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The Art of Command Line

Note: I'm planning to revise this and looking for a new co-author to help with expanding this into a more comprehensive guide. While it's very popular, it could be broader and a bit deeper. If you like to write and are close to being an expert on this material and willing to consider helping, please drop me a note at josh (0x40) holloway.com. –jlevy, Holloway. Thank you!

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 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 we'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.

This work is the result of many authors and translators. Some of this originally appeared on Quora, but it has since moved to GitHub, where people more talented than the original author have made numerous improvements. Please submit a question if you have a question related to the command line. Please contribute if you see an error or something that could be better!

Meta

Scope:

  • This guide is for both beginners and experienced users. 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" and "Windows only" sections. 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
    ,
    yum
    ,
    dnf
    ,
    pacman
    ,
    pip
    or
    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. The

    nano
    editor is one of the simplest for basic editing (opening, editing, saving, searching). However, for the power user in a text terminal, there is no substitute for Vim (
    vi
    ), the hard-to-learn but venerable, fast, and full-featured editor. Many people also use the classic Emacs, particularly for larger editing tasks. (Of course, any modern software developer working on an extensive project is unlikely to use only a pure text-based editor and should also be familiar with modern graphical IDEs and tools.)
  • Finding documentation:

    • Know how to read official 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
      . You can find out whether a command is an executable, shell builtin or an alias by using
      type command
      .
    • curl cheat.sh/command
      will give a brief "cheat sheet" with common examples of how to use a shell command.
  • 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 -hs *
    ). For filesystem management,
    df
    ,
    mount
    ,
    fdisk
    ,
    mkfs
    ,
    lsblk
    . Learn what an inode is (
    ls -i
    or
    df -i
    ).
  • Basic network management:

    ip
    or
    ifconfig
    ,
    dig
    ,
    traceroute
    ,
    route
    .
  • Learn and use a version control management system, such as

    git
    .
  • Know regular expressions well, and the various flags to

    grep
    /
    egrep
    . The
    -i
    ,
    -o
    ,
    -v
    ,
    -A
    ,
    -B
    , and
    -C
    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 or list all available commands and ctrl-r to search through command history (after pressing, type to search, press ctrl-r repeatedly to cycle through more matches, press Enter to execute the found command, or hit the right arrow to put the result in the current line to allow editing).

  • In Bash, use ctrl-w to delete the last word, and ctrl-u to delete the content from current cursor back to the start of the line. Use alt-b and alt-f to move by word, ctrl-a to move cursor to beginning of line, ctrl-e to move cursor to end of line, 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
    (and
    set -o emacs
    to put it back).
  • For editing long commands, after setting your editor (for example

    export EDITOR=vim
    ), ctrl-x ctrl-e will open the current command in an editor for multi-line editing. Or in vi style, escape-v.
  • To see recent commands, use

    history
    . Follow with
    !n
    (where
    n
    is the command number) to execute again. There are also many abbreviations you can use, the most useful probably being
    !$
    for last argument and
    !!
    for last command (see "HISTORY EXPANSION" in the man page). However, these are often easily replaced with ctrl-r and alt-..
  • Go to your home directory with

    cd
    . Access files relative to your home directory with the
    ~
    prefix (e.g.
    ~/.bashrc
    ). In
    sh
    scripts refer to the home directory as
    $HOME
    .
  • 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:
    bash
      find . -name '*.py' | xargs grep some_function
      cat hosts | xargs -I{} ssh [email protected]{} 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) or
    lsof -iTCP -sTCP:LISTEN -P -n
    (which also works on macOS).
  • See also

    lsof
    and
    fuser
    for open sockets and files.
  • See

    uptime
    or
    w
    to know how long the system has been running.
  • Use

    alias
    to create shortcuts for commonly used commands. For example,
    alias ll='ls -latr'
    creates a new alias
    ll
    .
  • Save aliases, shell settings, and functions you commonly use in

    ~/.bashrc
    , and arrange for login shells to source it. This will make your setup available in all your shell sessions.
  • Put the settings of environment variables as well as commands that should be executed when you login in

    ~/.bash_profile
    . Separate configuration will be needed for shells you launch from graphical environment logins and
    cron
    jobs.
  • Synchronize your configuration files (e.g.

    .bashrc
    and
    .bash_profile
    ) among various computers with Git.
  • Understand that care is needed when variables and filenames include whitespace. Surround your Bash variables with quotes, e.g.

