bash

• http://kvz.io/blog/2013/11/21/bash-best-practices/
• http://wiki.bash-hackers.org/commands/builtin/printf

Alapvető hibakezelés:

set -eufo pipefail

…

• Use set -o errexit (a.k.a. set -e) to make your script exit when a command fails.
• Then add || true to commands that you allow to fail.
• Use set -o nounset (a.k.a. set -u) to exit when your script tries to use undeclared variables.
• Use set -o xtrace (a.k.a set -x) to trace what gets executed. Useful for debugging.
• Use set -o pipefail in scripts to catch mysqldump fails in e.g. mysqldump |gzip. The exit status of the last command that threw a non-zero exit code is returned.

… kvz.io

• https://ss64.com/bash/syntax-keyboard.html

printf '"%b"\n' "$0" "$@" | nl -v0 -s": "

• http://stackoverflow.com/q/5799303/1108919
• http://stackoverflow.com/q/5349718/1108919

• 120 db szóköz:

  printf '%120s'

• 120 db tetszőleges karakter (pl. „=” jel):

  printf '%120s' | tr ' ' '='

• http://stackoverflow.com/a/26899206/1108919
• http://www.gnu.org/software/bash/manual/html_node/Shell-Parameter-Expansion.html#Shell-Parameter-Expansion

• Ha nincs deklarálva a $parameter változó, akkor legyen lecserélve a „default” szóra:

  echo "${parameter='default'}"

• Ha nincs deklarálva, vagy üres string a $parameter változó, akkor legyen lecserélve a „default” szóra:

  echo "${parameter:='default'}"

• http://unix.stackexchange.com/a/122848/21185
• http://www.thegeekstuff.com/2010/06/bash-array-tutorial

exec {out_fd}>/tmp/en_fajlom
 
echo "${out_fd}"
#például 10

• http://unix.stackexchange.com/a/64011/21185
• http://tldp.org/LDP/abs/html/process-sub.html

Parancs kimenetének használata, mint fájl leíró

• cat <( date )

• date >( cat )

• https://gist.github.com/andras-tim/f8aebf9243cecf3719d27d020a718ef8

TODO:

Solution:

| Component                     | Explanation            
| ------------- | --------------- | ------ |                                                                         | Note |
| `echo 2 >a`                   | Write `2` to file **a**.                                                                        | Creates **a** file with `2` content |
| `<(echo 3)`                   | Run `echo 3` command in a sub-shell and get the path of the it's stdout fd (e.g. `/dev/fd/63`). | Creates a subshell |
| `exec {b}< /dev/fd/63`        | ???                                                                                                | ??? |
| `cat -`                        | `cat` reads from stdin instead of a file and writes everything to stdout                                                     ||
| `echo 1 | cat -`              | stdout of `echo 1` piped to `cat`                             | #1 input of `cat` |
| `cat - <a`                    | Read **a** file into stdin of `cat`                                                         | #2 input of `cat` |
| `cat - <&${b}`                | ??? | #3 input of `cat` |
| ```
cat - <<-EOF
    4
EOF
``` | Read multiline string while `EOF` with removal of <TAB> indents | #4 input of `cat` |
| `cat - <<<5`                    | Write `5` to stdin of `cat` | #5 input of `cat` |
| `cat 2>&1`                | merge stderr of cat into it's stdout |
| `cat >c`                    | `cat` writes the input data to file **c** | creates **c** file |
| `cat >|d`                    | `cat` writes the input data to file **d** if the **d** is not existed ??? | creates **d** file |
| `cat >>e`                    | `cat` appends the input data to file **e** | creates **e** file |

• https://unix.stackexchange.com/questions/3886/difference-between-nohup-disown-and

• Háttérben indítás:

  # a szülő process megszűnésekor a parancs is leáll
  command &
   
  # a szülő process megszűnésekor a parancs tovább él
  command &!
  # ==
  nohup command

• Előtérben indított program háttérbe szüneteltetése: Ctrl+Z
• Szüneteltetett program kezeléseÉ

  # folytatás előtérben
  fg
   
  # folytatás háttérben
  bg

• Háttérben indított program utólagos leválasztás a szülőről:

  disown

• https://stackoverflow.com/a/41940626/1108919