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# The Art of Command Line

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- [Meta](#meta)
- [Basics](#basics)
- [Everyday use](#everyday-use)
- [Processing files and data](#processing-files-and-data)
- [System debugging](#system-debugging)
- [One-liners](#one-liners)
- [Obscure but useful](#obscure-but-useful)
- [More resources](#more-resources)
- [Disclaimer](#disclaimer)


![curl -s 'https://raw.githubusercontent.com/jlevy/the-art-of-command-line/master/README.md' | egrep -o '`\w+`' | tr -d '`' | cowsay -W50](cowsay.png)

커맨드라인을 능숙하게 다루는것은 도외시되거나 신비스럽게 여겨집니다. 하지만 커맨드라인은 명백하고도 미묘한 방법으로 엔지니어가 하는 작업의 유연성과 생산성을 향상시십니다. 이 문서는 리눅스에서 작업을 하면서 찾은 노트와 팁들의 모음입니다. 몇 가지는 기초적이고, 몇가지는 상당히 구체적이며, 세련되고, 잘 알려지지 않은 것입니다. 이 문서는 그리 길지 않지만, 여기 있는 모든것을 사용할 수 있게 되고, 기억해낼 수 있게 된다면, 많은 것을 알게되는 것입니다.

여기있는 대부분의 것은
[원래](http://www.quora.com/What-are-some-lesser-known-but-useful-Unix-commands)
[Quora에](http://www.quora.com/What-are-the-most-useful-Swiss-army-knife-one-liners-on-Unix)
 [올라온](http://www.quora.com/What-are-some-time-saving-tips-that-every-Linux-user-should-know) 것입니.
하지만 거기에 관심을 가지기보다, Github를 이용하는 것이 더 가치있는 것처럼 보입니다. 여기엔 더 재능있는 사람들이 손쉽게 개선안을 제안할 수 있는 곳이죠. 만약 문제가 있거나, 더 나아질 수 있는 내용이 보인다면, 이슈를 제출하거나 풀 리퀘스트를 보내주세요! (물론 meta 섹션과 이미 존재하는 풀 리퀘스트와 이슈를 봐주기를 바랍니다.)

## Meta

범위:

- 이 가이드는 초심자와 경험자 모두를 위한 것입니다. 목표는 범위(전부 다 중요합니다!), 구체성(대부분의 일반적인 케이스에 대한 구체적인 예제), 그리고 간결함(쉽게 마주치지 않는, 중요하지 않고, 지엽적인 것을 피함) 입니다. 모든 팁은 특정 상황에서 매우 중요하거나, 여러 대안들 사이에서의 시간을 확연하게 절약합니다.
- 이 문서는 리눅스를 위한것입니다. 일부는 MacOS에서 똑같이 적용되지 않습니다(Cygwin에서 조차 말이죠).
- 인터랙티브 Bash에 초점이 맞추어져있습니다만, 대부분의 팁은 다른 쉘이나, general Bash 스크립트에서도 동작합니다.
- 이 문서는 "스탠다드" 유닉스 커맨드와 특정 패키지 설치를 필요로 하는 것 둘 다 포함하고 있습니다. 여기서 다루는 스탠다드 커맨드와 특정 패키지에 대한 것은 포함될만큼 충분히 중요합니다.

노트:

- 이 문서를 한 파일로 유지하기 위해서, 컨텐츠들은 암시적인 레퍼런스 형태로 포함되어있습니다. 한 개념이나 명령어에 대해 알게 된 후에, 다른곳에서 그에대한 좀 더 자세한 정보를 찾을 수 있을만큼 당신은 똑똑할것입니다. `apt-get`, `yum`, `dnf`, `pacman`, `pip`, `brew` (혹은 적절한 다른 것)을 이용해 새 프로그램을 설치하세요.
- [Explainshell](http://explainshell.com/)을 이용해서 각각의 커맨드, 옵션, 파이프나 그 외 등등이 어떤것인지 알아보십시오.


