This is a very short basic description on Bash scripting - compiled from a book and various online sources. Refer to the reference section for a full reading/understanding.

Contents

• VARIABLES AND QUOTING
  • Variable Expansion
  • Command Execution
• SHELL BUILT-IN VARIABLES
• CONDITIONALS AND LOOPS
  • IF condition
  • WHILE loop
  • FOR loop
  • Boolean Operators
    • Numeric and String Operators
    • File Evaluation Operators
• PARENTHESES () and []
  • Arithmetic Expansion
• echo and printf
• References

  VARIABLES AND QUOTING

  In shell, variables are declared like prompt> etcdir='/etc'. Few things of note:

• Do NOT put space around = sign. Spaces are of big consequence in Shell Scripting.
  • prompt> etcdir =/etc interprets etcdir as a command

    Variable Expansion

• Reference a variable prepending $ sign to the variable (NO SPACES). Within double quoted strings, you need to use ${<varibleHere>} format to expand the variable. In single quoted strings, the variable will NOT expand.
  • prompt> echo $etcdir echoes /etc
  • prompt> echo "The etc directory is ${etcdir}" echoes The etc directory is /etc
  • prompt> echo 'This wont expand the variable ${etcdir} echoes This wont expand the variable ${etcdir} because

    Command Execution

• Back-quote or backticks (`) allow execution of commands within a string when they are surrounded by it.

  prompt> echo 'I am `whoami`'
  I am root 
  

• Varibles are CaSe-SeNsItIvE. ALLCAPS variables are often environment variables or global variables.

SHELL BUILT-IN VARIABLES

• $0: command that is invoked to run the current program/process. eg. python, gcc, etc.
• $1, $2, $3,… : 1st, 2nd, 3rd, etc. argument provided to the current function. eg. helloWorld.py, -o a.out a.c, etc
• $*: All arguments provided at once excluding $0
• $#: Number of arguments provided excluding $0
• $?: Exit code given by previous command execution

CONDITIONALS AND LOOPS

IF condition

• If conditionals can be written in multilines or otherwise.
• Multiline if conditions can be written as below:

  if [ CONDITION1 ]; then
    SOME_ACTION
  elif [ CONDITION2 ]; then
    SOME_OTHER_ACTION
  else
    DEFAULT ACTION
  fi
  

  WHILE loop

• While loop uses while ....; do ...... done format
• Example script that counts lines in a file

counter = 0
while read line; do
    counter = $((counter + 1))
done

FOR loop

• For loop has similar structure as while loop
• Some examples:

for script in *.sh; do
    cat $script > newfile.txt
done

#!/bin/sh
# Backup Script to backup all 

suffix=BACKUP--`date + %Y-%m-%d-%H%M`
for script in *.sh; do # *.sh wil automatically list all .sh files like ls does
    newname="$script.$suffix"
    echo "Copying $script to $newname"
    cp -p $script $newname
done

• for x in Y structure is not required if Y is a list
  • One can do for Y; do ... done

Boolean Operators

Numeric and String Operators

File Evaluation Operators

PARENTHESES () and []

• [] act as a shorthand of test command

  $ [ -f /etc/rc.local ] && echo "real file"
  real file
  
  -and-
  
  $ test -f /etc/rc.local && echo "real file"
  real file
  

• No WordSplitting or glob expansion will be done for [[ (and therefore many arguments need not be quoted)[4]:

  file="file name"
  [[ -f $file ]] && echo "$file is a regular file"
  

  will work even though $file is not quoted and contains whitespace. With [ the variable needs to be quoted:

  file="file name"
  [ -f "$file" ] && echo "$file is a regular file"
  

  This makes [[ easier to use and less error-prone.

  Parentheses in [[ do not need to be escaped:

  [[ -f $file1 && ( -d $dir1 || -d $dir2 ) ]]
  [ -f "$file1" -a \( -d "$dir1" -o -d "$dir2" \) ]
  

Arithmetic Expansion

• Arithmetic expansion allows the evaluation of an arithmetic expression and the substitution of the result. The format for arithmetic expansion is: $(( expression ))
• The format for a simple Arithmetic Evaluation is: (( expression ))
• The expression is treated as if it were within double quotes, but a double quote inside the parentheses is not treated specially. All tokens in the expression undergo parameter expansion, command substitution, and quote removal. Arithmetic substitutions can be nested. [5]

echo and printf

• Echo prints as-is without formatting while printf does formatting based on the string.

$ echo "\ta\tsd\n" 
\ta\sd\n
$ printf "\t\a\sd\n"
  a sd

References

1. Unix and System Administration Handboox, Fifth Edition
2. https://unix.stackexchange.com/questions/99185/what-do-square-brackets-mean-without-the-if-on-the-left
3. https://stackoverflow.com/questions/2188199/how-to-use-double-or-single-brackets-parentheses-curly-braces
4. http://mywiki.wooledge.org/BashFAQ/031
5. https://ss64.com/bash/syntax-brackets.html