if [ -r ~/.bashrc ]
then
  echo .bashrc is readable
else
  echo .bashrc is not readable
fi

while [ -z "$(pidof command1)" ]; do
  commands
done

while true
do
  command1 && continue  # jump to next iteration
  command2 || break     # exit loop
done

for i in 1 2 3 4 5; do
  command1
done

case ${1} in
  one)
    command1
    command2
    ;;
  2|two)
    command3
    ;;
  three*)
    command4
    ;;
  *)
    command7
    ;;
esac

command1 && command2
command1 || command2
command1 && { success_command || : ; } || failure_command

function func
{
  command1 $@
  return 0
}
func 1 2 3

function subcall
{
  return 0
}

if [ $# -eq 0 ]
then
  echo "USAGE: ${0} subcall"
else
  "$@"
fi

function subcall
{
  set -o errexit  # set 'e'
  echo "subcall options: $-"  # contains 'e' and 'u'
  echo ${ABC}     # errors since ABC is unset
}

set -o nounset   # set 'u'; is inherited by subshell
set +o errexit   # unset 'e'; don't SIGERR when subcall errors
( subcall )      # start function in subshell to separate its shell options
echo "main options: $-"   # no 'e' for errexit

# put single command in background
sleep 1s &

# put block of commands in background
{ sleep 2s; echo '2s done'; } &

# put block of commands with return code in background
{
  sleep 4s ; RET=$?
  echo '4s done'
  (exit $RET)
} &

wait # waits for all background jobs above

# find Makefiles and execute their default targets in parallel
find . -name Makefile -printf '%h\0' \
  | xargs --null --max-arg=1 --max-procs=4 \
    make --jobs=4 --directory

sudo dnf install parallel
parallel --citation <<<'will cite' # silence citation notice
man parallel_examples

# run ping in parallel (up to 4)
parallel --max-args=1 --jobs 4 'ping -c 1 {} > /dev/null && echo {}' 2>/dev/null < ipaddresses.txt
# run commands in parallel and show their outputs in order
parallel --max-args=1 --keep-order < commands.txt
# compress files in parallel
parallel gzip '{}' ::: file{1..5}.txt

set -o errexit
set -o nounset

while [ $# -gt 0 ]
do
  OPT=${1}

  case ${OPT} in
    -n|--name)
      NAME=${2}
      shift
    ;;
    -l|--level)
      LEVEL=${2}
      shift
    ;;
    *)
      echo unrecognized option ${OPT}
      false
    ;;
  esac
  shift || :
done

ARGS=("${@}")
# format input items as "item1" : "item2" ...
ARGS="${ARGS[@]//*/:\"&\"}"

# extract option items that begin with -
OPTIONS=$(awk -v RS=':' -v ORS=' ' '/^"-/' <<<"${ARGS}")
# extract other items y negating the pattern
OTHER=$(awk -v RS=':' -v ORS=' ' '$0 !~ /^"-/' <<<"${ARGS}")

set -o errexit

# run command on SIGERR
trap 'die' ERR

die()
{
  echo "Failed on line ${BASH_LINENO}"
  exit 1
}

false # run command that fails

# remove trap for SIGTERM and SIGINT
trap - TERM INT
# trap all signals
trap 'kill -9 ${SPIN_PID}' $(seq 0 15)

STRING='12345'
echo ${STRING:1} # 2345

ARRAY=( 1 2 3 4 5)
echo ${ARRAY[@]:1} # 2 3 4 5

q() {
  A=( $* )   ; echo ${A[@]@K} # 0 "a" 1 "a" 2 "b" 3 "b"
  A=( $@ )   ; echo ${A[@]@K} # 0 "a" 1 "a" 2 "b" 3 "b"
  A=( "$*" ) ; echo ${A[@]@K} # 0 " a a b b"
  A=( "$@" ) ; echo ${A[@]@K} # 0 " a a " 1 " b b"
}
# two arguments containing whitespace
q '  a  a '    \ \ b\ \ b

# move files to different directories
f() {
  cp "${@:1:1}" /path/for/first/
  cp "${@:2}" /path/for/rest/
}

# expand sequences
echo {0..9} # 0 1 2 3 4 5 6 7 8 9
echo {a..z} # a b c d ...
echo {a..c}{a..d} # aa ab ac ad ba bb ...
printf '%03d ' {1..100} ; echo # 001 002 003 004 ...
printf '%s, ' 10.1.{1..255}.{1..255} # 10.1.1.1, 10.1.1.2, 10.1.1.3, ...

