Shell scripting¶
Variables¶
Info
The action taken by the shell will be different on each value here are some samples:
FILE = "$PATH/this_is_a_file"
PATH = "/home/user/"
STRING = 'Hello World'
COMMAND = $( ls -l )
Built-in variables are automatically set by the shell and are typically used inside shell scripts.
| Variable | Description |
|---|---|
$# |
Number of command-line arguments |
$_ |
Options currently effect |
$? |
Exit value of the last exec command |
$$ |
Process number of current process |
$! |
Process number of the last bg command |
$0 |
First word, that is the command name |
$1-9 |
Individual arguments (positional) |
$* |
Location of the user’s mail spool |
"$@" |
Location of the user’s mail spool |
| Variable | Description |
|---|---|
var = value |
Set each variable var to a value |
${ var } |
Use value of var |
${ var :- value } |
Use var if set; otherwise, use value |
${ var := value } |
YUse var if set; otherwise, use value and assign value to var |
${ var :? value } |
Use var if set; otherwise, print value and exit |
${ var :+ value } |
Use value if var is set; otherwise, use nothing |
These variables are typically used in your ".profile" file, and can be shown running "echo $VAR".
| Variable | Description |
|---|---|
USER |
Your current username |
HOSTNAME |
Hostname of the computer |
SHELL |
Path to the current command shell |
PWD |
Current working directory |
HOME |
Home directory |
MAIL |
Location of the user’s mail spool |
LANG |
Current language |
TZ |
Time zone |
PS1 |
Default prompt in bash |
TERM |
Current terminal type |
DISPLAY |
Display used by X |
EDITOR |
Text editor |
MANPATH |
Path for Man pages |
OSTYPE |
Type of operating system |
SECONDS |
Seconds since the script was started |
RANDOM |
Returns a different random number each time |
Script samples
With a "getops" case you can add more logic to your scripts, making them more complex. It will take the command line arguments to but they have to be added before a "parameter"
Sample:
#!/bin/bash
while getopts :a:b:c: opt ; do
case ${opt} in
a)
arg1="${OPTARG}"
;;
b)
arg2="${OPTARG}"
;;
c)
arg3="${OPTARG}"
;;
\?)
echo "Invalid option: -$OPTARG" >&2
;;
esac
done
echo $arg1 $arg2 $arg3
Capture values from an output and use them in your script. Imagine that you want to parse a "free -m" command and get some values to work with. If you run this command it will show you a "table" with all the metrics so let's manipulate this output.
The standard output for free is this:
free -m
total used free shared buff/cache available
Mem: 15896 1300 12360 359 2236 13906
Swap: 2047 998 1049
As you know, to calculate avilable memory on a linux you must calculate with this expresion:
Free Memory = ( Total - Used + Buffer + Cache )
We must parse the command and get the 2nd, 3rd and 6th values. Here, we should use "set -- $FREE_MEM" to parse this output, check this out:
Sample:
#!/bin/bash
FREE_CMD=$(free -m | grep 'Mem:' )
echo $FREE_CMD # DEBUG COMMAND OUTPUT
echo ""
set -- $FREE_CMD # HERE WE PARSE ARGUMENTS FROM "$FREE_CMD" OUTPUT
# NOW I TAKE ARG VALUES AND SUBSTITUTE THEM AS INTEGERS
# WITH ": '\([0-9]\+\)'" AFTER THE EXPR VALUE TAKEN.
TOTAL_MEM=$(expr "${2}" : '\([0-9]\+\)') # ARG2 is TOTAL_MEM.
USED_MEM=$(expr "${3}" : '\([0-9]\+\)') # ARG3 is USED_MEM.
FREE_MEM=$(expr "${4}" : '\([0-9]\+\)') # ARG4 is FREE_MEM.
CACHE_MEM=$(expr "${6}" : '\([0-9]\+\)') # ARG4 is CACHE_MEM.
