Managing files in Linux

There are several commands and programs provided by Linux for viewing the contents of file. Working with files is one of the daunting task, most of the computer users be it newbie, regular user, advanced user, developer, admin, etc performs. Working with files effectively and efficiently is an art.

Today, in this article we will be discussing the most popular commands called head, tail and cat, most of us already aware of such commands, but very few of us implement it when needed.

1. head Command

The head command reads the first ten lines of a any given file name. The basic syntax of head command is:

head [options] [file(s)]

For example, the following command will display the first ten lines of the file named ‘/etc/passwd‘.

# head /etc/passwd 

root:x:0:0:root:/root:/bin/bash 
daemon:x:1:1:daemon:/usr/sbin:/bin/sh 
bin:x:2:2:bin:/bin:/bin/sh 
sys:x:3:3:sys:/dev:/bin/sh 
sync:x:4:65534:sync:/bin:/bin/sync 
games:x:5:60:games:/usr/games:/bin/sh 
man:x:6:12:man:/var/cache/man:/bin/sh 
lp:x:7:7:lp:/var/spool/lpd:/bin/sh 
mail:x:8:8:mail:/var/mail:/bin/sh 
news:x:9:9:news:/var/spool/news:/bin/sh

If more than one file is given, head will show the first ten lines of each file separately. For example, the following command will show ten lines of each file.

# head /etc/passwd /etc/shadow

==> /etc/passwd <== root:x:0:0:root:/root:/bin/bash bin:x:1:1:bin:/bin:/sbin/nologin daemon:x:2:2:daemon:/sbin:/sbin/nologin adm:x:3:4:adm:/var/adm:/sbin/nologin lp:x:4:7:lp:/var/spool/lpd:/sbin/nologin sync:x:5:0:sync:/sbin:/bin/sync shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown halt:x:7:0:halt:/sbin:/sbin/halt mail:x:8:12:mail:/var/spool/mail:/sbin/nologin uucp:x:10:14:uucp:/var/spool/uucp:/sbin/nologin ==> /etc/shadow <==
root:$6$85e1:15740:0:99999:7:::
bin:*:15513:0:99999:7:::
daemon:*:15513:0:99999:7:::
adm:*:15513:0:99999:7:::
lp:*:15513:0:99999:7:::
sync:*:15513:0:99999:7:::
shutdown:*:15513:0:99999:7:::
halt:*:15513:0:99999:7:::
mail:*:15513:0:99999:7:::
uucp:*:15513:0:99999:7:::

If it is desired to retrieve more number of lines than the default ten, then ‘-n‘ option is used along with an integer telling the number of lines to be retrieved. For example, the following command will display first 5 lines from the file ‘/var/log/yum.log‘ file.

# head -n5 /var/log/yum.log

Jan 10 00:06:49 Updated: openssl-1.0.1e-16.el6_5.4.i686
Jan 10 00:06:56 Updated: openssl-devel-1.0.1e-16.el6_5.4.i686
Jan 10 00:11:42 Installed: perl-Net-SSLeay-1.35-9.el6.i686
Jan 13 22:13:31 Installed: python-configobj-4.6.0-3.el6.noarch
Jan 13 22:13:36 Installed: terminator-0.95-3.el6.rf.noarch

In fact, there is no need to use ‘-n‘ option. Just the hyphen and specify the integer without spaces to get the same result as the above command.

# head  -5 /var/log/yum.log

Jan 10 00:06:49 Updated: openssl-1.0.1e-16.el6_5.4.i686
Jan 10 00:06:56 Updated: openssl-devel-1.0.1e-16.el6_5.4.i686
Jan 10 00:11:42 Installed: perl-Net-SSLeay-1.35-9.el6.i686
Jan 13 22:13:31 Installed: python-configobj-4.6.0-3.el6.noarch
Jan 13 22:13:36 Installed: terminator-0.95-3.el6.rf.noarch

The head command can also display any desired number of bytes using ‘-c‘ option followed by the number of bytes to be displayed. For example, the following command will display the first 45 bytes of given file.

# head -c45 /var/log/yum.log

Jan 10 00:06:49 Updated: openssl-1.0.1e-16.el

2. tail Command

The tail command allows you to display last ten lines of any text file. Similar to the head command above, tail command also support options  ‘n‘ number of lines and ‘n‘ number of characters.

