Some Linux commands

One of the steepest Unix/Linux learning curves is the sheer number of built-in commands, all of which have many options -- most of which you'll never use. Plus there are a number of advanced commands that are extremely powerful but also extremely complex.

To help address this, this page introduces a number of built-in Linux utilities along with some of their common options, by category.

And here's a Linux commands cheat sheet you may find useful:

• cat outputs all the contents of its input (one or more files and/or standard input) or the specified file

  • cat -n prefixes each line of output with its line number

  • CAUTION – only use on small files!

• zcat <file.gz> like cat, but understands the gzip (.gz) format, and decompresses the data before writing it to standard output

  • CAUTION – only use on small files! ..or pipe the output to more

  • Another CAUTION – does not understand .zip or .bz2 compression formats

• more and less pagers

  • both display their (possibly very long) input one Terminal "page" at a time

  • in more:

    • use spacebar to advance a page

    • use q or Ctrl-c to exit more

  • in less:

    • q – quit

    • left, right arrows - move back or forward one character

    • up, down arrows - move up or down one line

    • Ctrl-f or spacebar – page forward

    • Ctrl-b – page backward

    • /<pattern> – search for <pattern> in forward direction

      • n – next match

      • N – previous match

    • ?<pattern> – search for <pattern> in backward direction

      • n – previous match going back

      • N – next match going forward

  • use less -N to display line Numbers

  • use less -I to use case Insensitive pattern searches

  • less can be used directly on .gz format files

• head and tail

  • show you the first or last 10 lines (by default) of their input

  • head -n 20 or just head -20 shows the first 20 lines

  • tail -n 2 or just tail -2 shows the last 2 lines

  • tail -n +100 or just tail +100 shows all lines starting at line 100

  • tail -n +100 | head -20 shows 20 lines starting at line 100

  • tail -f shows the last lines of a file, then follows the output as more lines are written (Ctrl-c to quit)

• gzip <file> – compresses <file> contents creating a <file.gz> compressed file

  • and removes the original <file>

• gunzip <file.gz> – un-compresses <file.gz> contents creating a <file> file

  • and removes the <file.gz> file

  • gunzip -c <file.gz> | more (or less)

    • like zcat, un-compresses lines of <file.gz> and outputs them to standard output

    • <file.gz> is not altered on disk

    • always pipe the output to a pager!

• ls - list the contents of the specified directory

  • -l says produce a long listing (including file permissions, sizes, owner and group)

  • -a says show all files, even normally-hidden dot files whose names start with a period ( . )

  • -h says to show file sizes in human readable form (e.g. 12M instead of 12201749)

  • -t says to sort files on last modification time

  • -r says to reverse the current sort order

  • -d says to show directory listing information only, instead of directory contents

    • usually combined with -l, e.g.:  ls -ld <dirname>

• cd <whereto> - change the current working directory to <whereto>. Some special <wheretos>:

  •  .. (period period) means "up one level"

  •  ~ (tilde) means "my home directory"

  • - (dash) means "the last directory I was in"

  • . (period) means "the current directory"

• find <in_directory> [ operators ] -name <expression> [  tests ]

  • looks for files matching <expression> in <in_directory> and its sub-directories

  • <expression> can be a double-quoted string including pathname wildcards (e.g. "[a-g]*.txt")

  • there are tons of operators and tests:

    • -type f (file) and -type d (directory) are useful tests

    • -maxdepth NN is a useful operator to limit the depth of recursion

    • -iname ignores case when matching

• file <file> tells you what kind of file <file> is

• df shows you the top level directory structure of the system you're working on, along with how much disk space is available

  • -h says to show sizes in human readable form (e.g. 12G instead of 12318201749)

• pwd - display the present working directory

  • -P says to display the full absolute path

• tree <directory> -  shows the file system hierarchy of the specified directory

  • tree is not available on all Linux systems