    "$FOO"
    . Prefer the
    -0
    or
    -print0
    options to enable null characters to delimit filenames, e.g.
    locate -0 pattern | xargs -0 ls -al
    or
    find / -print0 -type d | xargs -0 ls -al
    . To iterate on filenames containing whitespace in a for loop, set your IFS to be a newline only using
    IFS=$'\n'
    .
  • In Bash scripts, use

    set -x
    (or the variant
    set -v
    , which logs raw input, including unexpanded variables and comments) for debugging output. Use strict modes unless you have a good reason not to: Use
    set -e
    to abort on errors (nonzero exit code). Use
    set -u
    to detect unset variable usages. Consider
    set -o pipefail
    too, to abort on errors within pipes (though read up on it more if you do, as this topic is a bit subtle). For more involved scripts, also use
    trap
    on EXIT or ERR. A useful habit is to start a script like this, which will make it detect and abort on common errors and print a message:
    bash
      set -euo pipefail
      trap "echo 'error: Script failed: see failed command above'" ERR
    
  • 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.

    bash
      # 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}
    . Using a default value if a variable is empty:
    ${name:-default}
    . If you want to have an additional (optional) parameter added to the previous example, you can use something like
    output_file=${2:-logfile}
    . If
    $2
    is omitted and thus empty,
    output_file
    will be set to
    logfile
    . Arithmetic expansion:
    i=$(( (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
    .
  • Brace expansion using

    {
    ...
    }
    can reduce having to re-type similar text and automate combinations of items. This is helpful in examples like
    mv foo.{txt,pdf} some-dir
    (which moves both files),
    cp somefile{,.bak}
    (which expands to
    cp somefile somefile.bak
    ) or
    mkdir -p test-{a,b,c}/subtest-{1,2,3}
    (which expands all possible combinations and creates a directory tree). Brace expansion is performed before any other expansion.
  • The order of expansions is: brace expansion; tilde expansion, parameter and variable expansion, arithmetic expansion, and command substitution (done in a left-to-right fashion); word splitting; and filename expansion. (For example, a range like

    {1..20}
    cannot be expressed with variables using
    {$a..$b}
    . Use
    seq
    or a
    for
    loop instead, e.g.,
    seq $a $b
    or
    for((i=a; i<=b; i++)); do ... ; done
    .)
  • The output of a command can be treated like a file via

     (known as process substitution). For example, compare local 
    /etc/hosts
    with a remote one:
    sh
      diff /etc/hosts 
  • When writing scripts you may want to put all of your code in curly braces. If the closing brace is missing, your script will be prevented from executing due to a syntax error. This makes sense when your script is going to be downloaded from the web, since it prevents partially downloaded scripts from executing:

    bash
    {
      # Your code here
    }
    
  • A "here document" allows redirection of multiple lines of input as if from a file:

    cat <
  • In Bash, redirect both standard output and standard error via:

    some-command >logfile 2>&1
    or
    some-command &>logfile
    . 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
    .
  • 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.
    byobu
    can enhance screen or tmux by providing more information and easier management. 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, uses 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/%[email protected]%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
  • Consider

    mosh
    an alternative to ssh that uses UDP, avoiding dropped connections and adding convenience on the road (requires server-side setup).

  • 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
    sh
      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 as another user, use

    sudo
    . Defaults to running as root; use
    -u
    to specify another user. Use
    -i
    to login as that user (you will be asked for your password).
  • For switching the shell to another user, use

    su username
    or
    su - username
    . The latter with "-" gets an environment as if another user just logged in. Omitting the username defaults to root. You will be asked for the password of the user you are switching to.
  • Know about the 128K limit on command lines. This "Argument list too long" error is common when wildcard matching large numbers of files. (When this happens alternatives like

    find
    and
    xargs
    may help.)
  • For a basic calculator (and of course access to Python in general), use the

    python
    interpreter. For example, ```

    2+3 5 ```

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, there are several options more advanced or faster than

    grep -r
    , including (in rough order from older to newer)
    ack
    ,
    ag
    ("the silver searcher"), and
    rg
    (ripgrep).
  • To convert HTML to text:

    lynx -dump -stdin
  • For Markdown, HTML, and all kinds of document conversion, try

    pandoc
    . For example, to convert a Markdown document to Word format:

    pandoc README.md --from markdown --to docx -o temp.docx
  • If you must handle XML,

    xmlstarlet
    is old but good.
  • For JSON, use

    jq
    . For interactive use, also see
    jid
    and
    jiq
    .

  • For YAML, use

    shyaml
    .

  • 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
    and the improved
    saws
    are 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
    .
  • For more complex calculations, including grouping, reversing fields, and statistical calculations, consider

    datamash
    .

