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The Art of Command Line

Meta
Basics
Everyday use
Processing files and data
System debugging
One-liners
Obscure but useful
More resources
Disclaimer

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

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 Quora에 올라온 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.)

Basics

기본 Bash를 배우세요. 말하자면, 최소한 man bash를 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).
man을 이용해서 문서를 읽는 법을 배우세요(호기심 많은 사람을 위해서 하는 얘기입니다만, man man은 섹션 번호들의 목록을 표시합니다. 예를 들어 1은 "regular" 커맨드, 5는 files/conventions, 그리고 8은 administration이죠). apropos를 히용해서 man 페이지를 찾으세요. 몇몇 커맨드는 실행가능한 커맨드가 아니라는 것을 알아두세요. 하지만 Bash 빌트인 함수들은 help와 help -d를 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.
*(그리고 아마도 ?과 {...})을 이용하는 파일 글롭(glob) 확장을 배우세요. 그리고 쌍따옴표"와 홑따옴표'의 차이를 배우세요. (변수 확장에 대해서 더 보려면 아래를 참조하세요)
Bash 작업 관리에 익숙해지세요. &, ctrl-z, ctrl-c, jobs, fg, bg, kill 등등.
ssh를 배우고, ssh-agent, ssh-add를 통해서 비밀번호 없는 인증 방식의 기본을 배우세요.
기본 파일 관리: ls와 ls -l(특별히, ls -l에서 각각의 열이 무슨 의미인지 배우세요), less, head, tail 그리고 tail -f(또는 더 좋은 less +F), ln과 ln -s(하드 링크와 소프트 링크의 차이와 각각의 장단점을 배우세요), chown, chmod, du( 디스크 사용량의 빠른 요약을 보려면 du -hk *). 파일 시스템 관리를 위해서는 df, mount, fdisk, mkfs, lsblk.
기본 네트워크 관리: ip 또는 ifconfig, dig.
정규표현식(regular expression)을 잘 알아두세요. 그리고 grep/egrep의 다양한 플래그도 알아두세요. -i, -o, -A와 -B 옵션은 알아둘 가치가 있습니다.
apt-get, yum, dnf 또는 pacman을 이용하여 패키지를 찾고 설치하는 법을 배우세요. 그리고 pip가 설치되어있는지 확인해서, 파이선 기반의 커맨드 라인 도구를 설치할 수 있도록 하세요(밑에 설명된 것중 몇가지는 pip를 이용해 설치하는게 제일 쉽습니다.

Everyday use

Bash 에서 Tab을 쓰면 argument를 완성하고, ctrl-r을 쓰면 커맨드 히스토리에서 검색합니다.
Bash에서 ctrl-w는 마지막 단어를 지웁니다. ctrl-u는 라인의 처음까지 전부다 지웁니다. alt-b와 alt-f를 이용해서 단어 단위로 이동할 수 있습니다. ctrl-k는 커서 위치부터 라인의 끝까지 지웁니다. ctrl-l은 화면을 깨끗하게 합니다. man readline을 이용해서 Bash의 기본 키 조합을 살펴보세요. 많은 것이 있습니다. 예를 들면 **alt-.**같은 경우, 이건 argument를 돌아가면서 나타내고 **alt-***는 글롭을 확장합니다.
vi 스타일의 키 조합을 사랑한다면, set -o vi를 사용할수도 있습니다.
최근 사용한 커맨드를 보려면 history를 입력하세요. !$(마지막 argument), !!(마지막 커맨드)와 같은 약어들이 매우 많습니다. 비록 이런 것들이 ctrl-r이나 **alt-.**명령어로 자주 대체되기 쉽지만요.
이전에 작업하던 디렉토리로 돌아가려면 cd -를 사용하세요.
커맨드를 타이핑 하던 도중에 마음이 바뀌었다면, **alt-#**을 쳐서 시작점에 #을 삽입하고, 엔터를 쳐서 코멘트로 여겨지게 하세요(또는 ctrl-a, #, enter). 나중에 커맨드 히스토리에서 찾아서 타이핑 중이었던 커맨드로 돌아올 수 있습니다.
xargs(혹은 parallel)를 사용하세요. 매우 강력합니다. 라인당 몇개의 아이템이 실행되게 할 것인지(-L) 그걸 병렬로 할 것인지(-P)를 제어할 수 있다는걸 기억하세요. 제대로 하고있는지 확신할 수 없다면 xargs echo를 먼저 실행해보세요. 또 -I{}도 간편합니다. 예시:

      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.

      # 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.
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).

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.
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. 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):

      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.
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.
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).

      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
      }

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: 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: A curated list of shell tools and resources.
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

This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

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