The basic syntax of tail command is:

# tail [options] [filenames]

For example, the following command will print the last ten lines of a file called ‘access.log‘.

# tail access.log 

1390288226.042      0 172.16.18.71 TCP_DENIED/407 1771 GET http://download.newnext.me/spark.bin? - NONE/- text/html
1390288226.198      0 172.16.16.55 TCP_DENIED/407 1753 CONNECT ent-shasta-rrs.symantec.com:443 - NONE/- text/html
1390288226.210   1182 172.16.20.44 TCP_MISS/200 70872 GET http://mahavat.gov.in/Mahavat/index.jsp pg DIRECT/61.16.223.197 text/html
1390288226.284     70 172.16.20.44 TCP_MISS/304 269 GET http://mahavat.gov.in/Mahavat/i/i-19.gif pg DIRECT/61.16.223.197 -
1390288226.362    570 172.16.176.139 TCP_MISS/200 694 GET http://p4-gayr4vyqxh7oa-3ekrqzjikvrczq44-if-v6exp3-v4.metric.gstatic.com/v6exp3/redir.html pg 
1390288226.402      0 172.16.16.55 TCP_DENIED/407 1753 CONNECT ent-shasta-rrs.symantec.com:443 - NONE/- text/html
1390288226.437    145 172.16.18.53 TCP_DENIED/407 1723 OPTIONS http://172.16.25.252/ - NONE/- text/html
1390288226.445      0 172.16.18.53 TCP_DENIED/407 1723 OPTIONS http://172.16.25.252/ - NONE/- text/html
1390288226.605      0 172.16.16.55 TCP_DENIED/407 1753 CONNECT ent-shasta-rrs.symantec.com:443 - NONE/- text/html
1390288226.808      0 172.16.16.55 TCP_DENIED/407 1753 CONNECT ent-shasta-rrs.symantec.com:443 - NONE/- text/html

If more than one file is provided, tail will print the last ten lines of each file as shown below.

# tail access.log error.log

==> access.log <== 1390288226.042      0 172.16.18.71 TCP_DENIED/407 1771 GET http://download.newnext.me/spark.bin? - NONE/- text/html 1390288226.198      0 172.16.16.55 TCP_DENIED/407 1753 CONNECT ent-shasta-rrs.symantec.com:443 - NONE/- text/html 1390288226.210   1182 172.16.20.44 TCP_MISS/200 70872 GET http://mahavat.gov.in/Mahavat/index.jsp pg DIRECT/61.16.223.197 text/html 1390288226.284     70 172.16.20.44 TCP_MISS/304 269 GET http://mahavat.gov.in/Mahavat/i/i-19.gif pg DIRECT/61.16.223.197 - 1390288226.362    570 172.16.176.139 TCP_MISS/200 694 GET http://p4-gayr4vyqxh7oa-3ekrqzjikvrczq44-if-v6exp3-v4.metric.gstatic.com/v6exp3/redir.html pg  1390288226.402      0 172.16.16.55 TCP_DENIED/407 1753 CONNECT ent-shasta-rrs.symantec.com:443 - NONE/- text/html 1390288226.437    145 172.16.18.53 TCP_DENIED/407 1723 OPTIONS http://172.16.25.252/ - NONE/- text/html 1390288226.445      0 172.16.18.53 TCP_DENIED/407 1723 OPTIONS http://172.16.25.252/ - NONE/- text/html 1390288226.605      0 172.16.16.55 TCP_DENIED/407 1753 CONNECT ent-shasta-rrs.symantec.com:443 - NONE/- text/html 1390288226.808      0 172.16.16.55 TCP_DENIED/407 1753 CONNECT ent-shasta-rrs.symantec.com:443 - NONE/- text/html ==> error_log <==
[Sun Mar 30 03:16:03 2014] [notice] Digest: generating secret for digest authentication ...
[Sun Mar 30 03:16:03 2014] [notice] Digest: done
[Sun Mar 30 03:16:03 2014] [notice] Apache/2.2.15 (Unix) DAV/2 PHP/5.3.3 mod_ssl/2.2.15 OpenSSL/1.0.0-fips configured -- resuming normal operations

Similarly, you can also print the last few lines using the ‘-n‘ option as shown below.