  • 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
    .
  • You can set a specific command's environment by prefixing its invocation with the environment variable settings, as in

    TZ=Pacific/Fiji date
    .
  • Know basic

    awk
    and
    sed
    for simple data munging. See One-liners for examples.
  • To replace all occurrences of a string in place, in one or more files:

    sh
      perl -pi.bak -e 's/old-string/new-string/g' my-files-*.txt
    
  • To rename multiple files and/or search and replace within files, try

    repren
    . (In some cases the

    rename
    command also allows multiple renames, but be careful as its functionality is not the same on all Linux distributions.)
    sh
      # Full rename of filenames, directories, and contents foo -> bar:
      repren --full --preserve-case --from foo --to bar .
      # Recover backup files whatever.bak -> whatever:
      repren --renames --from '(.*)\.bak' --to '\1' *.bak
      # Same as above, using rename, if available:
      rename 's/\.bak$//' *.bak
    
  • As the man page says,

    rsync
    really is a fast and extraordinarily versatile file copying tool. It's known for synchronizing between machines but is equally useful locally. When security restrictions allow, using
    rsync
    instead of
    scp
    allows recovery of a transfer without restarting from scratch. It also is among the fastest ways to delete large numbers of files:
    sh
    mkdir empty && rsync -r --delete empty/ some-dir && rmdir some-dir
    
  • For monitoring progress when processing files, use

    pv
    ,
    pycp
    ,
    pmonitor
    ,
    progress
    ,

    rsync --progress
    , or, for block-level copying,
    dd status=progress
    .
  • 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 and
    sdiff
    for a side-by-side diff. Note
    diff -r
    works for entire directories. Use
    diff -r tree1 tree2 | diffstat
    for a summary of changes. Use
    vimdiff
    to compare and edit files.
  • For binary files, use

    hd
    ,
    hexdump
    or
    xxd
    for simple hex dumps and
    bvi
    ,
    hexedit
    or
    biew
    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:
    sh
      # Displays hex codes or actual names of characters (useful for debugging):
      uconv -f utf-8 -t utf-8 -x '::Any-Hex;' < input.txt
      uconv -f utf-8 -t utf-8 -x '::Any-Name;' < input.txt
      # Lowercase 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).
  • Date and time: To get the current date and time in the helpful ISO 8601 format, use

    date -u +"%Y-%m-%dT%H:%M:%SZ"
    (other options are problematic). To manipulate date and time expressions, use
    dateadd
    ,
    datediff
    ,
    strptime
    etc. from
    dateutils
    .
  • Use

    zless
    ,
    zmore
    ,
    zcat
    , and
    zgrep
    to operate on compressed files.
  • File attributes are settable via

    chattr
    and offer a lower-level alternative to file permissions. For example, to protect against accidental file deletion the immutable flag:
    sudo chattr +i /critical/directory/or/file
  • Use

    getfacl
    and
    setfacl
    to save and restore file permissions. For example:
    sh
    getfacl -R /some/path > permissions.txt
    setfacl --restore=permissions.txt
    
  • To create empty files quickly, use

    truncate
    (creates sparse file),
    fallocate
    (ext4, xfs, btrfs and ocfs2 filesystems),
    xfs_mkfile
    (almost any filesystems, comes in xfsprogs package),
    mkfile
    (for Unix-like systems like Solaris, Mac OS).

System debugging

  • For web debugging,

    curl
    and
    curl -I
    are handy, or their
    wget
    equivalents, or the more modern
    httpie
    .
  • To know current cpu/disk status, the classic tools are

    top
    (or the better
    htop
    ),
    iostat
    , and
    iotop
    . Use
    iostat -mxz 15
    for basic CPU and detailed per-partition disk stats and performance insight.
  • For network connection details, use

    netstat
    and
    ss
    .
  • For a quick overview of what's happening on a system,

    dstat
    is especially useful. For broadest overview with details, use
    glances
    .
  • 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 
    and a full stack trace and heap summary (including generational garbage collection details, which can be highly informative) will be dumped to stderr/logs. The JDK's
    jps
    ,
    jstat
    ,
    jstack
    ,
    jmap
    are useful. SJK tools are more advanced.
  • 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
    ). Use trace child option (
    -f
    ) to avoid missing important calls.
  • Know about

    ldd
    to check shared libraries etc — but never run it on untrusted files.
  • 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/meminfo
    ,
    /proc/cmdline
    ,
    /proc/xxx/cwd
    ,
    /proc/xxx/exe
    ,
    /proc/xxx/fd/
    ,
    /proc/xxx/smaps
    (where
    xxx
    is the process id or pid).
  • 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).
  • If you delete a file and it doesn't free up expected disk space as reported by

    du
    , check whether the file is in use by a process:
    lsof | grep deleted | grep "filename-of-my-big-file"

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).
    sh
      sort a b | uniq > c   # c is a union b
      sort a b | uniq -d > c   # c is a intersect b
      sort a b b | uniq -u > c   # c is set difference a - b
    
