bash

pip install django-environ

if [ -x /etc/rc.local ] then

   -a file
          True if file exists.
   -b file
          True if file exists and is a block special file.
   -c file
          True if file exists and is a character special file.
   -d file
          True if file exists and is a directory.
   -e file
          True if file exists.
   -f file
          True if file exists and is a regular file.
   -g file
          True if file exists and is set-group-id.
   -h file
          True if file exists and is a symbolic link.
   -k file
          True if file exists and its ``sticky'' bit is set.
   -p file
          True if file exists and is a named pipe (FIFO).
   -r file
          True if file exists and is readable.
   -s file
          True if file exists and has a size greater than zero.
   -t fd  
          True if file descriptor fd is open 
          and refers to a terminal.
   -u file
          True if file exists and its set-user-id bit is set.
   -w file
          True if file exists and is writable.
   -x file
          True if file exists and is executable.

string
True if the length of string is non-zero.

string1 == string2
string1 = string2
True if the strings are equal. 
When used with the [[ command, this performs pattern matching 
as described above (see Conditional Constructs).

‘=’ should be used with the test command for POSIX conformance.

string1 != string2
True if the strings are not equal.

string1 < string2
True if string1 sorts before string2 lexicographically.

string1 > string2
True if string1 sorts after string2 lexicographically.

arg1 OP arg2
OP is one of ‘-eq’, ‘-ne’, ‘-lt’, ‘-le’, ‘-gt’, or ‘-ge’. 
These arithmetic binary operators return true if arg1 is equal to,
not equal to, less than, less than or equal to, greater than, 
or greater than or equal to arg2, respectively. 
Arg1 and arg2 may be positive or negative integers. 
When used with the [[ command, Arg1 and Arg2 
are evaluated as arithmetic expressions (see Shell Arithmetic).

date ='%Y-%m-%d %H:%M:%s'

#!/bin/bash

original_string="Hello, World!"
substring_to_replace="World"
replacement="Universe"

new_string="${original_string//substring_to_replace/replacement}"

echo "Original String: $original_string"
echo "New String: $new_string"

visudo
adduser newuser
usermod -aG sudo newuser

useradd -r -u 150 -g mail -d /var/vmail -s /sbin/nologin \
       -c "Virtual Mail User" vmail
# -r, --system                  create a system account

cat /etc/passwd

printf '%5s |%5s|%-5s|%-20.5f| %.5s|%5s \n' \
    1 2 3 4444,4444 kkkkhhh 6

awk -F':' '{printf "%-20s %s\n",$1,$3 }' /etc/passwd

tar -xvf файл.tar # extract
tar -xvzf файл.tar.gz # extract
unzip file.zip  # extract

top 
ctrl+z
htop
ctrl+z
jobs
bg %1
fg

usermod -l login-name old-name # change login of username

usermod -u UID username # change uid of username

cat /etc/passwd
cut -d: -f1 /etc/passwd

pkill -u tom pid
pkill -9 -u tom
usermod -l jerry tom # change name of user tom
groupmod -n jerry tom # change name of group tom

id tom
usermod -u 10000 tom # change uid of login
id tom

usermod -d /home/jerry -m jerry
id jerry
ls -ld /home/jerry

groupadd vmail
Usage: groupadd [options] GROUP
Options:
  -f, --force                   exit successfully if the group already exists,
                                and cancel -g if the GID is already used
  -g, --gid GID                 use GID for the new group
  -h, --help                    display this help message and exit
  -K, --key KEY=VALUE           override /etc/login.defs defaults
  -o, --non-unique              allow to create groups with duplicate
                                (non-unique) GID
  -p, --password PASSWORD       use this encrypted password for the new group
  -r, --system                  create a system account
  -R, --root CHROOT_DIR         directory to chroot into
  -P, --prefix PREFIX_DIR       directory prefix
      --extrausers              Use the extra users database