# PRINT VARIABLES
echo "Total: $TOTAL_MEM"
echo "Used: $USED_MEM"
echo "Buff/Cache: $CACHE_MEM"
echo "Free: $FREE_MEM"
# CALCULATE MEMORY
MEM_USED=$(($USED_MEM + $CACHE_MEM)) # CALCULATE $USED_MEM + $CACHE_MEM
AVAIL_MEM=$(($TOTAL_MEM - $MEM_USED)) # CALCULATE $TOTAL_MEM - $MEM_USED
echo "Available: $AVAIL_MEM" # PRINT AVAIL_MEM VALUE
echo ""
# TEST SCRIPT
if [ $AVAIL_MEM -eq $FREE_MEM ]; then
echo "Script is done"
else
echo "Something was wrong"
fi
#!/bin/bash
while getopts :a:b:c: opt ; do
case ${opt} in
a)
ANIMAL="${OPTARG}"
;;
b)
CITY="${OPTARG}"
;;
c)
OWNER="${OPTARG}"
;;
\?)
echo "Invalid option: -$OPTARG" >&2
;;
esac
done
ANIMAL=${ANIMAL:-"dog"}
OWNER=${OWNER:-"John"}
CITY=${CITY:-"Madrid"}
echo "$OWNER has a $ANIMAL and lives in $CITY"
Redirects¶
Info
Unix / Linux standard I/O streams with numbers:
| Handle | Name | Description |
|---|---|---|
| 0 | stdin | Get input from keyboard or program |
| 1 | stdout | Write information on screen or file |
| 2 | stderr | Show error message on screen or file |
Commands
command > filename Redirect stdout to file “filename.”
command >> filename Redirect and append stdout to file “filename.”
command 2> filename Redirect stderr to file “filename.”
command 2>> filename Redirect and append stderr to file “filename.”
command &> filename
command > filename 2>&1 Redirect both stdout and stderr to file “filename.”
command &>> filename
command >> filename 2>&1 Redirect both stdout and stderr append to file “filename.”
Operators¶
Info
+ -- echo `expr 3 + 4` # 7
+= -- n=3;echo $((n += 4)) # 7
- -- echo `expr 10 - 3` # 7
-= -- n=10;echo $((n -= 3)) # 7
* -- echo $((7 * 3)) # 21
*= -- n=7;echo $((n * 11)) # 77
** -- echo $((7 ** 3)) # 343
/ -- let n=21/3;echo $n # 7
/= -- n=21;let n=n/3;echo $n # 7
% -- echo `expr 21 % 4` # 1 (reminder)
%= -- n=21; expr `expr $n % 4` # 1 (reminder)
>,< -- String length compare
-z -- Zero length
-n -- Non-zero length
-eq,== -- Equal
-ne,!= -- Not equal
-gt,> -- Greater than
-ge,>= -- Greater than or equal
-lt,<= -- Less than
-le,< -- Less than or equal
-e -- File or folder exists
-f -- File exists
-d -- Folder exists
-s -- File size is more than zero
-b -- File is a block special
-c -- File is a character special
-p -- File is a pipe
-h,-L -- File is a symbolic link
-S -- File is a Socket
-t -- File is associated with a terminal
-r -- File has read permission
-w -- File has write permission
-x -- File has execute permission
-g -- File has SGID associated
-u -- File has SUID associated
-k -- File has Sticky bit
-O -- File has Owner ID
-G -- File has Group ID
-N -- File has been modified
-nt -- File is "Newer than" other
-ot -- File is "Older than" other
-ef -- File has two or more hardling pointing at same file
++ -- i=1;echo $((++i+7)) # 9 (PRE)
++ -- i=1;echo $((i++)) # 1 (POST)
-- -- i=7;echo $((--i-1)) # 5 (PRE)
-- -- i=7;echo $((--i)) # 6 (POST)
&& -- if [[ $INT = 7 && $STRING = "Lucky" ]]; then echo "Lucky Strike"; fi # AND
-a -- if [ 7 -gt 1 -a 7 -lt 8 ]; then echo "OK"; fi # AND
|| -- if [[ $INT = 7 || $STRING = "Lucky" ]]; then echo "Lucky Strike"; fi # OR
-o -- if [ 7 -gt 1 -a 8 -gt 7 ]; then echo "OK"; fi # OR
! -- if [[ !$STRING ]]; then echo "Not so lucky"; fi # NOT
‘?:’ operator can be used as an alternative of if statement. The logical condition is defined before ‘?’ and if the condition returns true then it will execute the statement that is defined before ‘:’ otherwise it will execute the statement that is defined after ‘:’. The following script shows the use of this operator.
#!/bin/bash
n=20
a=100
b=200
echo $(( n>=101 ? a : b )) # if n is "great or equal" to 101 then "echo" a, else "echo" b
&
&=
|
|=
>>
>>=
‘,’ operator is used to execute multiple statements in a line. The following command shows the use of this operator. The value of $n is assigned to 10, 30 is added with $n and the value of $n is printed.