  • Pretty-print two JSON files, normalizing their syntax, then coloring and paginating the result:

      diff 
  • Use

    grep . *
    to quickly examine the contents of all files in a directory (so each line is paired with the filename), or
    head -100 *
    (so each file has a heading). This can be useful for directories filled with config settings like those in
    /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):

    sh
      awk '{ x += $3 } END { print x }' myfile
    
  • To see sizes/dates on a tree of files, this is like a recursive

    ls -l
    but is easier to read than
    ls -lR
    :
    sh
      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
    :
    sh
      egrep -o 'acct_id=[0-9]+' access.log | cut -d= -f2 | sort | uniq -c | sort -rn
    
  • To continuously monitor changes, use

    watch
    , e.g. check changes to files in a directory with
    watch -d -n 2 'ls -rtlh | tail'
    or to network settings while troubleshooting your wifi settings with
    watch -d -n 2 ifconfig
    .
  • Run this function to get a random tip from this document (parses Markdown and extracts an item):

    sh
      function taocl() {
        curl -s https://raw.githubusercontent.com/jlevy/the-art-of-command-line/master/README.md |
          sed '/cowsay[.]png/d' |
          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 | iconv -t US
      }
    

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
    ,
    paste
    and
    join
    : data manipulation
  • fmt
    : format text paragraphs
  • pr
    : format text into pages/columns
  • fold
    : wrap lines of text
  • column
    : format text fields into aligned, fixed-width 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 traffic 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
  • time
    : execute and time a command
  • timeout
    : execute a command for specified amount of time and stop the process when the specified amount of time completes.
  • lockfile
    : create semaphore file that can only be removed by
    rm -f
  • logrotate
    : rotate, compress and mail logs.
  • watch
    : run a command repeatedly, showing results and/or highlighting changes
  • when-changed
    : runs any command you specify whenever it sees file changed. See

    inotifywait
    and
    entr
    as well.
  • tac
    : print files in reverse
  • comm
    : compare sorted files line by line
  • 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
    )
  • apg
    : generates random passwords
  • xz
    : high-ratio file compression
  • ldd
    : dynamic library info
  • nm
    : symbols from object files
  • ab
    or
    wrk
    : 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
    : Disk usage stats
  • mpstat
    : CPU usage stats
  • vmstat
    : Memory 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
  • sysctl
    : view and configure Linux kernel parameters at run time
  • hdparm
    : SATA/ATA disk manipulation/performance
  • lsblk
    : list block devices: a tree view of your disks and disk partitions
  • lshw
    ,
    lscpu
    ,
    lspci
    ,
    lsusb
    ,
    dmidecode
    : hardware information, including CPU, BIOS, RAID, graphics, devices, etc.
  • lsmod
    and
    modinfo
    : List and show details of kernel modules.
  • fortune
    ,
    ddate
    , and
    sl
    : um, well, it depends on whether you consider steam locomotives and Zippy quotations "useful"

macOS 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 enable the Option key in macOS Terminal as an alt key (such as used in the commands above like alt-b, alt-f, etc.), open Preferences -> Profiles -> Keyboard and select "Use Option as Meta key".

  • 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.
  • To get macOS release information, use

    sw_vers
    .

Windows only

These items are relevant only on Windows.

Ways to obtain Unix tools under Windows

  • Access the power of the Unix shell under Microsoft Windows by installing Cygwin. Most of the things described in this document will work out of the box.

  • On Windows 10, you can use Windows Subsystem for Linux (WSL), which provides a familiar Bash environment with Unix command line utilities.

  • If you mainly want to use GNU developer tools (such as GCC) on Windows, consider MinGW and its MSYS package, which provides utilities such as bash, gawk, make and grep. MSYS doesn't have all the features compared to Cygwin. MinGW is particularly useful for creating native Windows ports of Unix tools.

  • Another option to get Unix look and feel under Windows is Cash. Note that only very few Unix commands and command-line options are available in this environment.

Useful Windows command-line tools

  • You can perform and script most Windows system administration tasks from the command line by learning and using

    wmic
    .
  • Native command-line Windows networking tools you may find useful include

    ping
    ,
    ipconfig
    ,
    tracert
    , and
    netstat
    .
  • You can perform many useful Windows tasks by invoking the

    Rundll32
    command.

Cygwin tips and tricks

  • Install additional Unix programs with the Cygwin's package manager.

  • Use

    mintty
    as your command-line window.
  • Access the Windows clipboard through

    /dev/clipboard
    .
  • Run

    cygstart
    to open an arbitrary file through its registered application.
  • Access the Windows registry with

    regtool
    .
  • Note that a

    C:\
    Windows drive path becomes
    /cygdrive/c
    under Cygwin, and that Cygwin's
    /
    appears under
    C:\cygwin
    on Windows. Convert between Cygwin and Windows-style file paths with
    cygpath
    . This is most useful in scripts that invoke Windows programs.

More resources

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

Creative Commons License

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

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