## Basics

- 기본 Bash를 배우세요. 말하자면, 최소한 `man bash`를 실행하고, 전부를 훑어 보세요. 매뉴얼의 내용은 따라가기 쉬우며 그리 길지 않습니다. 다른 쉘들 또한 좋습니다만, Bash는 강력하고 언제나 사용가능합니다( *오직* zsh, fish, 그 외의 쉘만을 당신의 노트북에서 시도하면서 배우는 경우에는, 많은 경우 제한이 생길것입니다. 이미 존재하는 서버를 사용하는 것등의 일에서 말이죠).

- 텍스트 기반 에디터를 최소한 하나정도 다룰 수 있게 배우세요. Vim(`Vi`)가 이상적입니다. 터미널에서 온갖 작업을 하는데 다른 실질적인 경쟁자가 없기 때문이죠(Emacs, 대형 IDE 또는 모던 힙스터스러운 에디터를 대부분의 작업에 사용한다고 해도 말이죠).

- `man`을 이용해서 문서를 읽는 법을 배우세요(호기심 많은 사람을 위해서 하는 얘기입니다만, `man man`은 섹션 번호들의 목록을 표시합니다. 예를 들어 1은 "regular" 커맨드, 5는 files/conventions, 그리고 8은 administration이죠). `apropos`를 히용해서 man 페이지를 찾으세요. 몇몇 커맨드는 실행가능한 커맨드가 아니라는 것을 알아두세요. 하지만 Bash 빌트인 함수들은 `help`와 `help -d`를 이용해서 도움말을 볼 수 있습니다.

- `>`와 `<`, `|`를 이용한 파이프를 사용해서 입력과 출력의 리다이렉션을 배우세요. stdout(역주: 표준 출력)과 stderr(역주: 표준 에러 출력)에 대해서 배우세요. 

- Learn about file glob expansion with `*` (and perhaps `?` and `{`...`}`) and quoting and the difference between double `"` and single `'` quotes. (See more on variable expansion below.)

- Be familiar with Bash job management: `&`, **ctrl-z**, **ctrl-c**, `jobs`, `fg`, `bg`, `kill`, etc.

- Know `ssh`, and the basics of passwordless authentication, via `ssh-agent`, `ssh-add`, etc.

- Basic file management: `ls` and `ls -l` (in particular, learn what every column in `ls -l` means), `less`, `head`, `tail` and `tail -f` (or even better, `less +F`), `ln` and `ln -s` (learn the differences and advantages of hard versus soft links), `chown`, `chmod`, `du` (for a quick summary of disk usage: `du -hk *`). For filesystem management, `df`, `mount`, `fdisk`, `mkfs`, `lsblk`.

- Basic network management: `ip` or `ifconfig`, `dig`.

- Know regular expressions well, and the various flags to `grep`/`egrep`. The `-i`, `-o`, `-A`, and `-B` options are worth knowing.

- Learn to use `apt-get`, `yum`, `dnf` or `pacman` (depending on distro) to find and install packages. And make sure you have `pip` to install Python-based command-line tools (a few below are easiest to install via `pip`).


## Everyday use

- In Bash, use **Tab** to complete arguments and **ctrl-r** to search through command history.

- In Bash, use **ctrl-w** to delete the last word, and **ctrl-u** to delete all the way back to the start of the line. Use **alt-b** and **alt-f** to move by word, **ctrl-k** to kill to the end of the line, **ctrl-l** to clear the screen. See `man readline` for all the default keybindings in Bash. There are a lot. For example **alt-.** cycles through previous arguments, and **alt-*** expands a glob.

- Alternatively, if you love vi-style key-bindings, use `set -o vi`.

- To see recent commands, `history`. There are also many abbreviations such as `!$` (last argument) and `!!` last command, though these are often easily replaced with **ctrl-r** and **alt-.**.

- To go back to the previous working directory: `cd -`

- If you are halfway through typing a command but change your mind, hit **alt-#** to add a `#` at the beginning and enter it as a comment (or use **ctrl-a**, **#**, **enter**). You can then return to it later via command history.

- Use `xargs` (or `parallel`). It's very powerful. Note you can control how many items execute per line (`-L`) as well as parallelism (`-P`). If you're not sure if it'll do the right thing, use `xargs echo` first. Also, `-I{}` is handy. Examples:
```bash
      find . -name '*.py' | xargs grep some_function
      cat hosts | xargs -I{} ssh root@{} hostname
```

- `pstree -p` is a helpful display of the process tree.