# tail -5 access.log

1390288226.402      0 172.16.16.55 TCP_DENIED/407 1753 CONNECT ent-shasta-rrs.symantec.com:443 - NONE/- text/html
1390288226.437    145 172.16.18.53 TCP_DENIED/407 1723 OPTIONS http://172.16.25.252/ - NONE/- text/html
1390288226.445      0 172.16.18.53 TCP_DENIED/407 1723 OPTIONS http://172.16.25.252/ - NONE/- text/html
1390288226.605      0 172.16.16.55 TCP_DENIED/407 1753 CONNECT ent-shasta-rrs.symantec.com:443 - NONE/- text/html
1390288226.808      0 172.16.16.55 TCP_DENIED/407 1753 CONNECT ent-shasta-rrs.symantec.com:443 - NONE/- text/html

You can also print the number of characters using ‘-c’ argument as shown below.

# tail -c5 access.log

ymantec.com:443 - NONE/- text/html

3. cat Command

The ‘cat‘ command is most widely used, universal tool. It copies standard input to standard output. The command supports scrolling, if text file doesn’t fit the current screen.

The basic syntax of cat command is:

# cat [options] [filenames] [-] [filenames]

The most frequent use of cat is to read the contents of files. All that is required to open a file for reading is to type cat followed by a space and the file name.

# cat /etc/passwd 

root:x:0:0:root:/root:/bin/bash 
daemon:x:1:1:daemon:/usr/sbin:/bin/sh 
bin:x:2:2:bin:/bin:/bin/sh 
sys:x:3:3:sys:/dev:/bin/sh 
sync:x:4:65534:sync:/bin:/bin/sync 
games:x:5:60:games:/usr/games:/bin/sh 
man:x:6:12:man:/var/cache/man:/bin/sh 
lp:x:7:7:lp:/var/spool/lpd:/bin/sh 
…

The cat command also used to concatenate number of files together.

# echo 'Hi Tecmint-Team' > 1 
# echo 'Keep connected' > 2 
# echo 'Share your thought' > 3 
# echo 'connect us tecmint.com@gmail.com' > 4
# cat 1 2 3 4 > 5
# cat 5 

Hi Tecmint-Team 
Keep connected 
Share your thought 
connect us tecmint.com@gmail.com

It can be also used to create files as well. It is achieved by executing cat followed by the output redirection operator and the file name to be created.

# cat > tecmint.txt

Tecmint is the only website fully dedicated to Linux.

We can have custom end maker for ‘cat’ command. Here it is implemented.

# cat > test.txt << end 

I am Avishek 
Here i am writing this post 
Hope your are enjoying 
end
# cat test.txt 

I am Avishek 
Here i am writing this post 
Hope your are enjoying

Never underestimate the power of  ‘cat’ command and can be useful for copying files.

# cat avi.txt

I am a Programmer by birth and Admin by profession
# cat avi.txt > avi1.txt
# cat avi1.txt

I am a Programmer by birth and Admin by profession

Now what’s the opposite of cat? Yeah it’s ‘tac‘. ‘tac‘ is a command under Linux. It is better to show an example of ‘tac’ than to talk anything about it.

Create a text file with the names of all the month, such that one word appears on a line.

# cat month

January
February
March
April
May
June
July
August
September
October
November
December
# tac month

December
November
October
September
August
July
June
May
April
March
February
January

For more examples of cat command usage, refer to the 13 cat Command Usage

That’s all for now. I’ll be here again with another Interesting Article, worth Knowing. Till then stay tuned and connected to Tecmint. Don’t forget to provide us with your valuable feedback in our comment section.

Using the free Command in Linux

This article provides some useful examples of “free” commands with options, that might be useful for you to better utilize memory that you have.

1. Display System Memory

Free command used to check the used and available space of physical memory and swap memory inKB. See the command in action below.

# free

             total       used       free     shared    buffers     cached
Mem:       1021628     912548     109080          0     120368     655548
-/+ buffers/cache:     136632     884996
Swap:      4194296          0    4194296

2. Display Memory in Bytes

Free command with option -b, display the size of memory in Bytes.

# free -b

             total       used       free     shared    buffers     cached
Mem:    1046147072  934420480  111726592          0  123256832  671281152
-/+ buffers/cache:  139882496  906264576
Swap:   4294959104          0 4294959104

3. Display Memory in Kilo Bytes

Free command with option -k, display the size of memory in (KB) Kilobytes.

