the-art-of-command-line/README-ko.md

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The Art of Command Line

커멘드 라인 멋짐에 대해

• 메타
• 기본
• 자주 사용
• 파일과 데이터 처리
• 시스템 디버깅
• 한줄로
• 유명하진 않지만 사용하면 편한
• 맥용
• 더 많은 자료
• 면책 조항

Fluency on the command line is a skill often neglected or considered arcane, but it improves your flexibility and productivity as an engineer in both obvious and subtle ways. This is a selection of notes and tips on using the command-line that I've found useful when working on Linux. Some tips are elementary, and some are fairly specific, sophisticated, or obscure. This page is not long, but if you can use and recall all the items here, you know a lot.

Much of this originally appeared on Quora, but given the interest there, it seems it's worth using Github, where people more talented than I can readily suggest improvements. If you see an error or something that could be better, please submit an issue or PR! (Of course please review the meta section and existing PRs/issues first.)

메타

Scope:

• This guide is both for beginners and the experienced. The goals are breadth (everything important), specificity (give concrete examples of the most common case), and brevity (avoid things that aren't essential or digressions you can easily look up elsewhere). Every tip is essential in some situation or significantly saves time over alternatives.
• This is written for Linux, with the exception of the "MacOS only" section. Many of the other items apply or can be installed on other Unices or MacOS (or even Cygwin).
• The focus is on interactive Bash, though many tips apply to other shells and to general Bash scripting.
• It includes both "standard" Unix commands as well as ones that require special package installs -- so long as they are important enough to merit inclusion.

Notes:

• To keep this to one page, content is implicitly included by reference. You're smart enough to look up more detail elsewhere once you know the idea or command to Google. Use apt-get/yum/dnf/pacman/pip/brew (as appropriate) to install new programs.
• Use Explainshell to get a helpful breakdown of what commands, options, pipes etc. do.

기본

• Learn basic Bash. Actually, type man bash and at least skim the whole thing; it's pretty easy to follow and not that long. Alternate shells can be nice, but Bash is powerful and always available (learning only zsh, fish, etc., while tempting on your own laptop, restricts you in many situations, such as using existing servers).

• Learn at least one text-based editor well. Ideally Vim (vi), as there's really no competition for random editing in a terminal (even if you use Emacs, a big IDE, or a modern hipster editor most of the time).

• Know how to read documentation with man (for the inquisitive, man man lists the section numbers, e.g. 1 is "regular" commands, 5 is files/conventions, and 8 are for administration). Find man pages with apropos. Know that some commands are not executables, but Bash builtins, and that you can get help on them with help and help -d.

• Learn about redirection of output and input using > and < and pipes using |. Know > overwrites the output file and >> appends. Learn about stdout and stderr.

• 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).

자주 사용

• 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:

      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.

• Use alias to create shortcuts for commonly used commands. For example, alias ll='ls -latr' creates a new alias ll.

• 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.

      # 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:

      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 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

      stat -c '%A %a %n' /etc/timezone

• For interactive selection of values from the output of another command, use percol or fzf.

• For interaction with files based on the output of another command (like git), use fpp (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).

• For running a command with privileges, use sudo (for root) or sudo -u (for another user). Use su or sudo bash to actually run a shell as that user. Use su - to simulate a fresh login as root or another user.

파일과 데이터 처리

• 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.

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

• For Markdown, HTML, and all kinds of document conversion, try pandoc.

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

• For JSON, use jq.

• For Excel or CSV files, csvkit provides in2csv, csvcut, csvjoin, csvgrep, etc.

• For Amazon S3, s3cmd is convenient and s4cmd is faster. Amazon's aws 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:

      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 may help.

      # 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. For numbers, use -n, or -h for handling human-readable numbers (e.g. from du -h). 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.

• 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):

      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.

시스템 디버깅

• For web debugging, curl and curl -I are handy, or their wget equivalents, or the more modern 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. 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), perf, and sysdig.

• Check what OS you're on with uname or uname -a (general Unix/kernel info) or lsb_release -a (Linux distro info).

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

한줄로

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).

      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):

      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:

      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:

      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:

      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):

      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
      }

유명하진 않지만 사용하면 편한

• expr: 산술 또는 boolean 연산을 수행하거나 정규 표현식을 평가하는데 사용합니다.

• m4: 간단한 매크로 프로세서

• yes: 많은 양의 문자열 출력합니다.

• cal: 꽤 괜찮은 달력

• env: 명령어 실행합니다. (스크립트 작성시 유용함)

• printenv: 환경 변수 출력합니다. (디버깅하거나 스크립트 작성시 유용함)

• look: 문자열로 시작하는 영어 단어나 파일에 있는 라인을 찾습니다.

• cut, paste, join: 데이터를 조작합니다.

• fmt: 텍스트 구문 서식

• pr: 페이지 / 컬럼 텍스트 서식

• fold: 텍스트 줄 바꿈

• column: 열이나 테이블로 텍스트 서식 변경합니다.

• expand, unexpand: 탭에서 스페이스로, 스페이스에서 탭으로 변경

• nl: 줄번호를 추가합니다.

• seq: 숫자 출력를 출력합니다.

• bc: 계산기

• factor: 정수 인자

• gpg: 암호화한 서명 파일을 생성합니다.

• toe: terminfo 항목 표룰 나타냅니다.

• nc: 네트워크 디버깅과 데이터 변환합니다.

• socat: 소켓 릴레이 및 TCP 포트 전달자 (netcat과 유사)

• slurm: 네트워크 트래픽 시각화합니다.