- Use `pgrep` and `pkill` to find or signal processes by name (`-f` is helpful).

- Know the various signals you can send processes. For example, to suspend a process, use `kill -STOP [pid]`. For the full list, see `man 7 signal`

- Use `nohup` or `disown` if you want a background process to keep running forever.

- Check what processes are listening via `netstat -lntp` or `ss -plat` (for TCP; add `-u` for UDP).

- See also `lsof` for open sockets and files.

- In Bash scripts, use `set -x` for debugging output. Use strict modes whenever possible. Use `set -e` to abort on errors. Use `set -o pipefail` as well, to be strict about errors (though this topic is a bit subtle). For more involved scripts, also use `trap`.

- In Bash scripts, subshells (written with parentheses) are convenient ways to group commands. A common example is to temporarily move to a different working directory, e.g.
```bash
      # do something in current dir
      (cd /some/other/dir && other-command)
      # continue in original dir
```

- In Bash, note there are lots of kinds of variable expansion. Checking a variable exists: `${name:?error message}`. For example, if a Bash script requires a single argument, just write `input_file=${1:?usage: $0 input_file}`. Arithmetic expansion: `i=$(( (i + 1) % 5 ))`. Sequences: `{1..10}`. Trimming of strings: `${var%suffix}` and `${var#prefix}`. For example if `var=foo.pdf`, then `echo ${var%.pdf}.txt` prints `foo.txt`.

- The output of a command can be treated like a file via `<(some command)`. For example, compare local `/etc/hosts` with a remote one:
```sh
      diff /etc/hosts <(ssh somehost cat /etc/hosts)
```

- Know about "here documents" in Bash, as in `cat <<EOF ...`.

- In Bash, redirect both standard output and standard error via: `some-command >logfile 2>&1`. Often, to ensure a command does not leave an open file handle to standard input, tying it to the terminal you are in, it is also good practice to add `</dev/null`.

- Use `man ascii` for a good ASCII table, with hex and decimal values. For general encoding info, `man unicode`, `man utf-8`, and `man latin1` are helpful.

- Use `screen` or [`tmux`](https://tmux.github.io/) to multiplex the screen, especially useful on remote ssh sessions and to detach and re-attach to a session. A more minimal alternative for session persistence only is `dtach`.

- In ssh, knowing how to port tunnel with `-L` or `-D` (and occasionally `-R`) is useful, e.g. to access web sites from a remote server.

- It can be useful to make a few optimizations to your ssh configuration; for example, this `~/.ssh/config` contains settings to avoid dropped connections in certain network environments, use compression (which is helpful with scp over low-bandwidth connections), and multiplex channels to the same server with a local control file:
```
      TCPKeepAlive=yes
      ServerAliveInterval=15
      ServerAliveCountMax=6
      Compression=yes
      ControlMaster auto
      ControlPath /tmp/%r@%h:%p
      ControlPersist yes
```

- A few other options relevant to ssh are security sensitive and should be enabled with care, e.g. per subnet or host or in trusted networks: `StrictHostKeyChecking=no`, `ForwardAgent=yes`

- To get the permissions on a file in octal form, which is useful for system configuration but not available in `ls` and easy to bungle, use something like
```sh
      stat -c '%A %a %n' /etc/timezone
```

- For interactive selection of values from the output of another command, use [`percol`](https://github.com/mooz/percol).

- For interaction with files based on the output of another command (like `git`), use `fpp` ([PathPicker](https://github.com/facebook/PathPicker)).

- For a simple web server for all files in the current directory (and subdirs), available to anyone on your network, use:
`python -m SimpleHTTPServer 7777` (for port 7777 and Python 2) and `python -m http.server 7777` (for port 7777 and Python 3).


## Processing files and data

- To locate a file by name in the current directory, `find . -iname '*something*'` (or similar). To find a file anywhere by name, use `locate something` (but bear in mind `updatedb` may not have indexed recently created files).

- For general searching through source or data files (more advanced than `grep -r`), use [`ag`](https://github.com/ggreer/the_silver_searcher).

