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The Art of Command Line

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

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.)

Meta

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.

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).

• 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).

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-. inserts last argument of previous command, and **ctrl-x *** 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.

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. 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.

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.

• 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).

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

• 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

• sponge: read all input before writing it, useful for reading from then writing to the same file, e.g., grep -v something some-file | sponge some-file

• 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 or in local file system

• 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"

MacOS X only

These are items relevant only on MacOS.

• Package management with brew (Homebrew) and/or port (MacPorts). These can be used to install on MacOS many of the above commands.

• Copy output of any command to a desktop app with pbcopy and paste input from one with pbpaste.

• To open a file with a desktop app, use open or open -a /Applications/Whatever.app.

• Spotlight: Search files with mdfind and list metadata (such as photo EXIF info) with mdls.

• Be aware MacOS is based on BSD Unix, and many commands (for example ps, ls, tail, awk, sed) have many subtle variations from Linux, which is largely influenced by System V-style Unix and GNU tools. You can often tell the difference by noting a man page has the heading "BSD General Commands Manual." In some cases GNU versions can be installed, too (such as gawk and gsed for GNU awk and sed). If writing cross-platform Bash scripts, avoid such commands (for example, consider Python or perl) or test carefully.

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.