The Art of Command Line
- Basics
- Everyday use
- Processing files and data
- System debugging
- One-liners
- Obscure but useful
- 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!
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
/dnf
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.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, viassh-agent
,ssh-add
, etc.Basic file management:
ls
andls -l
(in particular, learn what every column inls -l
means),less
,head
,tail
andtail -f
(or even better,less +F
),ln
andln -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
orifconfig
,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
, ordnf
(depending on distro) to find and install packages. And make sure you havepip
to install Python-based command-line tools (a few below are easiest to install viapip
).
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-b and alt-f 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
(orparallel
). 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, usexargs 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
andpkill
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, seeman 7 signal
Use
nohup
ordisown
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. Useset -e
to abort on errors. Useset -o pipefail
as well, to be strict about errors (though this topic is a bit subtle). For more involved scripts, also usetrap
.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:
${name:?error message}
. For example, if a Bash script requires a single argument, just writeinput_file=${1:?usage: $0 input_file}
. Arithmetic expansion:i=$(( (i + 1) % 5 ))
. Sequences:{1..10}
. Trimming of strings:${var%suffix}
and${var#prefix}
. For example ifvar=foo.pdf
, thenecho ${var%.pdf}.txt
printsfoo.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
, andman latin1
are helpful.Use
screen
ortmux
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 isdtach
.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
), usefpp
(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).
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, uselocate something
(but bear in mindupdatedb
may not have indexed recently created files).For general searching through source or data files (more advanced than
grep -r
), useag
.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 ands4cmd
is faster. Amazon'saws
is essential for other AWS-related tasks.Know about
sort
anduniq
, including uniq's-u
and-d
options -- see one-liners below.Know about
cut
,paste
, andjoin
to manipulate text files. Many people usecut
but forget aboutjoin
.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, usingexport LC_ALL=C
.Know basic
awk
andsed
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 usesort -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 andbvi
for binary editing.Also for binary files,
strings
(plusgrep
, etc.) lets you find bits of text.To convert text encodings, try
iconv
. Oruconv
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) andcsplit
(to split by a pattern).Use
zless
,zmore
,zcat
, andzgrep
to operate on compressed files.
System debugging
For web debugging,
curl
andcurl -I
are handy, or theirwget
equivalents, or the more modernhttpie
.To know disk/cpu/network status, use
iostat
,netstat
,top
(or the betterhtop
), 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
andvmstat
. 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 likedu -sh *
.To find which socket or process is using bandwidth, try
iftop
ornethogs
.The
ab
tool (comes with Apache) is helpful for quick-and-dirty checking of web server performance. For more complex load testing, trysiege
.For more serious network debugging,
wireshark
,tshark
, orngrep
.Know about
strace
andltrace
. 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
, andsysdig
.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
. Supposea
andb
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 aboutLC_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 += $3 } END { print x }' myfile
- If want to see sizes/dates on a tree of files, this is like a recursive
ls -l
but is easier to read thanls -lR
:
find . -type f -ls
- Use
xargs
orparallel
whenever you can. Note you can control how many items execute per line (-L
) as well as parallelism (-P
). If you're not sure if it'll do the right thing, use xargs echo first. Also,-I{}
is handy. Examples:
find . -name '*.py' | xargs grep some_function
cat hosts | xargs -I{} ssh root@{} hostname
- Say you have a text file, like a web server log, and a certain value that appears on some lines, such as an
acct_id
parameter that is present in the URL. If you want a tally of how many requests for eachacct_id
:
cat access.log | egrep -o 'acct_id=[0-9]+' | cut -d= -f2 | sort | uniq -c | sort -rn
- Run this function to get a random tip from this document (parses Markdown and extracts an item):
function taocl() {
curl -s https://raw.githubusercontent.com/jlevy/the-art-of-command-line/master/README.md |
pandoc -f markdown -t html |
xmlstarlet fo --html --dropdtd |
xmlstarlet sel -t -v "(html/body/ul/li[count(p)>0])[$RANDOM mod last()+1]" |
xmlstarlet unesc | fmt -80
}
Obscure but useful
expr
: perform arithmetic or boolean operations or evaluate regular expressionsm4
: simple macro processorscreen
: powerful terminal multiplexing and session persistenceyes
: print a string a lotcal
: nice calendarenv
: run a command (useful in scripts)look
: find English words (or lines in a file) beginning with a stringcut
andpaste
andjoin
: data manipulationfmt
: format text paragraphspr
: format text into pages/columnsfold
: wrap lines of textcolumn
: format text into columns or tablesexpand
andunexpand
: convert between tabs and spacesnl
: add line numbersseq
: print numbersbc
: calculatorfactor
: factor integersgpg
: encrypt and sign filestoe
: table of terminfo entriesnc
: network debugging and data transferngrep
: grep for the network layerdd
: moving data between files or devicesfile
: identify type of a filestat
: file infotac
: print files in reverseshuf
: random selection of lines from a filecomm
: compare sorted files line by linehd
andbvi
: dump or edit binary filesstrings
: extract text from binary filestr
: character translation or manipulationiconv
or uconv: conversion for text encodingssplit
andcsplit
: splitting files7z
: high-ratio file compressionldd
: dynamic library infonm
: symbols from object filesab
: benchmarking web serversstrace
: system call debuggingmtr
: better traceroute for network debuggingcssh
: visual concurrent shellwireshark
andtshark
: packet capture and network debugginghost
anddig
: DNS lookupslsof
: process file descriptor and socket infodstat
: useful system statsglances
: high level, multi-subsystem overviewiostat
: CPU and disk usage statshtop
: improved version of toplast
: login historyw
: who's logged onid
: user/group identity infosar
: historic system statsiftop
ornethogs
: network utilization by socket or processss
: socket statisticsdmesg
: boot and system error messageshdparm
: SATA/ATA disk manipulation/performancelsb_release
: Linux distribution infolshw
: hardware informationfortune
,ddate
, andsl
: 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! ;)