Usage: groupmod [options] GROUP
Options:
  -g, --gid GID                 change the group ID to GID
  -h, --help                    display this help message and exit
  -n, --new-name NEW_GROUP      change the name to NEW_GROUP
  -o, --non-unique              allow to use a duplicate (non-unique) GID
  -p, --password PASSWORD       change the password to this (encrypted)
                                PASSWORD
  -R, --root CHROOT_DIR         directory to chroot into
  -P, --prefix PREFIX_DIR       prefix directory where are located the /etc/* files


groupadd -g 2000 vmail # add group with groupID
groupmode -g 2000 vmail # change gid of groupname
groupmode -n vmail2 vmail # change groupname of groupname
usermode -u 2000 vmail # change uid of username
usermode -g 2000 vmail # change group id of username

/etc/dovecot/dovecot-sql.conf.ext:user_query = \
    SELECT maildir, 150 AS uid, 8 AS gid FROM mailbox \
    WHERE username = '%u' AND active = true

$:~# grep -r 150 /etc/postfix
/etc/postfix/main.cf:virtual_uid_maps = static:150
/etc/postfix/main.cf:virtual_gid_maps = static:150

grep -r vmail -A 10 -B 10 /etc/dovecot /etc/postfix
history | grep -E 'nano|cd'
ls | xargs cat
ls | xargs -I % sh -c 'echo ===================; cat %; '
ls | xargs -I % sh -c 'cat ./%; echo ===================;'

grep -r Ln -A 5 -B 5 /etc/dovecot /etc/postfix
grep -rn Ln  /etc/dovecot /etc/postfix
grep -rn Ln -A 5 -B 5 /etc/dovecot /etc/postfix
/etc/dovecot/conf.d/10-director.conf:25:# %Ln
grep -rn %u  /etc/dovecot /etc/postfix

nano /etc/dovecot/conf.d/10-auth.conf
# set %Ln unstil %n
systemctl reload dovecot

# remove commented and empty line
grep -v '^//\|^$' defaults.inc.php

diff [options] file1 file2 -y --color='auto'
diff ~/.bash_aliases ~/.bash_aliases_dmp -y --color='auto' -q

sudo apt-get install imagemagick
pwd
ll
# resize and convert
convert poster_top.png -resize 1300x312 poster_top.jpeg

# rotate
convert image1.jpg -rotate 90 image1-rotated.jpg
for file in *.png; do convert $file -rotate 90 rotated-$file; done

#crop
convert image.jpg -crop 703x470+3+5 output.jpg

identify poster_top.jpeg

# ---------
$ convert apande.ru_2.png  apande.ru_2.jpeg
$ convert apande.ru_2.jpeg -resize 1980x1980 apande.ru_2_r1.jpeg
$ identify apande.ru_2_r1.jpeg
apande.ru_2_r1.jpeg JPEG 757x1980 757x1980+0+0 8-bit sRGB 332053B 0.000u 0:00.000

pwd
ll -al 2.py


f=2.py

t=0
w=`pwd`
c="stat ${f} -c %Z"
ts="pytest $f"
st1=`$c`
while true
do

sleep 1
st=`stat $f -c %Z`
if [[ $st1 != $st ]]; then
echo
stat ${f} -c %z
echo $st1
$ts
st1=$st
echo '=============================='
echo '=============================='
fi
# ((t++))
# echo ${t} $st $st1
# echo $w
done

$ . watch.sh

a=10;b=1;w='>';while [[ "$a" -ge 10 ]] && [[ "$a" -le 100 ]] ; do echo $a $b $w;a=$(($a+b)); if ((a==100)); then b=-1;w='<';fi;if ((a==10));then b=1;w='>';fi;sleep .05; done

a=10
b=1
w='>'
while [[ "$a" -ge 10 ]] && [[ "$a" -le 100 ]]
do
echo $a $b $w
a=$(($a+b))

if ((a==100))
then
b=-1
w='<'
fi

if ((a==10))
then
b=1
w='>'
fi

sleep .05
done