• shell – How to mkdir only if a directory does not already exist? – Stack Overflow

สร้าง directory ชื่อ log

#!/bin/bash

dir=log
if [ ! -d $dir ]
then
  mkdir $dir
  echo "create directory $dir"
else
  echo "Directory exists"
fi

• Display Date And Time In Linux – nixCraft (cyberciti.biz)

$ date --help
Usage: date [OPTION]... [+FORMAT]
  or:  date [-u|--utc|--universal] [MMDDhhmm[[CC]YY][.ss]]
Display the current time in the given FORMAT, or set the system date.

Mandatory arguments to long options are mandatory for short options too.
  -d, --date=STRING         display time described by STRING, not 'now'
  -f, --file=DATEFILE       like --date once for each line of DATEFILE
  -I[TIMESPEC], --iso-8601[=TIMESPEC]  output date/time in ISO 8601 format.
                            TIMESPEC='date' for date only (the default),
                            'hours', 'minutes', 'seconds', or 'ns' for date
                            and time to the indicated precision.
  -r, --reference=FILE      display the last modification time of FILE
  -R, --rfc-2822            output date and time in RFC 2822 format.
                            Example: Mon, 07 Aug 2006 12:34:56 -0600
      --rfc-3339=TIMESPEC   output date and time in RFC 3339 format.
                            TIMESPEC='date', 'seconds', or 'ns' for
                            date and time to the indicated precision.
                            Date and time components are separated by
                            a single space: 2006-08-07 12:34:56-06:00
  -s, --set=STRING          set time described by STRING
  -u, --utc, --universal    print or set Coordinated Universal Time (UTC)
      --help     display this help and exit
      --version  output version information and exit

FORMAT controls the output.  Interpreted sequences are:

  %%   a literal %
  %a   locale's abbreviated weekday name (e.g., Sun)
  %A   locale's full weekday name (e.g., Sunday)
  %b   locale's abbreviated month name (e.g., Jan)
  %B   locale's full month name (e.g., January)
  %c   locale's date and time (e.g., Thu Mar  3 23:05:25 2005)
  %C   century; like %Y, except omit last two digits (e.g., 20)
  %d   day of month (e.g., 01)
  %D   date; same as %m/%d/%y
  %e   day of month, space padded; same as %_d
  %F   full date; same as %Y-%m-%d
  %g   last two digits of year of ISO week number (see %G)
  %G   year of ISO week number (see %V); normally useful only with %V
  %h   same as %b
  %H   hour (00..23)
  %I   hour (01..12)
  %j   day of year (001..366)
  %k   hour, space padded ( 0..23); same as %_H
  %l   hour, space padded ( 1..12); same as %_I
  %m   month (01..12)
  %M   minute (00..59)
  %n   a newline
  %N   nanoseconds (000000000..999999999)
  %p   locale's equivalent of either AM or PM; blank if not known
  %P   like %p, but lower case
  %r   locale's 12-hour clock time (e.g., 11:11:04 PM)
  %R   24-hour hour and minute; same as %H:%M
  %s   seconds since 1970-01-01 00:00:00 UTC
  %S   second (00..60)
  %t   a tab
  %T   time; same as %H:%M:%S
  %u   day of week (1..7); 1 is Monday
  %U   week number of year, with Sunday as first day of week (00..53)
  %V   ISO week number, with Monday as first day of week (01..53)
  %w   day of week (0..6); 0 is Sunday
  %W   week number of year, with Monday as first day of week (00..53)
  %x   locale's date representation (e.g., 12/31/99)
  %X   locale's time representation (e.g., 23:13:48)
  %y   last two digits of year (00..99)
  %Y   year
  %z   +hhmm numeric time zone (e.g., -0400)
  %:z  +hh:mm numeric time zone (e.g., -04:00)
  %::z  +hh:mm:ss numeric time zone (e.g., -04:00:00)
  %:::z  numeric time zone with : to necessary precision (e.g., -04, +05:30)
  %Z   alphabetic time zone abbreviation (e.g., EDT)

By default, date pads numeric fields with zeroes.
The following optional flags may follow '%':

  -  (hyphen) do not pad the field
  _  (underscore) pad with spaces
  0  (zero) pad with zeros
  ^  use upper case if possible
  #  use opposite case if possible

After any flags comes an optional field width, as a decimal number;
then an optional modifier, which is either
E to use the locale's alternate representations if available, or
O to use the locale's alternate numeric symbols if available.

Examples:
Convert seconds since the epoch (1970-01-01 UTC) to a date
  $ date --date='@2147483647'

Show the time on the west coast of the US (use tzselect(1) to find TZ)
  $ TZ='America/Los_Angeles' date

Show the local time for 9AM next Friday on the west coast of the US
  $ date --date='TZ="America/Los_Angeles" 09:00 next Fri'

GNU coreutils online help: <http://www.gnu.org/software/coreutils/>
For complete documentation, run: info coreutils 'date invocation'

$ date --version
date (GNU coreutils) 8.22
Copyright (C) 2013 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Written by David MacKenzie.

ตัวอย่าง

Open a terminal application and type the following command:

$ date
Tue Feb  7 15:23:13 +07 2023

format the date

$ date +"%y-%m-%d"
23-02-07
$ date +"%Y-%m-%d"
2023-02-07

Simply display the current time on Linux:

]$ date "+%T"
15:24:15

To print the date of the day before yesterday, run:

$ date --date='2 days ago'
Sun Feb  5 15:25:37 +07 2023

Want to see the day of year of Christmas in the current year? Try:

$ date --date='25 Dec' +%j
359

Display the current full month name and the day of the month:

$ date '+%B %d'
February 07

• How to Use if-else in Shell Scripts? | DigitalOcean

How to use if-else in shell script

It is easy to see the syntax of a function and believe you know how to use it. But it is always a better choice to understand a function through examples because they help you understand the role that different aspects of a function play.