# make backup file (example.conf.YYYY-MM-dd.bak)
cp example.conf{,.$(date +%F).bak}

echo 'MY_VAR=${MY_VAIR}' > input.txt
export MY_VAR=abc
envsubst '$MY_VAR' < input.txt > output.txt

# output Unicode; \xHH \uHHHH \UHHHHHHHH
echo -e '\x41' '\u2728' '\U1F680' # A ✨ 🚀

# output more Unicode
UNICODE=$(printf '%u\n' 0x1F311)
for (( i=0; i < 8; i++ ))
do
  echo -ne '\U'$( printf '%X' $((UNICODE+i)) ) ' '
done

# convert hexadecimal to decimal
echo $((16#FF))
# convert decimal to hexadecimal
printf '%u\n' 0xFF

spin()
{
  while true
  do
    echo -n '\'; echo -en '\010'; sleep 0.2
    echo -n '|'; echo -en '\010'; sleep 0.2
    echo -n '/'; echo -en '\010'; sleep 0.2
    echo -n '-'; echo -en '\010'; sleep 0.2
  done
}
spin &
SPIN_PID=$!
# kill spinner on any signal including exit.
trap "kill -9 ${SPIN_PID}" $(seq 0 15)

sleep 2s # long running work

top -O # list fields
# mebibytes, sorted by memory, non-root
top -E m -e m -o %MEM -U '!0'

man 7 systemd.journal-fields

systemctl status --type=service
systemctl --all list-unit-files

# check for errors in dracut services logs
journalctl --since today --unit 'dracut*' --grep 'fatal|error|fail'

journalctl --follow --unit NetworkManager.service

journalctl --no-pager --since today --grep 'fail|error|fatal'

journalctl --user-unit=container-CONTAINER_NAME.service

# count error logs per executable
journalctl --no-pager --since today --priority=err --output json \
  | jq '._EXE' \
  | sort \
  | uniq -c \
  | sort --numeric --reverse --key='1,1'
  # syslog log levels:
  # 0=emerg, 1=alert, 2=crit, 3=err, 4=warning, 5=notice, 6=info, 7=debug

# log lines with explanation texts from the message catalog.
journalctl --catalog --pager-end --unit=firewalld

# send journal logs
python3 -c 'from systemd import journal; journal.send("foo\nbar")'
journalctl -n1 -o export

PID=$(pidof -s bash) # single pid
PIDS=$(pidof firefox) # one or more pid

# process selection
ps -x  # list all processs owned by own effective UID
ps -ax # list all processes
ps -A  # list all processes
ps -e  # list all processes
ps -A r # list all running processes
ps -A --deselect r # list all non-running processes
ps -C bash # select PID by command; NOTE: commands were max. 15 characters in older versions
ps --pid ${PIDS}  # select by PID
ps --ppid ${PIDS} # select by parent PID
ps --user $(id -u)  # select by effective UID
ps --User $(id -ur)  # select by real UID
ps --group $(id -g)  # select by effective GID
ps --Group $(id -gr) # select by real GID
ps --tty $(tty) # select by tty
# inverse the selection
ps --deselect --user 0 --user 1000
ps --deselect -d # select only session leaders

# output formats
ps -C bash -o pid --no-header # get PID of command
ps --pid ${PID} e -w -w -o args # show environment with unlimited width
ps c --forest # show true command with process trees
ps --context # show SELinux context
ps -f # full-format listing.
ps -F # extra full-format listing.
ps -f k +uid,-ppid,+pid # sort by columns
ps -f --sort user,-ppid,+pid # sort by columns (GNU)
ps -f n # output numeric UID and GID
ps -Z # add SELinux column
ps u # user-oriented format
ps s # signal format
ps v # virtual memory format
ps X # register format
ps --format pid,ppid,args # user-defined column format; short option -o
ps L # list all column format specifiers
ps -o pid,format,state,tname,time,command
ps -o pid,format,tname,time,cmd
ps -o pid,ruser=RealUser -o comm=Command # rename header columns
ps -o pid= -o comm= # remove header of columns
ps -o '%p %y %x %c' # AIX format specifiers
ps -T  # show threads with SPID (thread ID)
ps S --forest # sum resource information of process and its forked children

