Bash Shell Cheatsheet

NOTE: The content in this page is my personal note which I got from multiple sources and based on my personal experience.

File Redirection

> file      Create file
>> file     Append to file
< file      Read from file
a | b       Pipe a as input to b

Read text file line by line

while read f; do
    echo "line is $f"
done < file

While loop

a=0
while [ "$a" -lt 10 ]    # this is loop1
do
   b="$a"
   while [ "$b" -ge 0 ]  # this is loop2
   do
      echo -n "$b "
      b=`expr $b - 1`
   done
   echo
   a=`expr $a + 1`
done

For loop

for var in 0 1 2 3 4 5 6 7 8 9
do
   echo $var
done

for FILE in $HOME/.bash*
do
   echo $FILE
done

Until loop

a=0
until [ ! $a -lt 10 ]
do
   echo $a
   a=`expr $a + 1`
done

Select loop

The select loop provides an easy way to create a numbered menu from which users can select options. It is useful when you need to ask the user to choose one or more items from a list of choices.

select var in word1 word2 ... wordN
do
   Statement(s) to be executed for every word.
done

Here var is the name of a variable and word1 to wordN are sequences of characters separated by spaces (words). Each time the for loop executes, the value of the variable var is set to the next word in the list of words, word1 to wordN.

For every selection a set of commands would be executed with-in the loop. This loop was introduced in ksh and has been adapted into bash. It is not available in sh.

Here is a simple example to let the user select a drink of choice

#!/bin/ksh

select DRINK in tea cofee water juice appe all none
do
   case $DRINK in
      tea|cofee|water|all) 
         echo "Go to canteen"
         ;;
      juice|appe)
         echo "Available at home"
      ;;
      none) 
         break 
      ;;
      *) echo "ERROR: Invalid selection" 
      ;;
   esac
done

The menu presented by the select loop looks like the following

$./test.sh
1) tea
2) cofee
3) water
4) juice
5) appe
6) all
7) none
#? juice
Available at home
#? none
$

You can change the prompt displayed by the select loop by altering the variable PS3 as follows −

$PS3="Please make a selection => " ; export PS3
$./test.sh
1) tea
2) cofee
3) water
4) juice
5) appe
6) all
7) none
Please make a selection => juice
Available at home
Please make a selection => none
$

Get output of ls into variable

myvar=`ls`

Function declaration

doubleit(){
expr $1 \* 2
}

doubleit 3  # return 6

Run cmd1; if fails, run cmd2

cmd1 || cmd2

Run cmd1; if it works, run cmd2

cmd1 && cmd2

Test operators

if [ "$x" -lt "$y" ]; then
# do something
fi

Numeric tests

lt  less than
gt
eq
ne
ge
le

File tests

nt  newer than
d   is a directory
f   is a file
x   executable
r   redable
w   writable

String tests

=   equal to
z   zero length
n   not zero length

Logical test

&&
||
!

Arguments

$0
$1
...
$#  no of arguments
$*  all arguments

Variable substitution

${V:-default}   $V, or "default if unset
${V:=default}   $V (set to "default" if unset)
${V:?err}   $, or "err" if unset

Preset Varibales

$SHELL      What shell is running
$RANDOM     Provides random number
$SECONDS    Number of seconds from the time the shell is started
$$      PID of current process
$?      Return code from last command
$!      PID of last background cmd
$USER       The username of the user running the script.
$HOSTNAME   The hostname of the machine the script is running on.
$LINENO     Returns the current line number in the Bash script.

Passing an indirect reference to a function

#!/bin/bash
# ind-func.sh: Passing an indirect reference to a function.

echo_var ()
{
 echo "$1"
}

message=Hello
Hello=Goodbye

echo_var "$message"        # Hello
# Now, let's pass an indirect reference to the function.
echo_var "${!message}"     # Goodbye

echo "-------------"

# What happens if we change the contents of "hello" variable?
Hello="Hello, again!"
echo_var "$message"        # Hello
echo_var "${!message}"     # Hello, again!

exit 0

Read input from command line

read -s -p "Enter your password for encryption: " pass
echo -n ${pass} | my_encoding_script

Shell script with parameter expansion

Here is a bash script with parameter expansion which used for managing PostgreSQL database. You can use this script as a template to create a script to manage your systems like database, cloud, etc.

#!/bin/bash

[ "$_FAILHARD" == "false" ] || [ "$_FAILHARD" == "False" ] && set +e || set -e
[ "$_DEBUG" == "true" ] || [ "$_DEBUG" == "True" ]  && set -x || set +x

# REF LINK: http://mywiki.wooledge.org/BashSheet#Parameter_Operations

export PGHOST="localhost"
export PGUSER=${POSTGRES_USER}
export PGPASSWORD=${POSTGRES_PASSWORD}


usage () {
  echo "This script will create new database:
Usage: $(echo $0 | rev | cut  -f -1 -d '/' | rev) --database=<database> --db-user=<database-user> --db-password=<database-password>

Options:
  - database: database name to be created.
  - db-user: the owner of database.
  - db-password: the password for database user
"
}

# Get options from command line
for i in "$@"
do
case $i in
    --help)
    usage && exit
    shift 
    ;;
    -u=*|--db-user=*)
    DB_USER="${i#*=}"
    shift 
    ;;
    -P=*|--db-password=*)
    DB_PASSWORD="${i#*=}"
    shift 
    ;;
    -d=*|--database=*)
    DATABASE="${i#*=}"
    shift 
    ;;
    *)
    # unknown option
    ;;
esac
done

# Check input parameters
[ -z $DATABASE ] && echo "[+] --database option is needed." && usage && exit 1
[ -z $DB_USER ] && echo "[+] --db-use option is needed." && usage && exit 1
[ -z $DB_PASSWORD ] && echo "[+] --db-password option is needed." && usage && exit 1

# Create database
psql <<EOF
CREATE DATABASE $DATABASE;
CREATE USER $DB_USER WITH ENCRYPTED PASSWORD '$DB_PASSWORD';
ALTER DATABASE $DATABASE OWNER TO $DB_USER;
EOF