#!/bin/bash
echo "$(( n=10, n=n+30 ))"
Functions¶
Info
Think of a function as a small script within a script. It's a small chunk of code which you may call multiple times within your script. They are particularly useful if you have certain tasks which need to be performed several times.
function_name () {
commands
}
function_name () { commands; }
function function_name {
commands
}
function function_name { commands; }
Script samples
#!/bin/bash
hello_world () {
echo 'hello, world'
}
hello_world
Hello, world
#!/bin/bash
var1='A'
var2='B'
my_function () {
local var1='C'
var2='D'
echo "Inside function: var1: $var1, var2: $var2"
}
echo "Before executing function: var1: $var1, var2: $var2"
my_function
echo "After executing function: var1: $var1, var2: $var2"
Before executing function: var1: A, var2: B
Inside function: var1: C, var2: D
After executing function: var1: A, var2: D
#!/bin/bash
die(){
local m="$1" # the first arg
local e=$2 # the second arg
echo "$m"
exit $e
}
# if not enough args displayed, display an error and die
[ $# -eq 0 ] && die "Usage: $0 filename" 1
# Rest of script goes here
echo "We can start working the script..."
We can start working the script...
Usage: <script_filename> filename
Shell Expansions¶
Info
touch {file1,file2}.md
mkdir -p $HOME/{dir1,dir2}
| Pattern | Description |
|---|---|
* |
Match zero or more characters |
? |
Match any single character |
[...] |
Match any of the characters in a set |
?(patterns) |
Match zero or one occurrences of the patterns (extglob) |
*(patterns) |
Match zero or more occurrences of the patterns (extglob) |
+(patterns) |
Match one or more occurrences of the patterns (extglob) |
@(patterns) |
Match one occurrence of the patterns (extglob) |
!(patterns) |
Match anything that doesn't match one of the patterns (extglob) |
| Glob | Regular Expression Equivalent | Description |
|---|---|---|
?(patterns) |
(regex)? |
Match an optional regex |
*(patterns) |
(regex)* |
Match zero or more occurrences of a regex |
+(patterns) |
(regex)+ |
Match one or more occurrences of a regex |
@(patterns) |
(regex) |
Match the regex (one occurrence) |
Statements¶
Info
Run a command in between "$(command)" to iterate:
#!/bin/bash
for i in $( ls -1 ); do
echo $i
done
Run a sequence in between "seq 1 10" to iterate:
#!/bin/bash
for i in `seq 1 10`; do
echo $i
done
#!/bin/bash
COUNTER=0
while [ $COUNTER -lt 10 ]; do
echo The counter is $COUNTER
let COUNTER=COUNTER+1
done
#!/bin/bash
COUNTER=20
until [ $COUNTER -lt 10 ]; do
echo COUNTER $COUNTER
let COUNTER-=1
done
Arithmetic¶
Info
Make a variable equal to an expression.
Operator Operation
+, -, \*, / Addition, subtraction, multiply, divide
var++ Increase the variable var by 1
var-- Decrease the variable var by 1
% Modulus (Return the remainder after division)
#!/bin/bash
let a=5+4
echo $a # 9
let "a = 5 + 4"
echo $a # 9
let a++
echo $a # 10
let "a = 4 * 5"
echo $a # 20
let "a = $1 + 30"
echo $a # 30 + first command line argument
Print out the result of the expression.
#!/bin/bash
# Basic arithmetic using expr
expr 5 + 4
expr "5 + 4"
expr 5+4
expr 5 \* $1
expr 11 % 2
a=$( expr 10 - 3 )
echo $a # 7
Return the result of the expression.
#!/bin/bash
# Basic arithmetic using double parentheses
a=$(( 4 + 5 ))
echo $a # 9
a=$((3+5))
echo $a # 8
b=$(( a + 3 ))
echo $b # 11
b=$(( $a + 4 ))
echo $b # 12
(( b++ ))
echo $b # 13
(( b += 3 ))
echo $b # 16
a=$(( 4 * 5 ))
echo $a # 20
Return the length of the variable var.
#!/bin/bash
# Show the length of a variable.
a='Hello World'
echo ${#a} # 11
b=4953
echo ${#b} # 4
+ More¶
Scripts
#!/bin/bash
for x in {0..8}; do
for i in {30..37}; do
for a in {40..47}; do
echo -ne "\e[$x;$i;$a""m\\\e[$x;$i;$a""m\e[0;37;40m ";
done;
echo;
done;
done