# get PID of a command
PID="$(ps -C ${CMD} -o pid --no-header | tr -dc '[:digit:]')"

# send SIGTERM
kill -TERM ${PID}
kill -s TERM ${PID}
kill -n 15 ${PID}
# show a list of signals
kill -l

# access bits, access bits octal, owner user, owner group, bytes size, name, type
stat --format='%A (%a) %U:%g %s %n %F' file
# show mount point
stat --format='%m' file
# file system
stat --format='%T' --file-system file
# show SELinux context
stat --format='%n %C' file
# show update times
stat --printf='Birth:    %w \nAccess:   %x\nModified: %y\nStatus:   %z\n' file

# create hard link to associate another file name with the same inode
ln file1 file2
ls -i file1 file2
stat file1 file2
rm file1 # file2 remains with inode

# create a symbolic link
ln --symbolic target link
stat target link

ln -s script.sh s
basename "$(readlink -f s)" # script.sh

# link libraries so that name aliases soname when build-linking,
# and soname aliases versioned library when run-linking
ln -s --force lib${NAME}.so.${VERSION} lib${NAME}.so.${SONAME}   # file <- SONAME
ln -s --force lib${NAME}.so.${SONAME} lib${NAME}.so              # SONAME <- NAME

'.      " return cursor to last edit
ctrl+o  " move to previous jump position
ctrl+i  " move to next jump position
mX      " set global mark X (using uppercase makes it global)
'X      " go to global mark X (at start of line)
`X      " go to global mark X (at exact cursor position)
:marks  " show marks
ctrl+d  " half-page down
ctrl+u  " half-page up
zz      " center cursor in window
ctrl+e  " scroll down without moving cursor (though it sticks to top)
ctrl+y  " scroll down without moving cursor (though it sticks to bottom)
10|     " move to 10th column

"+P        " paste from system clipboard (use * instead of + for PRIMARY selection)
"+yy       " copy line to system clipboard
:'<,'>"+y  " copy visual selection to system clipboard (or use "+y in visual mode)

" run external command on visual selection (replaces text with output)
:'<,'>!awk -f script.awk
:'<,'>!sort
:'<,'>!column -t -s ','
" run external command on entire file (replaces text with output)
:%!clang-format
:!stat %   " use current file name in command

:read FILE       " insert contents of FILE at cursor
:read !COMMAND   " insert output of COMMAND at cursor

:Lexplore             " open netrw in left pane
:vertical resize -10  " resize window
ctrl+w ctrl+w         " cycle window focus
ctrl+w l              " switch window focus right
ctrl+w h              " switch window focus left
ctrl+w 10>            " increase window width by 10 columns
ctrl+w -10>           " decrease window width by 10 columns
ctrl+w q              " quit window

fx  " go to next character `x` inclusive
tx  " go to next character `x` exclusive
;   " continue forwards to next character `x`
,   " go backwards to previous character `x`

:g/regex  " search and list results with line numbers (go to line n with :n)
ctrl+n  " iterate autocompletions in insert mode

ctrl+r "    " paste from default register
ctrl+r +    " paste from system clipboard

chmod 2775 /path/to/dir
chmod g+s /path/to/dir

TEXT=text

tee file <<EOF
Has variable substitution
  ${TEXT}
EOF

tee file <<'EOF'
No variable substituion
  ${TEXT}
EOF

tee file <<<"
  ${TEXT}
"
# Single quote escape for variable substitution
tee file <<<'
  '${TEXT}'
'

tee <file.txt &>/dev/null \
  >(wc --lines) \
  >(grep --count --invert-match '^$')

# search without regular expressions
grep --fixed-strings '?+{|()' files
# extended regex interprets ? + { | ( ) without \-escapes
grep --extended-regex '(.*)' files

# specify pattern with --regex option or from file if it begins with -
grep ---regex='-+' --file='file.regex' files

# count non-empty lines
grep --count --invert-match '^$' files

# search in all files except dotfiles/dotdirectories
grep --recursive --ignore-case \
  --exclude='.*' --exclude-dir='.*' \
  ${PATTERN} ./

# perl-compatible regular expressions
man 3 pcre2pattern

# URL-encode
echo -n 'a is b?' | jq --slurp --raw-input --raw-output '@uri'
# HTML-entity-encode
echo -n '&<>"' | jq -sRr @html
# tab-separated values
echo -n '[{"a":"1"},{"a":"2"}]' | jq --raw-output 'map(.a) | @tsv'