• dd: 파일이나 장치간 데이터 이동합니다.

• file: 파일 유형 확인합니다.

• tree: 디렉토리와 하위 디렉토리를 중첩 트리로 표현. ls와 비슷하나 반복하여 보여줍니다.

• stat: 파일 정보를 표시합니다.

• tac: 역으로 파일을 출력합니다.

• shuf: 파일에서 줄을 무작위로 선택합니다.

• comm: 줄별로 정렬된 파일 줄을 비교합니다.

• pv: 파이프를 통해 처리되는 데이터를 감시합니다.

• hd, bvi: 바이너리 파일을 덤프하거나 수정합니다.

• strings: 바이너리 파일에서 텍스트를 추출합니다.

• tr: 문자를 변경하거나 조작합니다.

• iconv, uconv: 텍스트의 인코딩을 변경합니다.

• split, csplit: 파일을 분할합니다.

• sponge: 수정되는 내용을 읽어 입력합니다. 동일한 파일에 데이터를 변경하여 작성하기 유용합니다. 예로, grep -v something some-file | sponge some-file

• units: 단위를 계산하거나 변환합니다. 2주당 펄롱을 1깜박임당 트윕으로 변환합니다. (관련해서는 /usr/share/units/definitions.units를 확인하세요.)

• 7z: 높은 비율을 자랑하는 파일 압축기

• ldd: 동적 라이브러리 정보를 나타냅니다.

• nm: 오브젝트 파일의 심볼을 나타냅니다.

• ab: 웹서버 벤치마킹에 사용합니다.

• strace: 시스템 콜을 디버깅합니다.

• mtr: 네트워크 디버깅을위한 더 나은 경로를 추적합니다.

• cssh: 동시에 여러 쉘 작업을 할 수 있습니다.

• rsync: SSH를 이용하여 파일이나 폴더를 동기화합니다.

• wireshark, tshark: 패킷 캡처하고 네트워크를 디버깅합니다.

• ngrep: 네트워크 레이어를 grep 합니다.

• host, dig: DNS를 조회합니다.

• lsof: 프로세스 파일 서술자(descriptor)와 소켓 정보를 나타냅니다.

• dstat: 시스템 통계를 내는대 유용합니다.

• glances: 높은 수준의 다중 서브시스템 모니터링 툴

• iostat: CPU, 디스크 사용량 상태를 나타냅니다.

• htop: top의 개선 버전

• last: 로그인 이력을 확인합니다.

• w: 지금 누가 로그인 되어있는지를 확인합니다.

• id: 사용자 / 그룹 계정 정보를 확인합니다.

• sar: 시스템 상태에 대한 이력을 확인하는 도구입니다.

• iftop, nethogs: 소켓이나 프로세스용 네트워크 유틸입니다.

• ss: 소켓 통계를 보여줍니다.

• dmesg: 부팅과 시스템 에러 메시지를 확인할 수 있습니다.

• hdparm: SATA/ATA 디스크 조작하거나 성능을 확인할 수 있습니다.

• lsb_release: 리눅스 배포판 정보를 확인할 수 있습니다.

• lsblk: 블록 장치 목록(List block devices): 디스크와 디스크 파티션에 대한 목록을 트리형식으로 보여줍니다.

• lshw, lscpu, lspci, lsusb, dmidecode: CPU, BIOS, RAID, 그래픽, 장치 등이 포함되어있는 하드웨어 정보를 확인할 수 있습니다.

• fortune, ddate, sl: 음... 어떤 방식으로 사용하느냐에 따라 "유용하게" 사용할 수 있습니다.

맥용

MacOS에서만 해당되는 항목입니다.

• brew (Homebrew)나 port (MacPorts)를 패키지 메니저로 사용합니다. 보다 많은 명령어를 MacOS에 설치하여 사용할 수 있습니다.

• pbcopy를 이용하여 데스크탑 어플리케이션에 명령어 출력물을 복사하거나 pbpaste를 이용해 붙여넣기를 할 수 있습니다.

• 데스크탑 어플리케이션에서 파일을 열기위해, open 또는 open -a /Applications/Whatever.app을 사용하면 됩니다.

• Spotlight: mdfind를 이용해 파일을 찾고, mdls를 이용해 메타데이타 (사진 EXIF 정보와 같은) 목록을 볼 수 있습니다.

• MacOS는 BSD Unix 기반이며 많은 명령어들을 (예로 ps, ls, tail, awk, sed) 사용할 수 있으며, 이것들은 Linux 버전들과 미묘한 차이가 있습니다. 그리고 크게는 System V-style Unix와 GNU 도구들에 많은 영향을 받았습니다. 이런 내용들을 man 페이지 상단의 "BSD General Commands Manual." 라는 문구를 통해 알 수 있습니다. 가끔은 GNU 버전이 설치되기도합니다. (예로, GNU awk와 sed인 gawk와 gsed에서). 만약 이종 플랫폼간 Bash 스크립트를 작성하려면, 동일한 명령어 (예로, 파이썬이나 perl과 같은)나 테스트시 주의해야합니다.

더 많은 자료

• awesome-shell: 쉘 도구와 리소스를 모아둔 목록
• 더 좋은 쉘 스크립트 작성을 위한 Strict 모드

면책 조항

매우 작은 작업을 제외한 코드들은 다른 사람이 읽을 수 있도록 작성됩니다. 그러니 이 내용은 작성자 전원에게 책임이 있습니다. Bash에서 뭔가를 할 수 있다는 것이 당신이 뭔가를 해야된다는 것을 강요하는 것이 아니다! ;)

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