# free -k

             total       used       free     shared    buffers     cached
Mem:       1021628     912520     109108          0     120368     655548
-/+ buffers/cache:     136604     885024
Swap:      4194296          0    4194296

4. Display Memory in Megabytes

To see the size of the memory in (MB) Megabytes use option as -m.

# free -m

             total       used       free     shared    buffers     cached
Mem:           997        891        106          0        117        640
-/+ buffers/cache:        133        864
Swap:         4095          0       4095

5. Display Memory in Gigabytes

Using -g option with free command, would display the size of the memory in GB(Gigabytes).

# free -g
             total       used       free     shared    buffers     cached
Mem:             0          0          0          0          0          0
-/+ buffers/cache:          0          0
Swap:            3          0          3

6. Display Total Line

Free command with -t option, will list the total line at the end.

# free -t

            total       used       free     shared    buffers     cached
Mem:       1021628     912520     109108          0     120368     655548
-/+ buffers/cache:     136604     885024
Swap:      4194296          0    4194296
Total: 5215924 912520 4303404

7. Disable Display of Buffer Adjusted Line

By default the free command display “buffer adjusted” line, to disable this line use option as -o.

# free -o

            total       used       free     shared    buffers     cached
Mem:       1021628     912520     109108          0     120368     655548
Swap:      4194296          0    4194296

8. Dispaly Memory Status for Regular Intervals

The -s option with number, used to update free command at regular intervals. For example, the below command will update free command every 5 seconds.

# free -s 5

             total       used       free     shared    buffers     cached
Mem:       1021628     912368     109260          0     120368     655548
-/+ buffers/cache:     136452     885176
Swap:      4194296          0    4194296

9. Show Low and High Memory Statistics

The -l switch displays detailed high and low memory size statistics.

# free -l

             total       used       free     shared    buffers     cached
Mem:       1021628     912368     109260          0     120368     655548
Low:        890036     789064     100972
High:       131592     123304       8288
-/+ buffers/cache:     136452     885176
Swap:      4194296          0    4194296

10. Check Free Version

The -V option, display free command version information.

# free -V

procps version 3.2.8

Using crontab

The crontab is a list of commands that you want to run on a regular schedule, and also the name of the command used to manage that list.

crontab stands for “cron table,” because it uses the job scheduler cron to execute tasks;cron itself is named after “chronos,” the Greek word for time.

Overview

cron is the system process which will automatically perform tasks for you according to a set schedule. The schedule is called the crontab, which is also the name of the program used to edit that schedule.

Let’s say you have a script which backs up important files, or creates a report about system statistics, for example. Let’s say the script is called/home/myname/scripts/do-every-day.sh, and you want to run it every morning at 5 A.M.

To edit the crontab, use this command:

crontab -e

This will open the crontab in a text editor (Usually this is vi or vim, but it may be something else depending on your Linux distribution).

The default crontab file looks like this:

# Edit this file to introduce tasks to be run by cron.
#·
# Each task to run has to be defined through a single line
# indicating with different fields when the task will be run
# and what command to run for the task
#·
# To define the time you can provide concrete values for
# minute (m), hour (h), day of month (dom), month (mon),
# and day of week (dow) or use '*' in these fields (for 'any').#·
# Notice that tasks will be started based on the cron's system
# daemon's notion of time and timezones.
#·
# Output of the crontab jobs (including errors) is sent through
# email to the user the crontab file belongs to (unless redirected).
#·
# For example, you can run a backup of all your user accounts
# at 5 a.m every week with:
# 0 5 * * 1 tar -zcf /var/backups/home.tgz /home/
#·
# For more information see the manual pages of crontab(5) and cron(8)
#·
# m h  dom mon dow   command

These lines all start with a # because they are comments; they are ignored by cron, and are just there for you to read.

So, now let’s add our job to the crontab. Each job you add should take up a single line.

But how do we format our job entry line? Above, you can see that the last comment line is there to remind you how to format your entry. The format is very simple: six pieces of information, each separated by a space; the first five pieces of information tell cron whento run the job, and the last piece of information tells cron what the job is.