# merge two objects
jq -s '.[0] * .[1]' defaults.json overrides.json

xmllint --auto --encode UTF8 --output example.xml  # generate an XML file
xmllint --format --recover example.xml             # format an XML file
xmllint -xpath '/parent/child[1]' example.xml      # first <child>
xmllint -xpath '//*[@id="example"]' example.xml    # any node with attribute value
xmllint -xpath '/parent/child = "a" or /parent/child = "b"' example.xml  # first <child>

# delete comment lines
sed -i '/^#/d' file.txt
# delete non-comment lines
sed -i '/^#/!d' file.txt
# append newline if missing ($ matches EOF)
sed -i '$a\' file.txt
# print every fourth line starting at line 1 (first~step)
sed -n '1~3p' file.txt
# print lines 100-106
sed -n '100,+6p' file.txt
# transliterate letters to numbers
sed 'y/abcdefghij/0123456789/' file.txt
# add carriage return to end of line
sed -i 's/$/\r/' file.txt

# regex
sed -E -n '/^a+b$/p' <<<$'a\nb\naab\n' # aab
sed -E -n '/^[^[:space:]]+$/p' <<<$'\na\n \n\t\n\n' # a
# regex back-reference (print lines where uid=gid)
sed -E -n '/^\w+:x:(\w)+:\1/p' /etc/passwd

# exit 1 if a line starts with 'foo'
sed -n '/^foo/q1' file.txt

# encode.sed
s:%:%25:g
s: :%20:g
s:!:%21:g
s:#:%23:g
s:\$:%24:g
s:&:%26:g
s:':%27:g
s:(:%27:g
s:):%27:g
s:*:%2A:g
s:+:%2B:g
s:,:%2C:g
s:/:%2F:g
s^:^%3A^g
s:;:%3B:g
s:<:%3C:g
s:=:%3D:g
s:>:%3E:g
s:?:%3F:g
s:@:%40:g
s:\[:%5B:g
s:\\:%5C:g
s:]:%5D:g
s:\^:%5E:g
s:{:%7B:g
s:|:%7C:g
s:}:%7D:g

# URL-encode
VALUE='% !#$&()*+,/:;<=>?@[\]^{|}'
sed --file=encode.sed <<<"${VALUE}'"

# grep on markdown files (runs: grep file1 file2 ...)
find notes/ -name '*.md' -type f -print0 \
  | xargs --verbose --null grep --fixed-strings --color 'ausearch'

# process files directly in starting point, but not the starting point itself
find . -mindepth 1 -maxdepth 1

# run find only on starting points (which are read from stdin)
ls --zero . | find -files0-from - -maxdepth 0 -regex 'PATTERN'

# skip directories lib and obj
find . '(' -path ./lib -o -path ./obj ')' -a -prune -o -print

# exit after finding a file (implicit -a)
find / -name needle -print -quit

# run make in each folder with a Makefile
find . \
  -name Makefile \
  -execdir make ';'
  # command exit code only affects find-expression; find exits with 0
  # use xargs to exit with error when a commands exits with error

# execute make processes in parallel
find . \
  -name Makefile \
  -printf '%h\0' \
  | xargs --null --max-arg=1 --max-procs=0 \
    make --directory

# exit on first error by returning 255 in command
find . \
  -name Makefile \
  -printf '%h\0' \
  | xargs --null -I '{}' \
    sh -c 'make --directory {} || exit 255'
  # parallel commands (using --max-procs) are waited for to terminate on error

# explain debug options
find -D help
# show diagnostics of exec
find -D exec . -type f -exec ls '{}' ';'

# find processes with a specific files open (LINK -> FILE \n USER TYPE ACCESS KB-SIZE CONTEXT)
sudo find /proc/*/fd/ -ilname '/path/*' -printf '%p -> %l\n  %u %y %a %k %Z\n'

cat /etc/redhat-release
dnf upgrade --refresh

dnf repolist --all
dnf repolist --enabled --verbose

dnf group list
dnf group info 'Development Tools'
sudo dnf group install 'Development Tools'
sudo dnf group remove 'Development Tools'

dnf list --installed
dnf whatprovides /usr/bin/gzip
dnf list --upgrades [<package-file-spec>...]

