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The Art of Command Line

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

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

Fluency on the command line is a skill now often neglected or considered archaic, 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!

Scope:

The goals are breadth and brevity. Every tip is essential in some situation or significantly saves time over alternatives.
This is written for Linux. Many but not all items apply equally to MacOS (or even Cygwin).
The focus is on interactive Bash, though many tips apply to other shells and to general Bash scripting.
Descriptions are intentionally minimal, with the expectation you'll use man, apt-get/yum to install, and Google for more background.

Basics

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).
Learn about redirection of output and input using > and < and pipes using |. Learn about stdout and stderr.
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, 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 -sk *), df, mount.
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 or yum (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 ctrl-r to search through command history.
In Bash, use ctrl-w to delete the last word, and ctrl-u to delete the whole line. Use alt-Left and alt-Right to move by word, and ctrl-k to kill to the end of the line. 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.
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.
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.
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, and use compression (which is helpful with scp over low-bandwidth connections):

      TCPKeepAlive=yes
      ServerAliveInterval=15
      ServerAliveCountMax=6
      Compression=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).

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

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 or tshark.
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.

      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

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
screen: powerful terminal multiplexing and session persistence
yes: print a string a lot
cal: nice calendar
env: run a command (useful in 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
dd: moving data between files or devices
file: identify type of a file
stat: file info
tac: print files in reverse
shuf: random selection of lines from a file
comm: compare sorted files line by line
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
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
wireshark and tshark: packet capture and network debugging
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
lshw: hardware information
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! ;)

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