- To convert HTML to text: `lynx -dump -stdin`

- For Markdown, HTML, and all kinds of document conversion, try [`pandoc`](http://pandoc.org/).

- If you must handle XML, `xmlstarlet` is old but good.

- For JSON, use `jq`.

- For Excel or CSV files, [csvkit](https://github.com/onyxfish/csvkit) provides `in2csv`, `csvcut`, `csvjoin`, `csvgrep`, etc.

- For Amazon S3, [`s3cmd`](https://github.com/s3tools/s3cmd) is convenient and [`s4cmd`](https://github.com/bloomreach/s4cmd) is faster. Amazon's [`aws`](https://github.com/aws/aws-cli) is essential for other AWS-related tasks.

- Know about `sort` and `uniq`, including uniq's `-u` and `-d` options -- see one-liners below. See also `comm`.

- Know about `cut`, `paste`, and `join` to manipulate text files. Many people use `cut` but forget about `join`.

- Know about `wc` to count newlines (`-l`), characters (`-m`), words (`-w`) and bytes (`-c`).

- Know about `tee` to copy from stdin to a file and also to stdout, as in `ls -al | tee file.txt`.

- Know that locale affects a lot of command line tools in subtle ways, including sorting order (collation) and performance. Most Linux installations will set `LANG` or other locale variables to a local setting like US English. But be aware sorting will change if you change locale. And know i18n routines can make sort or other commands run *many times* slower. In some situations (such as the set operations or uniqueness operations below) you can safely ignore slow i18n routines entirely and use traditional byte-based sort order, using `export LC_ALL=C`.

- Know basic `awk` and `sed` for simple data munging. For example, summing all numbers in the third column of a text file: `awk '{ x += $3 } END { print x }'`. This is probably 3X faster and 3X shorter than equivalent Python.

- To replace all occurrences of a string in place, in one or more files:
```sh
      perl -pi.bak -e 's/old-string/new-string/g' my-files-*.txt
```

- To rename many files at once according to a pattern, use `rename`. For complex renames, [`repren`](https://github.com/jlevy/repren) may help.
```sh
      # Recover backup files foo.bak -> foo:
      rename 's/\.bak$//' *.bak
      # Full rename of filenames, directories, and contents foo -> bar:
      repren --full --preserve-case --from foo --to bar .
```

- Use `shuf` to shuffle or select random lines from a file.

- Know `sort`'s options. Know how keys work (`-t` and `-k`). In particular, watch out that you need to write `-k1,1` to sort by only the first field; `-k1` means sort according to the whole line. Stable sort (`sort -s`) can be useful. For example, to sort first by field 2, then secondarily by field 1, you can use `sort -k1,1 | sort -s -k2,2`. For handling human-readable numbers (e.g. from `du -h`) use `sort -h`.

- If you ever need to write a tab literal in a command line in Bash (e.g. for the -t argument to sort), press **ctrl-v** **[Tab]** or write `$'\t'` (the latter is better as you can copy/paste it).

- The standard tools for patching source code are `diff` and `patch`. See also `diffstat` for summary statistics of a diff. Note `diff -r` works for entire directories. Use `diff -r tree1 tree2 | diffstat` for a summary of changes.

- For binary files, use `hd` for simple hex dumps and `bvi` for binary editing.

- Also for binary files, `strings` (plus `grep`, etc.) lets you find bits of text.

- For binary diffs (delta compression), use `xdelta3`.

- To convert text encodings, try `iconv`. Or `uconv` for more advanced use; it supports some advanced Unicode things. For example, this command lowercases and removes all accents (by expanding and dropping them):
```sh
      uconv -f utf-8 -t utf-8 -x '::Any-Lower; ::Any-NFD; [:Nonspacing Mark:] >; ::Any-NFC; ' < input.txt > output.txt
```

- To split files into pieces, see `split` (to split by size) and `csplit` (to split by a pattern).

- Use `zless`, `zmore`, `zcat`, and `zgrep` to operate on compressed files.


## System debugging

- For web debugging, `curl` and `curl -I` are handy, or their `wget` equivalents, or the more modern [`httpie`](https://github.com/jakubroztocil/httpie).

- To know disk/cpu/network status, use `iostat`, `netstat`, `top` (or the better `htop`), and (especially) `dstat`. Good for getting a quick idea of what's happening on a system.

