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Shell Syntax

Introduction

start with a she bang

#!/bin/bash
#!/usr/bin/env bash

Printing

echo hi
echo "hello"
a="1"
echo $a

Variables

a="this is it"
a = "sdaf" # this is wrong no space

Parameter expansion

a="a string"
echo "${a}"
echo "${a/string/strings/}" # replace string with strings
echo ${a:0:3} # print form 0, and then 3 characters
echo ${a:-5} # return last five character form the string
echo ${#a} # the length of the string
echo ${a:-"or print this is a is missing or empty"}

Integer

a=7

Arrays

array0=(one two three four five six)
echo $array # one
echo ${array0[0]} # one
echo ${array0[1]} # two
echo ${array0[@]} # entire array
echo ${#array0[1]} # 3 length of two
echo #{array0[@]:3:2} # print from 3rd index and two elements

# print the array
for i in "${array0[@]}";do
    echo "$i"
done

brace expansion

echo {1..10} # 1 2 3 4 5 6 7 8 9 10
echo {a..z}

built in variables

  • $? - last program return value
  • $$ - script pid
  • $# - number of argument passed to script
  • $@ - all arguments passed to script
  • $1 - first argument,
  • $2 - second argument, and so on...

run external command

echo "$(pwd)"

input

echo "enter amount: "
read amount
echo "amount: $amount"

flow control

if else

if [ $amount == 1000 ];then
    echo "expensive"
fi

if [ $amount == 1000 ];then
    echo "expensive"
else
    echo "cheap"
fi

if [ $amount == 1000 ];then
    echo "expensive"
elif [ $amount == 500 ];then
    echo "nice"
else
    echo "cheap"
fi

echo "Always executed" || echo "Only executed if first command fails"
echo "Always executed" && echo "Only executed if first command does NOT fail"

conditionals

  • -a file - True if file exists.
  • -b file - True if file exists and is a block special file.
  • -c file - True if file exists and is a character special file.
  • -d file - True if file exists and is a directory.
  • -e file - True if file exists.
  • -f file - True if file exists and is a regular file.
  • -g file - True if file exists and its set-group-id bit is set.
  • -h file - True if file exists and is a symbolic link.
  • -k file - True if file exists and its "sticky" bit is set.
  • -p file - True if file exists and is a named pipe (FIFO).
  • -r file - True if file exists and is readable.
  • -s file - True if file exists and has a size greater than zero.
  • -t fd - True if file descriptor fd is open and refers to a terminal.
  • -u file - True if file exists and its set-user-id bit is set.
  • -w file - True if file exists and is writable.
  • -x file - True if file exists and is executable.
  • -G file - True if file exists and is owned by the effective group id.
  • -L file - True if file exists and is a symbolic link.
  • -N file - True if file exists and has been modified since it was last read.
  • -O file - True if file exists and is owned by the effective user id.
  • -S file - True if file exists and is a socket.
  • file1 -ef file2
    • True if file1 and file2 refer to the same device and inode numbers.
  • file1 -nt file2 -
    • True if file1 is newer (according to modification date) than file2, or if file1 exists and file2 does not.
  • file1 -ot file2
    • True if file1 is older than file2, or if file2 exists and file1 does not.
  • -o optname - True if the shell option optname is enabled. The list of options appears in the description of the -o option to the set builtin.
  • -v varname - True if the shell variable varname is set.
  • -R varname - True if the shell variable varname is set and is a name reference.
  • -z string - True if the length of string is zero.
  • -n string or string True if the length of string is non-zero.
  • string1 == string2 or string1 = string2 - True if the strings are equal. When used with the [[ command, this performs pattern matching. = should be used with the test command for POSIX conformance.
  • string1 != string2 True if the strings are not equal.
  • string1 < string2 True if string1 sorts before string2 lexicographically.
  • string1 > string2 True if string1 sorts after string2 lexicographically.
  • arg1 OP arg2 OP is one of
    • -eq, equal to
    • -ne, not equal to
    • -lt, less than
    • -le, less than or equal to
    • -gt, greater than
    • -ge. greater than or equal to
    • When used with the [[ command, Arg1 and Arg2 are evaluated as arithmetic expressions.

and, or

if [ "$Name" == "Steve" ] && [ "$Age" -eq 15 ]
then
    echo "This will run if $Name is Steve AND $Age is 15."
fi

if [ "$Name" == "Daniya" ] || [ "$Name" == "Zach" ]
then
    echo "This will run if $Name is Daniya OR Zach."
fi

regex

  • ~= is used
Email=me@example.com
if [[ "$Email" =~ [a-z]+@[a-z]{2,}\.(com|net|org) ]]
then
    echo "Valid email!"
fi

case

case "$Variable" in
    # List patterns for the conditions you want to meet
    0) echo "There is a zero.";;
    1) echo "There is a one.";;
    *) echo "It is not null.";;  # match everything
esac

run command in background

sleep 30 &
  • how to check the running background jobs use jobs command
  • to bring a job to foreground fg pid
  • ctrl+c to kill a process
  • ctrl+z to pause a process
  • bg resume process in bg which have been paused

  • kill %2 kill job 2

  • run a command and print its file descriptor

    • echo <(echo "#helloworld")

expressions

echo $(( 10 + 5 )) # 15
  • id++ id-- variable post-increment and post-decrement
  • ++id --id variable pre-increment and pre-decrement
  • - + unary minus and plus
  • ! ~ logical and bitwise negation
  • ** exponentiation
  • * / % multiplication, division, remainder
  • + - addition, subtraction
  • << >> left and right bitwise shifts
  • <= >= < > comparison
  • == != equality and inequality
  • & bitwise AND
  • ^ bitwise exclusive OR
  • | bitwise OR
  • && logical AND
  • || logical OR
  • expr ? expr : expr conditional operator
  • = *= /= %= += -= <<= >>= &= ^= |= assignment
  • expr1 , expr2 comma

loops

for Variable in {1..3}
do
    echo "$Variable"
done

for ((a=1; a <= 3; a++))
do
    echo $a
done

for Variable in file1 file2
do
    cat "$Variable"
done

for Output in $(ls)
do
    cat "$Output"
done

while [ true ]
do
    echo "loop body here..."
    break
done

functions

function foo ()
{
    echo "Arguments work just like script arguments: $@"
    echo "And: $1 $2..."
    echo "This is a function"
    returnValue=0    # Variable values can be returned
    return $returnValue
}

exit shell script

  • do the cleanup, remove temporary files you created
  • The trap command allows you to execute a command whenever your script receives a signal.
  • Here, trap will execute rm if it receives any of the three listed signals.
trap "rm $TEMP_FILE; exit" SIGHUP SIGINT SIGTERM