be nested. Thus, "~source/s1/{oldls,ls}.c" expands to "/usr/source/s1/oldls.c /usr/source/s1/ls.c" without chance of error if the home directory for "source" is "/usr/source". Similarly "../{memo,*box}" might expand to "../memo ../box ../mbox". Note that "memo" was not sorted with the results of the match to "*box". As a special case {, }, and {} are passed undisturbed.


The standard input and the standard output of a command may be redirected with the following syntax:

<nameOpen file name (which is first variable, command, and file name expanded) as the standard input.
<<wordRead the shell input up to a line that is identical to word. The word is not subjected to variable, command, or file name substitution, and each input line is compared to word before any substitutions are done on the input line. Unless a quoting \, ", ' or ` appears in word, variable and command substitution is performed on the intervening lines, allowing \ to quote $, \ and `. Commands that are substituted have all blanks, tabs, and newlines preserved, except for the final newline that is dropped. The resultant text is placed in an anonymous temporary file that is given to the command as its standard input.



The file name is used as the standard output. If the file does not exist then it is created; if the file exists, it is truncated; its previous contents are lost.

If the variable noclobber is set, then the file must not exist or be a character special file (e.g., a terminal or /dev/null) or an error results. This helps prevent accidental destruction of files. Here, the ! forms can suppress this check.

The forms involving & route the standard error output to the specified file and the standard output. The name is expanded in the same way as < input file names are.



Uses file name as the standard output; like > but places output at the end of the file. If the variable noclobber is set, then it is an error for the file not to exist unless one of the ! forms is given. Otherwise, similar to >.

A command receives the environment in which the shell was invoked as modified by the input-output parameters and the presence of the command in a pipeline. Thus, unlike some previous shells, commands run from a file of shell commands have no access to the text of the commands by default; instead they receive the original standard input of the shell. The << mechanism should be used to present inline data. This permits shell command scripts to function as components of pipelines and allows the shell to block read its input. Note that the default standard input for a command run detached is not modified to be the empty file /dev/null; instead the standard input remains as the original standard input of the shell. If this is a terminal and if the process attempts to read from the terminal, then the process blocks and the user is notified (see Jobs above).

The standard error output may be directed through a pipe with the standard output. Use the form Contents Next grep Jul

In the example, the output of the ls command is the input for the grep command.

Displaying Program Output and Copying It to a File (tee command)

The tee command, used with a pipe, reads standard input, then writes the output of a program to standard output and simultaneously copies it into the specified file or files. This gives you the advantage of viewing your output immediately and storing it for future use at the same time.

For example, to view and save the output from a command at the same time, enter:

ps -ef 

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Csh capture error
Csh capture error

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