Chapter 3:Command Line Utilities

Doin’ stuff

In this chapter …• Special characters

• Redirection

• More utilities than you shake a stick at

Typing Commands• Beware of special characters

• Characters that have special meaning to the shell

• Shell expands, modifies and interprets special characters before issuing the command

Special Characters• & ; | * ? ‘ “ ` [ ] ( ) $ < > { } ^ # / \ % ! ~ +

• Plus whitespace (tabs, spaces, newlines)

• Do not use these in filenames unless you have to

• To use them, either put in single quotes, or proceed with a backslash– ls ‘filename with special chars!!’– ls \[cat\]

Special Characters con’t• All special characters have special meaning

to the shell

• We’ll explore these in great detail in upcoming chapters

Utilities• Linux & Unix come with thousands of utilities

• Some used explicitly, others implicitly

• Some text-based, some GUI, some both

Some tips before we start• Tab completion

– When typing a filename or command name, you can type the first few letters then hit TAB to auto-complete the command

• Pipe (|) symbol– Used to chain commands together– The output of one command becomes the input

of another– We’ll revisit this in detail later

ls: LiSt files• Used to list files contained in a directory

• Can narrow the search using pattern matching

• Examples– ls displays ‘all’ the files in the directory– ls cats displays the file cats in the directory– ls ca* displays files starting with ‘ca’

cat: catenate a file• Displays the contents of one or more files

• Beware – don’t try with binary files

• Examples– cat myfile displays contents of myfile– cat file1 file2 displays contents of file1

followed by contents of file2

rm: ReMoves a file• Similar to del in DOS

• Use the –i option to invoke interactive mode, which prompts you if you’re sure

• Examples:– rm myfile deletes myfile– rm –i myfile prompts you before deleting

more and less: pagers• more and less are similar in that they both

break up long files into page long chunks

• Press h to display possible commands

• Examples– less myfile displays myfile one page at a

time

hostname: Where am I?• hostname will display the name of the

system you are currently logged onto

• Usually a fully qualified domain name (FQDN)

• Example:– hostname displays:

ares.bcs.solano.cc.ca.us

cp: CoPies files• Usage: cp sourcefile destinationfile

• Creates a copy, leaves sourcefile intact

• If destinationfile exists, it will be overwritten– Unless you use –i option

• Example:– cp myfile myfile.backup

mv: MoVe files / change name• Usage: mv existingfile newfile

• Just like cp, can overwrite with –i option

• Renames a file, which can also move it to another directory

• Examples:– mv myfile foshizzle– mv /dir1/myfile /dir2/myfile

lpr: Line PRinter• Places files into the print queue

• Usage: lpr [-Pprintername] files

• You can check the status of queue with lpq

• You can delete a job with lprm

• Sorry, we don’t have a printer

grep: global regular expression print

• Used to search for strings in files/output

• Usage: grep expression filename

• Returns lines with expression in filename

• Example:– grep ‘automagically’ myfile

head: display beginning• Displays beginning of file

• head -X filename displays first X lines

• Check out pg 727/691 for more options

tail: duh• Displays the end of a file

• tail -X myfile displays the last X lines

• Check out pg 843/783 for more options

sort: displays sorted info• sort displays data in a sorted manner,

without altering the original file

• Lots of options – sort alphabetically, numerically, with or without repeats, reverse order, etc

• Check out pg 817/762

uniq: removes duplicates• uniq displays data, omitting successive

repeat entries

• Have to sort file first – otherwise it might not find all duplicates

• Does not alter original file

file: what kind of file is this?• Usage: file filename

• Tells you what kind of file you’re working with and what kind of data is in it

• Examples include program, shell builtin, ASCII text, compressed data, etc

echo: display text• Displays (echoes) text back to the terminal

screen

• Can print out contents of shell variables

• Useful in shell scripts

• In other words, seems dumb now but we’ll use it a lot down the road

date: displays time and date• Command options can change formatting

• Privileged accounts can use date to change date and time

• Can be useful for scripting

script: captures session• Captures all input and output on the terminal

and saves to a file

• A good way to document your work, or capture errors for analysis

• Type script to start capture, exit to quit

• By default stores everything in the file typescript

Text Converters• unix2dos and dos2unix

• Unix and DOS use different end of line characters

• Use these utilities when moving text files back and forth between Windows and Linux systems

• Weird script error? Try dos2unix

Compressing files• bzip2 files

• gzip files

• compress files

• Each use their own algorithms and have their uses

Uncompressing files• bunzip2 compressed-file

• gunzip compressed-file

• ucompress compressed-file

tar: Tape ARchive• Packs and unpacks files from archives

• *Does not compress, only assembles*

• Tons of options, allowing you to add or remove files from archive, and also apply compression using third party support

which: locates utilties• Will display the location of a utility

• which ls displays location of ls command you’re using

• In case of there being multiple locations, which only displays the first (i.e., the one you will be using)

whereis: locates utilities• Similar to which, but displays the utilities in a

standard set of locations

• The first one listed may not be the one you will issue when you enter the command

• All depends on your PATH (chapter 4)

Sidenote• which and whereis do not list shell builtins

• Shell builtins are functions that are internal to the shell itself – no binary executable

• To see if you’re using a builtin, use type

apropos: what do I use?• Not sure what utility you’re looking for?

• Try apropos keyword

• Displays utilities and libraries related to your keyword

• Found one, but not sure? whatis utility to show what it does, or check man page

locate: search for files• System maintains a database of files

• Your system administrator should configure a job to regularly update this database

• Searches for any kind of file – not just utilities

• Some systems use slocate (secure)

• Latest distros use mlocate via locate

who: Who’s online?• Displays what users are logged on

• Also displays when they logged on, and with what device (terminal or console, etc)

• Also try who am i

finger: reach out and touch …• finger by itself displays users logged on like

who, but also shows idle time and office location

• finger username shows info about that user, like home directory, last logon, their shell, if they have unread mail, and .plan and .project files

w: What’s up?• w is similar to who by showing who’s logged

on

• Also shows system uptime, and memory and CPU load averages

• Good overall status of the system

write: send a message• write username opens up a text-based chat

with the user

• Type message

• Wait for response

• CTRL-D to exit write

mesg: Enable/disable write• Usage: mesg y|n

• Turns off whether users can write you or not

• Useful if you don’t want to be bugged

mail: system mail• Our system is a closed system

• You can send mail to other users on the system

• No public mail