Learning the Shell

  • df - free disk space
  • free - free memory
  • date - current date
  • cal - show a calendar

User prompt ends with $, super user prompt ends with #

Common ls Options

Option Long Option Description
-a --all all files, including hidden
-d --directory with -l show details about the directory rather than its contents
-F --classify append an indicator character to end of each item (e.g. forward slash if name is a directory).
-h --human-readable with -l display file sizes in human-readable format rather than in bytes
-l long format
-r --reverse display results in reverse order
-S sort results by file size
-t sort by modification time

Fields in long listing

-rw-r--r-- 1 root root 453764 2012-04-03 11:05 oo-welcome.odt

1 = File's number of hard links (often 1) root = user who owns the file root = group who owns the file 453764 = size in bytes 2012-04-03 11:05 = last modified date

Directories Found on Linux Systems

Directory Comments
/ root directory
/bin binaries for the system to boot and run
/boot Linux kernel (/boot/vmlinuz), initial RAM disk image (drivers needed at boot time), boot loader
/dev list of all devices
/etc system wide config files, shell scripts for starting system files
/lib shared library files for core system programs
/lost+found used in partial recovery situations, usually empty
/media mount point for CD drives etc
/mnt older systems manual mount point for removable drives
/opt optional software
/proc virtual file system maintained by the kernel - not a real files system - but all files are readable
/root home directory for the root account
/sbin system binaries
/tmp temporary files
/usr all programs and support files used by regular users
/usr/bin binaries installed by the linux distro - usually 1000s of programs
/usr/lib shared libraries for /usr/bin
/usr/local system wide, but usually not via the linux distro, but via a sys admin user e.g. /usr/local/bin is where programs compiled from source usually go
/usr/sbin more system binaries (TODO what is the difference between here and /sbin?)
/usr/share shared data for programs in /usr/bin e.g. default config files, icons
/usr/share/doc documentatino
/var storage for data which is likely to change e.g. logs, databases, email
/var/log log files - /var/log/messages is important

Wildcards

  • * any character
  • ? any single character
  • [characters] any character in set 'characters'
  • [!characters] any character NOT in set 'characters'
  • [[:class]] any character of the specified class e.g. [:alnum], [:alpha], [:digit], [:lower], [:upper]

Manipulating files

cp

  • cp -a item1 item2 additionally copy attributes including ownership and permissions (usually copies take on the default attributes of the user performing the copy)
  • cp -u item1 item2 copy only files which don't exist or are newer
  • cp -i item1 item2 interactive
  • cp -r item1 item2 recursive
  • cp -v item1 item2 verbose
  • cp file1 file2 dir1 copy file1 and file2 to dir1 (dir1 must exist)
  • cp dir1/* dir2 all files in dir1 are copied to dir2 (dir2 must exist)
  • cp -r dir1 dir2 recursively copy files in dir1 to dir2
    • IMPORTANT if dir2 exists, dir1 will be copied within it, if dir2 doesn't exist it will be created and will hold the same contents as dir1

mv

  • mv dir1 dir2 move dir1 and its contents to dir2
    • IMPORTANT if dir2 doesn't exists, dir1 will effectively be renamed dir2

rf

  • rf -f ignore nonexistent files and do not prompt - TODO what does this mean?

Commands

A command is one of:

  • an executable binary or script

  • a command built into the shell itself e.g. cd is a bash shell builtin

  • a shell function - minature shell scripts incorporated into the environment

  • an alias

  • type shows which of the 4 a particular command is e.g. type cd gives cd is a shell builtin

  • which shows the location of an executable (doesn't necessarily work for shell builtins)

  • help help command for shell builtins e.g. help cd

  • --help often supported by executable programs to show similar usage information e.g. mkdir --help

  • man display a commands man page - if no section is specified the first section available is shown - most likely section 1 - to show a specific section use e.g. man 5 passwd - man pages are stored in /usr/share/man under the appropriate section diretory

    Section Contents
    1 User commands
    2 Programming interfaces for kernel system calls
    3 Programming interfaces to the C library
    4 Special files such as device nodes and drivers
    5 File formats e.g. man 5 passwd
    6 Games and amusements such as screensavers
    7 Miscellaneous
    8 System administrator commands e.g. man mount

  • apropos search man titles and descriptions for a keyword or similar term (man -k does the same thing)

  • whatis matches man titles and descirption for a keyword exactly

  • info GNU alternative to man e.g. info ls - tree-structured - most commands are in the coreutils.info

  • alias name='string' set up an alias - remove with unalias - alias on its own lists all defined aliases

I/O Redirection

stdout, stderr and stdin can be thought of as files where the output for stdout and stderr is by default connected to the screen and the input for stdin is, by default, connected to the keyboard. These can be redirected using I/O redirection.

  • ls -l /usr/bin > ls-output.txt redirects stdout to a file (if any errors these still go to stderr, which by default is still printed to the screen - in this case the file will be created or truncated before the error happens since > replaces content rather than appending content)
  • >> appends content e.g. ls -l /usr/bin >> ls-output.txt
  • File descriptors can alternatively be used to redirect stdin, stdout and stderr (and is the only way to redirect stderr). These are 0, 1 and 2 respectively. 2> redirects stderr e.g. ls -l /usr/idontexist 2> ls-error.txt
  • 2>&1 redirects stderr to file descriptor 1 i.e. stdout so ls -l /bin/usr > ls-output.txt 2>&1 will redirect stdout to a file and then stderr to the same destination i.e. also a file
  • &> shorthand for 2>&1 (recent versions of bash only)
  • 2> /dev/null to discard anything sent to stderr
  • cat concatenates files and can be used with redirection to rejoin files which have been split e.g. cat movie.00* > movie.mp4 (crucially for this wildcards expand in sorted order)
  • cat without any arguments uses stdin - ctrl-d (EOF) to terminate
  • < redirects stdin e.g. cat < lazy-dog.txt redirects the file lazy-dog.txt to stdin with the result that cat prints the contents of the file (cat uses stdin when no arguments are passed)
  • | pipes the standard output of one command to the standard input of another command
    • e.g. ls -l /usr/bin | less
  • uniq removes duplicates from a sorted list
  • uniq -d only displays duplicates from a sorted list
  • wc prints line, word and byte counts (all 3, tab seperated)
    • without arguments uses stdin (so often used at the end of a pipeline to count things)
  • grep pattern [files...] where pattern is a regex
    • option -i ignores case
    • option -v prints lines which DON'T match
  • tail -f [file] to tail a file real time
  • tee reads from stdin and redirects to stdout AND files (hence a T on the pipeline)
    • e.g. ls /usr/bin | tee ls.txt | grep zip will capture the output of ls /usr/bin to a file as well as passing it into grep zip

Configuration and the Environment

A Gentle Introduction to VI

  • Most commands can be prefixed with a number e.g. 5j to move down 5 lines
  • u undo
  • a append after end of word
  • A append to end of current line
  • o open a line below the current line
  • O open a line above the current line
  • G go to the last line in the document
  • dd cut the current line
  • dw cut to start of next word
  • d$ cut to end of line
  • d0 cut to beginning of line
  • p paste after current line
  • P paste before current line
  • yy 'yank' the current line (i.e. copy) - then similar combinations to cut