Examples of Variables and Arrays in Bash
- Create a variable, ${var} is the same as $var, but not ambiguous.
var="http://wiki.plecko.hr/" echo $var http://wiki.plecko.hr/
- Return string length
echo ${#var} 31
- Execute the contents of $var (same as 'eval $$var')
echo ${!var}
- Returns variable names beginning with 'U'
echo ${!U*} UID USER USERNAME
- Returns the text from position 7
echo ${var:7} wiki.plecko.hr/
</code bash Returns 6 characters from position 12> echo ${var:11:6} plecko </code>
- Cut 'http://' from beginning of string
echo ${var#http://} wiki.plecko.hr//
- Cut '.hr/' from end of string
echo ${var%.hr/} http://wiki.plecko
- Replaces 'plecko' with 'eurekamoment' once
echo ${var/plecko/eurekamoment} http://wiki.eurekamoment.hr/
- Replaces 'o' with 'O', always"
echo ${var//o/O} http://wiki.pleckO.hr/
- If string starts with 'http', replace 'http' with 'Site'
echo ${var/#http/Site} Site://wiki.plecko.hr/
- If string ends with 'st', replace 'st' with 'STING'
var2="test" echo ${var2/%st/STING} teSTING
- "" (double quotation marks) protect a string, but recognize $, \ and ` as specials
echo "$var" http://wiki.plecko.hr/
- $'…' (dollar sign before single quotes) protects a string completely, but interprets \n, \t, \a, etc.
echo $'$var\n' $var
- ... (between crases) Execute commands in a subshell, returning the result
echo `ls` Desktop Documents Downloads Images Templates Music Public Videos
- Execute commands on a subshell
(ls) Desktop Documents Downloads Images Templates Music Public Videos
- Execute commands on a subshell, returning the result
echo $(ls) Desktop Documents Downloads Images Templates Music Public Videos
- Returns the result of an arithmetic operation.
echo $((11-9)) 2
- Tests an expression, returning 0 or 1 (alias of command ‘test’)
[[ $var ]] && echo 'Its bigger' Its bigger
- Tests an expression, returning 0 or 1 (can use && and ** ** )
[[ $var ]] && echo 'Is there this variable' Is there this variable
- Special Variables
Variable Positional Parameters $0 Parameter Number 0 (Name of Command or Function) $1 Parameter Number 1 (from command line or function) ... Parameter number N … $9 Parameter Number 9 (from command line or function) ${10} Parameter Number 10 (from command line or function) ... Parameter number NN … $# Total number of command line or function parameters $* All parameters as a single string $@ All parameters, such as multiple protected strings Variable Miscellania $$ PID number of current process (from script itself) $! PID number of last background job $_ Last argument of last command executed $? Return code from last command executed
- Special escapes to use at prompt (PS1)
Escape Reminder Expands to … \a Alert Alert (beep) \d Date Date in "Weekday Month Day" format (Sat Jan 15) \e Escape Esc Character \h Hostname Machine Name Without Domain (dhcp11) \H Hostname Full Machine Name (dhcp11.company) \j Jobs Number of Active Jobs \l Tty Current Terminal Name (ttyp1) \n Newline Newline \r Return Return by car \s Shell Name of the shell (basename $ 0) \t Time Time in 24-hour format HH: MM: SS \T Time 12-hour format HH: MM: SS \@ At Time in 12-hour format HH: MM am/pm \A At Time in 24-hour format HH:MM \u User Current user login \v Version Bash Version (2.00) \V Version Bash Version Subversion (2.00.0) \w Working Dir Current directory, full path ($PWD) \W Working Dir Current directory, only the last one (basename $PWD) \! History ico Current command number in history \# Number Current command number \$ ID > Show "#" if root, "$" if normal user \nnn Octal Character whose octal is nnn \\ Backslash Backslash \ literal \[ Escapes Starts a sequence of escapes (color coded type) \] Escapes Ends an escape sequence
- Formatters of the date Command
Format Description %a Abbreviated Weekday Name (Sun..Sab) %A Name of the day of the week (Sunday..Saturday) %b Abbreviated Month Name (Jan. Dec) %B Name of the month (January.December) %c Complete date (Sat Nov 04 12:02:33 EST 1989) %y Year (two digits) %Y Year (four digits) %m Month (01..12) %d Day (01..31) %j Day of the year (001..366) %H Hours (00..23) %M Minutes (00..59) %S Seconds (00..60) %s Seconds since January 1, 1970 %% A % literal %t One TAB %n A line break
- Printf Command Formatters
Format Description %d Decimal number %o Octal Number %x Hexadecimal Number (a-f) %X Hexadecimal Number (A-F) %f Floating-point number %e Number in scientific notation (e + 1) %E Number in scientific notation (E + 1) %s String
- To know all local variables, execute
set #or, and open txt to see later set > VariablesLocations.txt
- Global Variables, to know all global variables, execute
env #or printenv
- To assign a value to a local variable
LINUX=free echo $LINUX free
- check if the variable created in the previous item appears in the list of local variables
set | grep LINUX LINUX=free
- Now let’s make this local variable a global variable.
export LINUX env | grep LINUX LINUX=free
- Delete a ‘local environment variable’ from memory using unset command
unset LINUX echo $LINUX
- Create an alias (nickname for a command or program), you can still include it in your ~/.bashrc
alias list='ls -la color=auto'
- Destroying an aliase
unalias list
- Verify all commands entered
history #run command by his number in history !468 #execute last command typed !! #they stay in bash_history cat ~/.bash_history #clear history history -c
- Command Line Interpreters
$ - Shell of an normal user; # - Superuser shell root (administrator) #Check of Available Shells cat /etc/shells #Variable that shows the SHELL you use echo $SHELL
- Creating an Array (Array) "Variable Set"
DISTROS=("Debian" "Trisquel" "Ubuntu" "RedHat") #If you print the DISTROS Array as a variable, it prints variable 0, the array displays the variables contained in it starting at 0 (zero), so it would be the same as printing at ${DISTROS[0]} echo $DISTROS
- Print element 1 of array DISTROSS
echo ${DISTROS[1]} Trisquel
- You can also create an array by inserting element one at a time.
DISTROS[0]="Debian" DISTROS[1]="Trisquel" DISTROS[2]="Ubuntu" DISTROS[3]="RedHat"
- Change element 2 of array DISTROSS
DISTROS[2]="Linux Mint" echo ${DISTROS[2]} Linux Mint
- Display Element 2 Distro to End
echo ${DISTROS[@]:2} Ubuntu RedHat
- This displays the elements starting at position 1 followed by two more elements consecutive to position 1
echo ${DISTROS[@]:1:2} Trisquel Ubuntu
- Know how many elements the array has DISTROSS
echo ${#DISTROS[@]} 4
Discussion