Section 10 - Special-use Commands

Comparing Two Files

Using the diff Command

The diff command is used to display line-by-line differences between pairs of text files. The format for the diff command is:

diff is a differential file-comparing command. When run on regular files, diff tells what lines must be changed in the files to bring them into agreement. Except in rare circumstances, diff finds the smallest sufficient set of differences. If neither filename1 nor filename2 is a directory, either may be given as "-", in which case the standard input is used. If filename1 is a directory, a file in that directory, whose filename is the same as the filename of filename2, is used (and vice versa).

There are several options for output format; the default output format contains lines of these forms:

Following each of these lines come all the lines that are affected in the first file flagged by "<", then all the lines that are affected in the second file are flagged by ">".

If both arguments are directories, diff sorts the contents of the directories by name and then runs the regular file diff program as described above on text files that are different. Binary files that differ, common subdirectories, and files that appear in only one directory are listed.

diff Options

Searching for a Text String, Counting Words and Lines

Using the grep Command

The grep, egrep, and fgrep commands are used to search a file for a string or regular expression. The formats for the commands are:

egrep [-bchilnsv][-e expression][-f filename][expression][filename... ]

fgrep [-bchilnsvx][-e string][-f filename][string][filename... ]

Commands of the grep family search the input filenames (the standard input default) for lines matching a pattern. Normally, each line found is copied to the standard output. grep patterns are limited, regular expressions in the style of ed. egrep patterns are full, regular expressions including alternation. fgrep patterns are fixed strings--no regular expression metacharacters are supported.

In general, egrep is the fastest of these programs.

Take care when using the characters $, *, [, ^, |, (, ), and \ in the expression, since these characters are also meaningful to the shell. It is safest to enclose the entire expression argument in single quotes ('...').

When any one of the grep utilities is applied to more than one input file, the name of the file is displayed preceding each line that matches the pattern. The filename is not displayed when processing a single file, so if you actually want the filename to appear, use /dev/null as a second file in the list.

grep Family Options

Using the wc Command

The wc command is used to display a count of lines, words, and characters. The format of the wc command is:

wc counts lines, words, and characters in filenames or in the standard input, if no filename appears. It also keeps a total count for all named files. A word is a string of characters delimited by space, tab, or newline characters.

wc Options

When filenames are specified on the command line, their names will be printed along with the counts.

The default is -lwc (count lines, words, and characters).

Searching for a File

Using the find Command

The find command is used to find files by name or by other characteristics. The format of the findcommand is:

find recursively descends the directory hierarchy for each pathname in the pathname-list, seeking files that match a logical expression written using the operators listed under the next topic.

find does not follow symbolic links to other files or directories; it applies the selection criteria to the symbolic links themselves as if they were ordinary files (see ln for a description of symbolic links).

Using find Operators

In the descriptions, the argument n is used as a decimal integer where +n means more than n, -n means less than n, and n means exactly n.

Searching for a Command

UNIX which Command

The which command is used to locate a command and display its pathname or alias. The format of the which command is:

The which command takes a list of names and looks for the files that would be executed had these names been given as commands. Each argument is expanded if it is aliased and searched for along the user's path. Both aliases and the path are taken from the user's .cshrc file.

Making a Tape Archive

Using the tar Command

The tar command is used to make a tape (or other media) archive file. The format of the tar command is:

tar provides a way to store many files into a single archive. This can be kept in another UNIX file; stored on an I/O device, such as tape, floppy, cartridge, or disk; sent over a network; or piped to another program. It is useful for making backup copies or for packaging up a set of files to move them to another system.

tar has existed since Version 7 UNIX with very little change. It has been proposed as the standard format for interchange of files among systems that conform to the IEEE P1003 "Portable Operating System" standard.

tar Options

Execution of a Command Prior to Each Prompt

T-shell supports a special alias, precmd, which, if set, holds a command that will be executed before printing each prompt. For example, if the user has done

then the program date will be run just before the shell prompts for each command. There are no limitations on what precmd can be set to do, although discretion should be used.

Periodic Command Execution

T-shell is now capable of providing periodic command execution through the use of the shell variable tperiod and the alias periodic. When these items are set, the alias periodic will be executed every tperiod minutes. This provides a convenient means for checking on common but infrequent changes, such as new messages. For example:

This will cause the checknews program to be run every 30 minutes. Having the alias periodic set, but with an unset tperiod (or a value of 0 for tperiod), will cause periodic to degenerate to another form of precmd.

T-shell Prompt Format

The format for the prompt shell variable includes many things, such as the current time of day, and current working directory. The new format uses %<char> to signal an expansion, much like printf. The available sequences are:

The sequences for standout are often used to indicate that this is an enabled (running as root) shell. An example:

In addition, there is a variable, prompt2, which is used to prompt for the body of while and for loops (wherever normal, C-shell prompts with a question mark). The default for prompt2 is %R?--the status of the parser followed by a question mark. This alternate prompt is also used when the parser is waiting for more input; i.e., when the previous line ended in a \. The prompt3 variable is used when displaying the corrected command line when automatic spelling correction is in effect; it defaults to CORRECT>%R (y|n|e)?.

T-shell Timed

Event List

T-shell supports a scheduled-event list through the use of the command sched. This command gives the user a mechanism by which to arrange for other commands to be executed at given times. An event is added to the scheduled-event list by saying

as in

This will make an entry in the list at 11:00 am for the echo command to be run with the given arguments. The time may be specified in either absolute or relative time, and absolute times may have a morning/afternoon specification as well using "am" or "pm."

For example,

     > sched +2:15 /usr/lib/uucp/uucico -r1 -sother

     > sched 5pm set prompt='[%h] It\'s after 5; go home: >'

     > sched +3am echo This syntax doesn\'t work.

     Relative time inconsistent with am/pm.

     >

NOTE: T-shell will complain, if you try to make faulty time specifications.

Printing the current time-event list is accomplished by giving the sched command with no arguments:

     > sched

        1 Wed Apr 4 15:42 /usr/lib/uucp/uucico -r1 -sother

        2 Wed Apr 4 17:00 set prompt=[%h] It's after 5; go home: >

     >

There is also a mechanism by which the user can remove an item from the list:

     > sched --3

     Usage for delete: sched -<item#>.

     > sched -3

     Not that many scheduled events.

     > sched -2

     > sched

        1 Wed Apr 4 15:42 /usr/lib/uucp/uucico -r1 -sother

     >

All commands specified on the scheduled-event list will be executed just prior to printing the first prompt immediately following the time when the command is to be run. Hence, it is possible to miss the exact time when the command is to be run, but T-shell will definitely get around to all commands that are overdue at its next prompt. Scheduled-event list items that come due while T-shell is waiting for user input will be executed immediately. In no case, however, will normal operation of already-running commands be interrupted so that a scheduled-event list element may be run.

This mechanism is similar to, but not the same as, the at command on some UNIX systems. Its major disadvantage is that it does not necessarily run a command at exactly the specified time (but only if another command is already being run). Its major advantage is that commands that run directly from T-shell, as sched commands are, have access to shell variables and other structures. This provides a mechanism for changing one's working environment based on the time of day.

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