Here are some useful examples of if-else in shell scripts to give you a better idea of how to use this tool.

1. Using if-else to check whether two numbers are equal

When trying to understand the working of a function like if-else in a shell script, it is good to start things simple. Here, we initialize two variables a and b, then use the if-else function to check if the two variables are equal. The bash script should look as follows for this task.

#!/bin/bash
m=1
n=2

if [ $n -eq $m ]
then
        echo "Both variables are the same"
else
        echo "Both variables are different"
fi

Output:

Both variables are different

2. Using if-else to compare two values

The more common use of if-else in shell scripts is for comparing two values. Comparing a variable against another variable or a fixed value helps is used in a variety of cases by all sorts of programmers.

For the sake of this example, we will be initializing two variables and using the if-else function to find the variable which is greater than the other.

#!/bin/bash
a=2
b=7
if [ $a -ge $b ]
then
  echo "The variable 'a' is greater than the variable 'b'."
else
  echo "The variable 'b' is greater than the variable 'a'."
fi

Output:

The variable 'b' is greater than the variable 'a'.

3. Using if-else to check whether a number is even

Sometimes we come across situations where we need to deal with and differentiate between even and odd numbers. This can be done with if-else in shell scripts if we take the help of the modulus operator.

The modulus operator divides a number with a divisor and returns the remainder.

As we know all even numbers are a multiple of 2, we can use the following shell script to check for us whether a number is even or odd.

#!/bin/bash
n=10
if [ $((n%2))==0 ]
then
  echo "The number is even."
else
  echo "The number is odd."
fi

Output:

The number is even

As you can see, we’ve enclosed a part of the condition within double brackets. That’s because we need the modulus operation to be performed before the condition is checked.

Also, enclosing in double brackets runs statements in C-style allowing you to process some C-style commands within bash scripts.

4. Using if-else as a simple password prompt

The if-else function is known for its versatility and range of application. In this example, we will use if-else in shell script to make the interface for a password prompt.

To do this, we will ask the user to enter the password and store it in the variable pass.

If it matches the pre-defined password, which is ‘password’ in this example, the user will get the output as -“The password is correct”.

Else, the shell script will tell the user that the password was incorrect and ask them to try again.

#!/bin/bash
echo "Enter password"
read pass
if [ $pass="password" ]
then
  echo "The password is correct."
else
  echo "The password is incorrect, try again."
fi

5. นับจำนวนไฟล์ว่าเท่ากับจำนวนที่กำหนดหรือไม่

• How To Assign Output of a Linux Command to a Variable (tecmint.com)

#!/bin/bash
a=NUM_FILE=`find . -type f -print | wc -l`
b=4
if [ $a -eq $b ]
then
  echo "Exactly the same."
else
  echo "Wrong number"
fi

6. start, stop process

ถ้า process ไม่ได้รันอยู่ ให้ start process

#!/bin/bash

a=`ps -ef | grep PROCESS_NAME | wc -l`
b=1

if [ $a -gt $b ]
then
  echo "Found existing process"
else
  echo "Not found existing, START PROCESS_NAME"
  start.sh
fi

ถ้า process รันอยู่ ให้ stop process

#!/bin/bash

a=`ps -ef | grep PROCESS_NAME | wc -l`
b=1

if [ $a -gt $b ]
then
  echo "Found existing, STOP PROCESS_NAME "
  stop.sh
else
  echo "Process not found"
fi

• The pgrep Command Tutorial (linuxhint.com)

ทดสอบบน Red Hat 7.7

The pgrep command allows a user to find process IDs in the running program in the system’s current state.

$ pgrep --version
pgrep from procps-ng 3.3.10

$ pgrep --help

Usage:
 pgrep [options] <pattern>

Options:
 -d, --delimiter <string>  specify output delimiter
 -l, --list-name           list PID and process name
 -a, --list-full           list PID and full command line
 -v, --inverse             negates the matching
 -w, --lightweight         list all TID
 -c, --count               count of matching processes
 -f, --full                use full process name to match
 -g, --pgroup <PGID,...>   match listed process group IDs
 -G, --group <GID,...>     match real group IDs
 -n, --newest              select most recently started
 -o, --oldest              select least recently started
 -P, --parent <PPID,...>   match only child processes of the given parent
 -s, --session <SID,...>   match session IDs
 -t, --terminal <tty,...>  match by controlling terminal
 -u, --euid <ID,...>       match by effective IDs
 -U, --uid <ID,...>        match by real IDs
 -x, --exact               match exactly with the command name
 -F, --pidfile <file>      read PIDs from file
 -L, --logpidfile          fail if PID file is not locked
 --ns <PID>                match the processes that belong to the same
                           namespace as <pid>
 --nslist <ns,...>         list which namespaces will be considered for
                           the --ns option.
                           Available namespaces: ipc, mnt, net, pid, user, uts

 -h, --help     display this help and exit
 -V, --version  output version information and exit

For more details see pgrep(1).

ตัวอย่าง

หา process ID ของ nginx

$ pgrep nginx
1085
1086

specify output delimiter ด้วย -d

$ pgrep -d: nginx
1085:1086

match exactly with the command name ด้วย -x

$ pgrep -x nginx
1085
1086

หา process ID จาก user name

$ pgrep -u jack