sed summary (cont)

Some basic POSIX groups:

\d = [[:digit:]], \D = [^[:digit:]].
\s = [[:whitespace:]], including space, tab .. ; \S = ?
\w = [[:alnum:]], including 0-9,a-z, A-Z; \W = ?

executing multiple commands with sed -e command

One method of combining multiple commands is to use a -e before each command:

sed -e 's/a/A/' -e 's/b/B/'
new

A "-e" isn't needed in the earlier examples because sed knows that there must always be one command. If you give sed one argument, it must be a command, and sed will edit the data read from standard input.

Reversing the restriction with !
Sometimes you need to perform an action on every line except those that match a regular expression, or those outside of a range of addresses. The "!" character, which often means not in Unix utilities, inverts the address restriction. You remember that

sed -n '/match/ p'acts like the grep command. The "-v" option to grep prints all lines that don't contain the pattern. Sed can do this with sed -n '/match/ !p'

Ranges by Line Number

You can specify a range on line numbers by inserting a comma between the numbers. To restrict a substitution to the first 100 lines, you can use:

sed '1,100 s/A/a/'

If you know exactly how many lines are in a file, you can explicitly state that number to perform the substitution on the rest of the file. In this case, assume you used wc to find out there are 532 lines in the file:

sed '101,532 s/A/a/'

An easier way is to use the special character "$," which means the last line in the file.

sed '101,$ s/A/a/'

The "$" is one of those conventions that mean "last" in utilities like cat -e, vi, and ed. "cat -e" Line numbers are cumulative if several files are edited. That is,

sed '200,300 s/A/a/' f1 f2 f3 >new

is the same as

cat f1 f2 f3 | sed '200,300 s/A/a/' >new

Transform with Y

If you wanted to change a word from lower case to upper case, you could write 26 character substitutions, converting "a" to "A," etc. Sed has a command that operates like the tr program. It is called the "y" command. For instance, to change the letters "a" through "f" into their upper case form, use:

sed 'y/abcdef/ABCDEF/' file

I could have used an example that converted all 26 letters into upper case, and while this column covers a broad range of topics, the "column" prefers a narrower format.

If you wanted to convert a line that contained a hexadecimal number (e.g. 0x1aff) to upper case (0x1AFF), you could use:

sed '/0x[0-9a-zA-Z]*/ y/abcdef/ABCDEF' file

This works fine if there are only numbers in the file. If you wanted to change the second word in a line to upper case, you are out of luck - unless you use multi-line editing. (Hey - I think there is some sort of theme here!)

the check archive command for tar and gzip

TAR:
tar -tvf file.tar WILL LIST ALL the files in the file.tar archive.

GZIP
gzip -l

extract a file from the .tar file

Under stdsun:
Take a file named with backup.tar for example. We want to extract the file named update_log inside the backup directory.

tar -xvf backup.tar backup/update__log

Done!
If the file is .tar.gz, we can similarly use
tar -xzvf backup.tar.gz backup/update__log

a comment about the find command

To find all the directories in a directory and remove them, use the following command:
find /home/ye/Backup/ -type d -name apattern -exec rm -rf '{}' \;

-name pattern

Base of file name (the path with the leading directories removed) matches shell pattern pattern. The metacharacters (`*', `?', and `[]') do not match a `.' at the start of the base name. To ignore a directory and the files under it, use -prune; see an example in the description of -path.

-exec command ;

Execute command; true if 0 status is returned. All following arguments to find are taken to be arguments to the command until an argument consisting of `;' is encountered. The string `{}' is replaced by the current file name being processed everywhere it occurs in the arguments to the command, not just in arguments where it is alone, as in some versions of find. Both of these constructions might need to be escaped (with a `\') or quoted to protect them from expansion by the shell. The command is executed in the starting directory.

How to run linux command regularly?

Using cron to schedule the jobs that need to be run regularly.

cron is a Linux system process that will execute a program at a preset time. To use cron you must prepare a text file that describes the program that you want executed and the times that cron should execute them. Then you use the crontab program to load the text file that describes the cron jobs into cron.

Here is the format of a cron job file:

[min] [hour] [day of month] [month] [day of week] [program to be run]

where each field is defined as

[min]	Minutes that program should be executed on. 0-59. Do not set as * or the program will be run once a minute.
[hour]	Hour that program should be executed on. 0-23. * for every hour.
[day of month]	Day of the month that process should be executed on. 1-31. * for every day.
[month]	Month that program whould be executed on. 1-12 * for every month.
[day of week]	Day of the week. 0-6 where Sunday = 0, Monday = 1, ...., Saturday = 6. * for every day of the week.
[program]	Program to be executed. Include full path information.

Here are some examples:

0,15,30,45 * * * * /usr/bin/foo

Will run /usr/bin/foo every 15 minutes on every hour, day-of-month, month, and day-of-week. In other words, it will run every 15 minutes for as long as the machine it running.

10 3 * * * /usr/bin/foo

Will run /usr/bin/foo at 3:10am on every day.

10 * 1 * * /usr/bin/foo

Will run /usr/bin/foo at 12:10am on the first day of the month.

10 * * 1 * /usr/bin/foo

Will run /usr/bin/foo at 12:10am on the first month of the year.

10 14 * * 1 /usr/bin/foo

Will run /usr/bin/foo at 2:10pm on every Monday.

There are more options for these. See man man crontab -S 5.

You must use crontab to load cron jobs into cron. First create a text file that uses the above rule to describe the cron job that you want to load into cron. But before you load it, type crontab -l to list any jobs that are currently loaded in crontab.

If none are listed, then it is safe to load your job. Example. If you wanted to run /usr/local/bin/foo once a day at 3:10am, then create a text file

10 3 * * * /usr/bin/foo

Save it as foo.cron. Then type crontab foo.cron. Check to see if it was loaded by typing crontab -l. It should display something like this:

# DO NOT EDIT THIS FILE - edit the master and reinstall.
# (ipwatch.cron installed on Thu Nov 18 11:48:02 1999)
# (Cron version -- $Id: crontab.c,v 2.13 1994/01/17 03:20:37 vixie Exp $)
10 3 * * * /usr/bin/foo

If you want to edit the cron job, then edit foo.cron and then remove the existing cron job (crontab -r) and load it again (crontab foo.cron). You can have multiple jobs. Just put each different one on a seperate line in foo.cron.

contab jobs will run under the user that was in effect when you loaded the job in crontab.

See man cron, man crontab and man crontab -S 5 for more information.