dnf history --reverse # show command history
dnf history info
dnf history info 5
dnf history list # latest history ID
dnf history list 101 # specific history ID
dnf history list <package-spec>

dnf repoquery '*-devel'
dnf repoquery PACKAGE_NAME --list
dnf repoquery PACKAGE_NAME --requires

dnf repoquery --queryformat '[%{name}] "%{summary}".\n> %{description}' package
dnf repoquery 'PACKAGENAME' --qf '%{NAME} @%{REPONAME} \t "%{SUMMARY}"\n\tDescription: %{DESCRIPTION}\n\n'

dnf repoquery --userinstalled

[main]
gpgcheck=1
installonly_limit=3
clean_requirements_on_remove=True
best=False
skip_if_unavailable=True
fastestmirror=True
max_parallel_downloads=10

cachedir=/dnfcache
install_weak_deps=False
tsflags=nodocs

sudo cat > /etc/yum.repos.d/vscode.repo <<<'
[code]
name=Visual Studio Code
baseurl=https://packages.microsoft.com/yumrepos/vscode
enabled=1
gpgcheck=1
gpgkey=https://packages.microsoft.com/keys/microsoft.asc'

sudo dnf install \
  https://mirrors.rpmfusion.org/free/fedora/rpmfusion-free-release-$(rpm -E %fedora).noarch.rpm +
  https://mirrors.rpmfusion.org/nonfree/fedora/rpmfusion-nonfree-release-$(rpm -E %fedora).noarch.rpm

CACHEDIR=$(mktemp -d --tmpdir=/tmp -t temp-XXXXXXXX) # default is /var/cache/dnf
dnf -y makecache --releasever=31 --setopt=cachedir=${CACHEDIR}
sudo chcon --reference /var/cache/dnf -R ${CACHEDIR}  # SELinux type=rpm_var_cache_t

# remake cache
dnf clean all --setopt=cachedir=${CACHEDIR}
dnf makecache --refresh --setopt=cachedir=${CACHEDIR}

# install with dependencies to temporary directory
INSTALLROOT=$(mktemp -d --tmpdir=/tmp -t temp-XXXXXXXX)
sudo dnf --installroot ${INSTALLROOT} \
  --disablerepo='*' --enablerepo=<repoid> \
  --releasever 31 \
  --setopt=cachedir=${CACHEDIR} \
  --cacheonly \
  --nodocs \
  --setopt=install_weak_deps=False \
  install httpd

rpm --install xmlstarlet-x.x.x-1.i386.rpm
rpm --erase xmlstarlet

rpm --query --list --package package.rpm
rpm --install --nodeps --noplugins --prefix=/path/to/dir/ package.rpm

# extract rpm contents
rpm2cpio php-5.1.4-1.esp1.x86_64.rpm | cpio -idmv
  # -i = extract
  # -d = make directories
  # -m = preserve modification time
  # -v = verbose

# rebuild corrupted database
sudo rpm --rebuilddb

yum --disablerepo='*' --enablerepo='rhel-7-server-rpms'
yum update
yum install -y httpd
yum remove httpd
yum groupinstall 'Development Tools'
yum install pcre pcre-devel zlib zlib-devel openssl openssl-devel

# show update information summary
yum updateinfo
yum updateinfo list --security
yum updateinfo RHSA-2018:1453
yum update --security
yum update --security --sec-severity=Critical,Important
yum update --advisory RHSA-2018:1318 # updates to latest version that contains fix
yum update-minimal --advisory RHSA-2018:1318 # updates to exact version that contains fix

yum updateinfo list --cve CVE-2018-1111
yum update --cve CVE-2018-1111
yum update-minimal --cve CVE-2018-1111

apt-get clean
apt-get update
apt-get -u upgrade --assume-no
apt-get upgrade
apt-get dist-upgrade

apt-get install build-essential
apt-get install libpcre3 libpcre3-dev zlib1g zlib1g-dev libssl-dev

apt-cache search 'search text'
apt-cache depends package # list package dependencies
apt-cache pkgnames
apt-cache show package

apt-get upgrade package2
apt-get remove package1
apt-get purge package2
apt-get --purge remove package2

# remove packages that were automatically installed
apt-get autoremove
apt-get --purge autoremove

# don't upgrade these packages without intervention
apt-mark hold package1 package2

cat /etc/apt/sources.list
cat /etc/apt/sources.list.d/*.list
sudo apt update # update sources

add-apt-repository ppa:certbot/certbot

dpkg -l # list installed
dpkg --info package
dpkg -L package # list package files of installed package
dpkg --contents file.deb # list package files of archive
dpkg -S /path/to/file # show which package provides a file

dpkg --install file.deb
dpkg --remove package
dpkg --purge package

# extract .deb package
ar x package.deb
dpkg -x package.deb /tmp

sudo pacman -Syy # refresh
sudo pacman -Syu # update
sudo pacman -Rsn # remove
sudo packer -Ss  # search