The information you must include is (in order of appearance):

  1. A number (or list of numbers, or range of numbers), m, representing the minute of the hour;
  2. A number (or list of numbers, or range of numbers), h, representing the hour of the day;
  3. A number (or list of numbers, or range of numbers), dom, representing the day of the month;
  4. A number (or list, or range), or name (or list of names), mon, representing the month of the year;
  5. A number (or list, or range), or name (or list of names), dow, representing the day of the week; and
  6. command, which is the command to be run, exactly as it would appear on the command line.

A “number” is an integer, for example 5. A “list of numbers” is a set of integers separated by commas, for example 15,30,45, which would represent just those three numbers. A “range of numbers” is a set of numbers separated by a hyphen, for example 10-20, which would represent all the numbers from 10 through 20, inclusive.

We want our job to run at 5 A.M., which would be minute 0, hour 5, every day of the month, every month, every day of the week. We need to add a line to the bottom of the file which looks like this:

0 5 * * * /home/myname/scripts/do-every-day.sh

In vi or vim, you can add this line by typing G to go to the end of the file, and o to add a new line and enter insert mode.

The asterisks (“*“) in our entry tell cron that for that unit of time, the job should be run “every”. You can now save the file and exit the text editor. In vi, this is done by pressingESCAPE and then typing :wq (for “write and quit”) and pressing ENTER. crontab will give you the following message:

crontab: installing new crontab

…and return you to the command line. Your script will now run automatically at 5 A.M., every day.

To view your crontab, you can use this command:

crontab -v

…or, to remove your crontab so that there no jobs are ever executed by cron, use this command:

crontab -r

For more examples of how to configure your crontab, see our Examples section below.

Syntax

crontab [-u user] file
crontab [-u user] [-l | -r | -e] [-i] [-s]

Technical Description

crontab is the program used to edit, remove or list the tables used to drive the crondaemon. Each user can have their own crontab. Although these files are located in/var/spool/, they are not intended to be edited directly, and that’s where the crontabcommand comes in.

cron jobs can be allowed or disallowed for individual users, as specified in the filescron.allow and cron.deny, located in the directory /etc. If the cron.allow file exists, a user must be listed there in order to be allowed to use a given command. If thecron.allow file does not exist but the cron.deny file does, then a user must not be listed there in order to use a given command. If neither of these files exists, only the superuserwill be allowed to use a given command. Another option is using PAM (pluggable authentication module) authentication to set up users who may or may not use crontaband system cron jobs, as configured in /etc/cron.d/.

The temporary directory for cron jobs can be set in environment variables (see below); if not, /tmp is used as the temporary directory.

Options

-u Append the name of the user whose crontab is to be tweaked. If this option is not given,crontab examines “your” crontab, i.e., the crontab of the person executing the command. Note that su can confuse crontab and that if you are running it inside of su you should always use the -u option for safety’s sake. The first form of this command is used to install a new crontabfrom some named file, or from standard input if the filename is given as ““.
-l Display the current crontab.
-r Remove the current crontab.
-e Edit the current crontab, using the editor specified in the VISUAL or EDITOR environment variables.
-i Same as -r, but gives the user a “Y/n” prompt before actually removing the crontab.
-s SELinux only: appends the current SELinux security context string as an MLS_LEVEL setting to the crontab file before editing or replacement occurs. See your SELinux documentation for details.

More About crontab Files

Blank lines and leading spaces and tabs are ignored. Lines whose first non-space character is a pound-sign (#) are interpreted as comments, and are ignored. Note that comments are not allowed on the same line as cron commands, since they will be taken to be part of the command. Similarly, comments are not allowed on the same line as environment variable settings.

An active line in a crontab will be either an environment setting or a cron command. An environment setting is of the form

name = value

where the spaces around the equal sign (=) are optional, and any subsequent non-leading spaces in value will be part of the value assigned to name. The value string may be placed in quotes (single or double, but matching) to preserve leading or trailing blanks.

Several environment variables are set up automatically by the cron daemon. SHELL is set to /bin/sh, and LOGNAME and HOME are set from the /etc/passwd line of the crontab’s owner. HOME and SHELL may be overridden by settings in the crontab;LOGNAME may not.

(Another note: the LOGNAME variable is sometimes called USER on BSD systems. On these systems, USER will be set also.)

In addition to LOGNAME, HOME, and SHELL, cron will look at MAILTO if it has any reason to send mail as a result of running commands in “this” crontab. If MAILTO is defined (and non-empty), mail is sent to the named user. If MAILTO is defined but empty (‘MAILTO=””‘), no mail will be sent. Otherwise mail is sent to the owner of the crontab. This option is useful if you decide on /bin/mail instead of /usr/lib/sendmail as your mailer when you install cron, because /bin/mail doesn’t do aliasing.

