bash_notes.txt

# == commands
Ctrl+P - previous command in history
Ctrl+N - next command in history

printf "%d, %.2f\n" 5 3.2
type [cmd]
type ls -> 'alias'
type pwd -> 'shell builtin'
type expr -> '/usr/bin/expr'

ls -1           # files in 1 column
ls --recursive

compgen -c if    # get autocomplete list of commands starting with "if"
compgen -a l     # autocomplete list of aliases
compgen -v P     # autocomplete list of shell variables
compgen -A directory t    # autocomplete list of directories

free -h | grep "^Mem" | tr -s ' ' | cut -d' ' -f2        # get total RAM

xxd -g 1 file.txt    # print hex content of a file, group by 2 hex digits


# == user info
finger
users
who
w


# == history
date
!d   # executes a previous command matching the 'd'

!!   # repetas the last command
!-2  # executes a command in position -2 in the history
!#   # repeats the content of the current command line
  date; sleep 2; !#

cd -  # switch between the current directory and the last directory
cd    # return to home directory


# == const variable
readonly text="123"


# == user input
read -p "Enter vars: " var1 var2
echo $var1 $var2
read
echo $REPLY

read -s -p "Enter password: "


# == built-ins
BASH - bash path
BASH_VERSION
BASH_PID
UID
HOME
HOSTNAME
PATH
PPID - shell parent pid
PWD
OLDPWD
RANDOM
REPLY
SECONDS - seconds since the shell start
HISTFILE
FUNCNAME - name of the current function
LINENO - current line number


# == arithmetics
expr "4 + 7"
let "a = 4 * 8"
echo $a


# == logical && || !
# && - execute the next command if the exit code is 0
# || - execute the next command if the exit code is not 0
! cd /tmp/new && mkdir /tmp/new
cd /tmp/new || mkdir /tmp/new


# == wildcards
~ - home directory (from the $HOME variable)
~user_name - home for a user
~+ - pwd
~- - old pwd

* - 0 or more characters

? - 1 character
ls ?? ????? - files with name length 2 or 5

[] - a set of characters
ls [a-d]*
ls [!0-9]*

{} - multiple patterns
ls {*.sh,*.c}
echo {1..50}
touch hello{0..9}.cpp


# == eval
params="-lah"
eval "ls $params"


# == set
# exit on the first failure in a bash script
set -e
# enable/disable symbolic links resolution (for 'cd')
set -P
set +P


# == special params
$0 - script name
$# - number of args
$$ - current process pid
$? - exit code for the last command
$_ - the last arg for the last command
$! - pid of the last background command


# == redirection
echo 'msg' > file
echo 'appended' >> file
wc -l < file
wc -l << EOF
  line1
  line2
  line3
EOF

cat /var/log/messages | grep 'DHCPOFFER'

# these are identical
echo 'abc' | tr [:lower:] [:upper:]
tr [:lower:] [:upper:] <<< 'abc'

# streams
/dev/stdin    # &0
/dev/stdout   # &1
/dev/stderr   # &2

q.sh 2> log_errors   # redirect error stream
q.sh > log_all >&2   # redirect stdin and stderr to a file
q.sh &> log_all      # same

ls folder 1>&1   # stdin
ls folder 2>&1   # stderr to stdin
ls folder 1>log1 2>&1   # stdin to file and stderr to stdin


# == test
# files

-f file - file exists and is a regular file
-d file - file is a directory
-e file - file exists
-s file - file exists and is not empty
-r file - file is readable (by your script)
-w file - file is writable (by your script)
-x file - file is executable (by your script)

-b file - file is a block device file
-c file - file is a character device file
-h file - file is a symbolic link
-p file - file is a pipe
-S file - file is a socket
-t fd   - file descriptor is opened on a terminal
-N file - file has new content (since the last time it was read)

f1 -nt f2 - f1 is newer than f2
f1 -ot f2 - f1 is older than f2
f1 -ef f2 - f1 is a hard link to f2

# strings
-z - the string is empty
-n - the string is not empty
s1 = s2
s1 != s2
s1 < s2
s1 > s2

# numbers
n1 -eq n2
n1 -ne n2
n1 -lt n2
n1 -le n2
n1 -gt n2
n1 -ge n2


if test -f "$TMP" -a ! -w "$TMP"; then
  printf "The file '$TMP' exists and is not writable\n"
fi

if [ -f "$TMP" -a ! -w "$TMP" ]; then

if [ "$VAR" \< "M" ]; then


# == globbing
if [[ $V = A* ]]; then printf "'$V' starts with A\n"; fi
if [[ $V = [A-Z]* ]]; then printf "'$V' starts with a letter\n"; fi

# character classes
[:alnum:] - Alphanumeric
[:alpha:] - Alphabetic
[:ascii:] - ASCII characters
[:blank:] - Space or tab
[:cntrl:] - Control characters
[:digit:] - Decimal digits
[:graph:] - Non-blank characters
[:lower:] - Lowercase characters
[:print:] - Non-control characters
[:punct:] - Punctuation charactersn
[:space:] - Whitespace
[:upper:] - Uppercase characters
[:xdigit:] - Hexadecimal digits