- For a more in-depth system overview, use [`glances`](https://github.com/nicolargo/glances). It presents you with several system level statistics in one terminal window. Very helpful for quickly checking on various subsystems.

- To know memory status, run and understand the output of `free` and `vmstat`. In particular, be aware the "cached" value is memory held by the Linux kernel as file cache, so effectively counts toward the "free" value.

- Java system debugging is a different kettle of fish, but a simple trick on Oracle's and some other JVMs is that you can run `kill -3 <pid>` and a full stack trace and heap summary (including generational garbage collection details, which can be highly informative) will be dumped to stderr/logs.

- Use `mtr` as a better traceroute, to identify network issues.

- For looking at why a disk is full, `ncdu` saves time over the usual commands like `du -sh *`.

- To find which socket or process is using bandwidth, try `iftop` or `nethogs`.

- The `ab` tool (comes with Apache) is helpful for quick-and-dirty checking of web server performance. For more complex load testing, try `siege`.

- For more serious network debugging, `wireshark`, `tshark`, or `ngrep`.

- Know about `strace` and `ltrace`. These can be helpful if a program is failing, hanging, or crashing, and you don't know why, or if you want to get a general idea of performance. Note the profiling option (`-c`), and the ability to attach to a running process (`-p`).

- Know about `ldd` to check shared libraries etc.

- Know how to connect to a running process with `gdb` and get its stack traces.

- Use `/proc`. It's amazingly helpful sometimes when debugging live problems. Examples: `/proc/cpuinfo`, `/proc/xxx/cwd`, `/proc/xxx/exe`, `/proc/xxx/fd/`, `/proc/xxx/smaps`.

- When debugging why something went wrong in the past, `sar` can be very helpful. It shows historic statistics on CPU, memory, network, etc.

- For deeper systems and performance analyses, look at `stap` ([SystemTap](https://sourceware.org/systemtap/wiki)), [`perf`](http://en.wikipedia.org/wiki/Perf_(Linux)), and [`sysdig`](https://github.com/draios/sysdig).

- Confirm what Linux distribution you're using (works on most distros): `lsb_release -a`

- Use `dmesg` whenever something's acting really funny (it could be hardware or driver issues).


## One-liners

A few examples of piecing together commands:

- It is remarkably helpful sometimes that you can do set intersection, union, and difference of text files via `sort`/`uniq`. Suppose `a` and `b` are text files that are already uniqued. This is fast, and works on files of arbitrary size, up to many gigabytes. (Sort is not limited by memory, though you may need to use the `-T` option if `/tmp` is on a small root partition.) See also the note about `LC_ALL` above and `sort`'s `-u` option (left out for clarity below).
```sh
      cat a b | sort | uniq > c   # c is a union b
      cat a b | sort | uniq -d > c   # c is a intersect b
      cat a b b | sort | uniq -u > c   # c is set difference a - b
```

- Use `grep . *` to visually examine all contents of all files in a directory, e.g. for directories filled with config settings, like `/sys`, `/proc`, `/etc`.


- Summing all numbers in the third column of a text file (this is probably 3X faster and 3X less code than equivalent Python):
```sh
      awk '{ x += $3 } END { print x }' myfile
```

- If want to see sizes/dates on a tree of files, this is like a recursive `ls -l` but is easier to read than `ls -lR`:
```sh
      find . -type f -ls
```

- Use `xargs` or `parallel` whenever you can. Note you can control how many items execute per line (`-L`) as well as parallelism (`-P`). If you're not sure if it'll do the right thing, use xargs echo first. Also, `-I{}` is handy. Examples:
```sh
      find . -name '*.py' | xargs grep some_function
      cat hosts | xargs -I{} ssh root@{} hostname
```

- Say you have a text file, like a web server log, and a certain value that appears on some lines, such as an `acct_id` parameter that is present in the URL. If you want a tally of how many requests for each `acct_id`:
```sh
      cat access.log | egrep -o 'acct_id=[0-9]+' | cut -d= -f2 | sort | uniq -c | sort -rn
```