By default, cron will send mail using the ‘Content-Type:‘ header ‘text/plain‘ with the ‘charset=‘ parameter set to the charmap / codeset of the locale in which crond is started up: either the default system locale (if no LC_* environment variables are set) or the locale specified by the LC_* environment variables. You can use different character encodings for mailed cron job output by setting the CONTENT_TYPE andCONTENT_TRANSFER_ENCODING variables in crontabs.

The MLS_LEVEL environment variable provides support for multiple per-job SELinux security contexts in the same crontab. By default, cron jobs execute with the default SELinux security context of the user that created the crontab file. When using multiple security levels and roles, this may not be sufficient, because the same user may be running in a different role or at a different security level. You can set MLS_LEVEL to the SELinux security context string specifying the SELinux security context in which you want the job to run, and crond will set the execution context of the or jobs to which the setting applies to the specified context. (See the description of crontab -s in the options section.)

cron Command Format

Each cron command in the crontab file has five time and date fields, followed by a user name if it is the system crontab file, followed by a command. Commands are executed bycron when the minute, hour, and month of year fields match the current time, and at least one of the two day fields (day of month, or day of week) match the current time. Note that this means that nonexistent times, such as “missing hours” during daylight savings conversion, will never match, causing jobs scheduled during the “missing times” not to be run. Similarly, times that occur more than once during daylight savings will cause matching jobs to be run twice.

cron examines crontab entries once every minute.

The time and date fields are:

field allowed values
minute 059
hour 023
day of month 131
month 112 (or names; see example below)
day of week 07 (0 or 7 is Sunday, or use names; see below)

A field may be an asterisk (*), which always stands for “first through last”.

Ranges of numbers are allowed. Ranges are two numbers separated with a hyphen. The specified range is inclusive; for example, 8-11 for an “hours” entry specifies execution at hours 8, 9, 10 and 11.

Lists are allowed. A list is a set of numbers (or ranges) separated by commas. Examples: “1,2,5,9“, “0-4,8-12“.

Step values can be used in conjunction with ranges. For example, “0-23/2” can be used in the hours field to specify command execution every other hour. Steps are also permitted after an asterisk, so if you want to say “every two hours”, you can use “*/2“.

Names can also be used for the “month” and “day of week” fields. Use the first three letters of the particular day or month (case doesn’t matter). Ranges or lists of names are not allowed.

The “sixth” field (the rest of the line) specifies the command to be run. The entire command portion of the line, up to a newline or % character, will be executed by /bin/shor by the shell specified in the SHELL variable of the cronfile. Percent-signs (%) in the command, unless escaped with backslash (\), will be changed into newline characters, and all data after the first % will be sent to the command as standard input.

Note that the day of a command’s execution can be specified by two fields: day of month, and day of week. If both fields are restricted (in other words, they aren’t *), the command will be run when either field matches the current time. For example, “30 4 1,15 * 5” would cause a command to be run at 4:30 am on the 1st and 15th of each month, plus every Friday.

Files

/etc/cron.allow
/etc/cron.deny

Example crontab Command

crontab -e

Edit your crontab.

crontab -l

Display (“list”) the contents of your crontab.

crontab -r

Remove your crontab, effectively un-scheduling all crontab jobs.

sudo crontab -u charles -e

Edit the crontab of the user named charles. The -u option requires administrator privileges, so the command is executed using sudo.

sudo crontab -u jeff -l

View the crontab of user jeff.

sudo crontab -u sandy -r

Remove the crontab of user sandy.

Examples Of crontab Entries

15 6 2 1 * /home/melissa/backup.sh

Run the shell script /home/melissa/backup.sh on January 2 at 6:15 A.M.

15 06 02 Jan * /home/melissa/backup.sh

Same as the above entry. Zeroes can be added at the beginning of a number for legibility, without changing their value.

0 9-18 * * * /home/carl/hourly-archive.sh

Run /home/carl/hourly-archive.sh every hour, on the hour, from 9 A.M. through 6 P.M., every day.

0 9,18 * * Mon /home/wendy/script.sh

Run /home/wendy/script.sh every Monday, at 9 A.M. and 6 P.M.