# == let
let "SUM=20 + 10"
let "SUM=20 - 10"
let "SUM=20 * 10"
let "SUM=20 / 10"
let "SUM=20 % 10"
let "VAR+=5"
let "VAR++"
let "RESULT=(5+3)*2"
let "RESULT=VALUE > 1 ? 1 : 0"



# == expressions
# substring
V=TestString
printf "${V:1:3}\n"  # est
V=`ls -l | wc -l`
V=$(ls -l | wc -l)
printf "Result: $((2+9))\n"


# == loops
while (( X-- > 0 )); do

while true; do

for item in i1 i2 i3; do
  echo $item
done

for item in $(cat file); do

for file in /var/log/httpd/*; do

for (( i=1; i<10; i++ )); do


# == debugging
bash -n q.sh      # check for errors without executing
bash -x q.sh      # prints each command before its execution
shopt -o -s nounset    # shows error if unset vars used
shopt -o -s xtrace     # as bash -x
trap ': VAR is $VAR' DEBUG         # prints the text after executing each line


# == read script arguments
R=`getopt --name "q2" --options "-h, -c:" -- "$@"`
eval set -- "$R"
printf "1: $1; 2: $2; 3: $3\n"


# == jobs
sleep 10 &
jobs
jobs -l   # pids
disown %2   # remove bg task
fg  # move task to foreground
Ctrl+Z -> bg   # move fg task to bg and resume its execution

kill -SIGSTOP 123
kill -SIGCONT 123
suspend   # in a script

trap 'printf "term_sig_trapped\n"' SIGTERM
trap 'printf "user_sig_trapped\n"' SIGUSR1


# == text files
basename /home/home/f.txt         # f.txt
basename /home/home/f.txt .txt    # f
dirname /home/home/f.txt          # /home/home
pathchk "/home/home/progs" && echo "Correct path"

file /var/f.txt     # get type of file
stat -t f.txt | cut -d" " -f 2      # get file size
statftime -f "Size: %_s" f.txt
md5sum f.txt
split -l 1000 log.txt out_prefix-
mktemp              # create temp files/folders with unique names
head -5 f.txt       # show first 5 lines
tail -5 f.txt       # show last 5 lines
wc --bytes f.txt
wc --lines f.txt
ls -l log | cut -d' ' -f1       # separate a string by delimiters
ls -l log | cut -c1-5           # get characters from a string

# open file for reading
exec 3< f.txt
while read LINE <&3; do printf "%s\n" $LINE; done

# open file for writing
exec 4> f.txt
printf "123\n" >&4

# open file for reading and writing
exec 3<> f.txt


# == text processing
grep -in "^F" f.txt              # lines starting with F, ignore case, show line numbers
grep -c "^F" f.txt               # print number of matches

tr -d '\r' < dos.txt > linux.txt     # delete \r character from a file
tr -s ' ' < f.txt                    # replace multiple character repetitions with a single character

sed "s/^package:/--/Ig" file.txt
sed "/^package/d" file.txt           # delete line that contains text
sed "/^$/d" file.txt                 # delete empty lines
sed "1d" file.txt                    # delete the first line
sed "/package/i*****" file.txt       # insert text before the matching line
sed -n "l" file.txt                  # show unprintable chars
sed -n "/package/p" file.txt         # print found lines

tar -c -f pack.tar pack.txt
tar -c -f pack.bz --bzip pack.txt
tar -c -f pack.gz --gzip pack.txt
tar -xvf pack.gz --gzip


# == console
showkey --ascii     # shows typed keys, in a native console only
setterm -foreground yellow    # set foreground color
setterm -background red
setterm -cursor off           # hide cursor
setterm -repeat off           # disable auto repeat


# == file processing
find . -name "*.txt" -type f
find . ! -name "*.txt" -type f    # not .txt files
find . "(" -name "*.txt" -or -name "*.sh" ")" -type f    # .txt or .sh files
find . -type d -or type f
find . -type f -name "*.log" -size +1024k       # files > 1MB
find . -type f -name "*.txt" -printf "%f, size: %s, access_time: %a, mod_time: %TH:%TM\n"


# == shell security
id
chown -R usr:grp file.txt
chmod 0755 file.txt
chmod ugo+r file.txt
chmod a+rwx file.txt
chroot /home/usr/new_root
bash -r            # restricted shell (no redirection, modify files only in the PWD)
wipe file.txt      # securely erae a file


# == network
host en.wikipeda.org
lynx en.wikipeda.org          # console browser

# raw request through TCP socket
cat </dev/tcp/time.nist.gov/13
exec 5<>/dev/tcp/www.net.cn/80; echo -e "GET / HTTP/1.0\n" >&5; cat <&5