- Run this function to get a random tip from this document (parses Markdown and extracts an item):
```sh
      function taocl() {
        curl -s https://raw.githubusercontent.com/jlevy/the-art-of-command-line/master/README.md |
          pandoc -f markdown -t html |
          xmlstarlet fo --html --dropdtd |
          xmlstarlet sel -t -v "(html/body/ul/li[count(p)>0])[$RANDOM mod last()+1]" |
          xmlstarlet unesc | fmt -80
      }
```


## Obscure but useful

- `expr`: perform arithmetic or boolean operations or evaluate regular expressions

- `m4`: simple macro processor

- `yes`: print a string a lot

- `cal`: nice calendar

- `env`: run a command (useful in scripts)

- `printenv`: print out environment variables (useful in debugging and scripts)

- `look`: find English words (or lines in a file) beginning with a string

- `cut `and `paste` and `join`: data manipulation

- `fmt`: format text paragraphs

- `pr`: format text into pages/columns

- `fold`: wrap lines of text

- `column`: format text into columns or tables

- `expand` and `unexpand`: convert between tabs and spaces

- `nl`: add line numbers

- `seq`: print numbers

- `bc`: calculator

- `factor`: factor integers

- `gpg`: encrypt and sign files

- `toe`: table of terminfo entries

- `nc`: network debugging and data transfer

- `socat`: socket relay and tcp port forwarder (similar to `netcat`)

- `slurm`: network trafic visualization

- `dd`: moving data between files or devices

- `file`: identify type of a file

- `tree`: display directories and subdirectories as a nesting tree; like `ls` but recursive

- `stat`: file info

- `tac`: print files in reverse

- `shuf`: random selection of lines from a file

- `comm`: compare sorted files line by line

- `pv`: monitor the progress of data through a pipe

- `hd` and `bvi`: dump or edit binary files

- `strings`: extract text from binary files

- `tr`: character translation or manipulation

- `iconv` or `uconv`: conversion for text encodings

- `split `and `csplit`: splitting files

- `units`: unit conversions and calculations; converts furlongs per fortnight to twips per blink (see also `/usr/share/units/definitions.units`)

- `7z`: high-ratio file compression

- `ldd`: dynamic library info

- `nm`: symbols from object files

- `ab`: benchmarking web servers

- `strace`: system call debugging

- `mtr`: better traceroute for network debugging

- `cssh`: visual concurrent shell

- `rsync`: sync files and folders over SSH

- `wireshark` and `tshark`: packet capture and network debugging

- `ngrep`: grep for the network layer

- `host` and `dig`: DNS lookups

- `lsof`: process file descriptor and socket info

- `dstat`: useful system stats

- [`glances`](https://github.com/nicolargo/glances): high level, multi-subsystem overview

- `iostat`: CPU and disk usage stats

- `htop`: improved version of top

- `last`: login history

- `w`: who's logged on

- `id`: user/group identity info

- `sar`: historic system stats

- `iftop` or `nethogs`: network utilization by socket or process

- `ss`: socket statistics

- `dmesg`: boot and system error messages

- `hdparm`: SATA/ATA disk manipulation/performance

- `lsb_release`: Linux distribution info

- `lsblk`: List block devices: a tree view of your disks and disk paritions

- `lshw`, `lscpu`, `lspci`, `lsusb`, `dmidecode`: hardware information, including CPU, BIOS, RAID, graphics, devices, etc.

- `fortune`, `ddate`, and `sl`: um, well, it depends on whether you consider steam locomotives and Zippy quotations "useful"


## More resources

- [awesome-shell](https://github.com/alebcay/awesome-shell): A curated list of shell tools and resources.
- [Strict mode](http://redsymbol.net/articles/unofficial-bash-strict-mode/) for writing better shell scripts.


## Disclaimer

With the exception of very small tasks, code is written so others can read it. With power comes responsibility. The fact you *can* do something in Bash doesn't necessarily mean you should! ;)


## License

[![Creative Commons License](https://i.creativecommons.org/l/by-sa/4.0/88x31.png)](http://creativecommons.org/licenses/by-sa/4.0/) 

This work is licensed under a [Creative Commons Attribution-ShareAlike 4.0 International License](http://creativecommons.org/licenses/by-sa/4.0/).