30 22 * * Mon,Tue,Wed,Thu,Fri /usr/local/bin/backup

Run /usr/local/bin/backup at 10:30 P.M., every weekday.

Using FTP and SFTP

FTP is File Transfer Protocol. SFTP is secure FTP. In this article let us review how to connect and login to a remote ftp server for downloading and uploading files using ftp or sftp command. Most of the ftp commands are applicable to sftp. So, wherever ftp is mentioned, you can use sftp also.

1. Connect to a FTP site

Connect to a particular FTP server using ftp command as shown below.
Syntax:

$ ftp IP/hostname

or 

$ ftp
ftp> open IP/hostname

You can directly open connection with a remote host using it’s IP or host name from the command line. You can also go to ftp prompt and use open command to connect with remote host.

It will ask you for the user name and password to login. On some public domain FTP server, you can use “anonymous” username with any email address as the password to connect.

2. Download a file using ftp

Use the get command to download file from a remote ftp server as shown below.

ftp> get FILENAME

You have to be in the right mode to download files. i.e binary or ascii mode. Use ascii mode for transferring text files, and binary mode for all other type of files.

Download the file and save it with another name. In the following example, index.html file will be downloaded and saved as my.html on the local server.

ftp> get index.html my.html
Fetching /home/groups/index.html to my.html
/home/groups/index.html                          100% 2886     1.4KB/s   00:02    

3. Changing FTP Mode to binary or ascii

Go to ftp Ascii mode

ftp> ascii
200 Type set to A.

Go to ftp Binary mode

ftp> binary
200 Type set to I.

4. Uploading a file to FTP server

Use put command to upload a file to a remote ftp server as shown below.

ftp> put filename

5. Changing the remote and local directory

Apart from downloading or uploading a file, you may want to change either the remote or local directory, which you can do using cd and lcd respectively.

Change the remote server current directory using cd command

ftp> pwd
257 "/myftpserver" is current directory.
ftp> cd dir1
250 CWD command successful. "/myftpserver/dir1" is current directory.
ftp> pwd
257 "/myftpserver/dir1" is current directory.

Change the local machine current directory using lcd command

ftp> !
$ pwd
/home/sathiya/FTP
$ exit
exit
ftp> lcd /tmp
Local directory now /tmp
ftp> !
$ pwd
/tmp

Note:

  • executing ! takes you to the shell.
  • prompt starts with ftp> is ftp prompt.
  • prompt starts with $ is shell command line.

6. Listing the contents of remote directory from FTP

You can view the content of a remote directory using the ls / dir command.

ftp> ls

7. FTP Help

Type help or ? to view list of all available ftp commands.

For a detailed help on a particular ftp command use:

ftp> help COMMAND

8. Downloading multiple files with mget command

mget is for fetching multiple files from ftp server. You can use globs to download multiple files. For example, *.html will download all html files. The glob expansion are done on the remote server. So, it depends on the operating system of the remote server.

ftp> mget *.html
Fetching /ftptest/features.html to features.html
/ftptest/features.html                       100% 2256     2.2KB/s   00:01    
Fetching /ftptest/index.html to index.html
/ftptest/index.html                          100% 2886     2.8KB/s   00:01    
Fetching /ftptest/othertools.html to othertools.html
/ftptest/othertools.html                     100% 2282     2.2KB/s   00:01    
Fetching /ftptest/samplereport.html to samplereport.html
/ftptest/samplereport.html                   100%   15KB   7.3KB/s   00:02    
Fetching /ftptest/usage.html to usage.html
/ftptest/usage.html                          100% 2340     2.3KB/s   00:01    

To view the file names before downloading, you can also use mls command as shown below.

ftp> mls *.html -
/ftptest/features.html         
/ftptest/index.html            
/ftptest/othertools.html       
/ftptest/samplereport.html     
/ftptest/usage.html            

9. Uploading multiple files with mput command

Use mput to upload multiple files together. This works similar to the mget command. The following example uploads all the *.html file from local server to remote server.

ftp> mput *.html

10. Close a FTP connection

Without exiting the ftp prompt you may want to open a connection to another server. In that case, execute close command.

ftp> open ftp.your_server.com
Already connected to NNN.com, use close first.
ftp> close
221 Goodbye.
ftp> open ftp.your_server.com

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