学习笔记|Shell Script

Usage of shell script

References: Learn Shell, [Shell 教程

菜鸟教程 (runoob.com)](https://www.runoob.com/linux/linux-shell.html)

The first line of shell script file begins with #!, followed by the full path where the shell interpreter is located. For example,

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#!/bin/bash

To find out the currently active shell and its path, type the commands followed,

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ps | grep $$
which bash

1 Variables

• Variable name can consist of a combination of letters and the underscore “_”.
• Value assignment is done using the “=” sign and there is no space permitted on either side of = sign when initializing variables.
• A backslash “\” is used to escape special character meaning. (转义特殊字符含义)
• Encapsulating the variable name with ${} is used to avoid ambiguity.
• Encapsulating the variable name with “” will preserve any white space values

Example

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BIRTHDATE="Dec 13, 2001"
Presents=10
BIRTHDAY='date -d "$BIRTHDATE" +%A'

if [[ "$BIRTHDATE" == "Dec 13, 2001" ]]; then
	echo "BIRTHDATE is correct, it is ${BIRTHDATE}"
fi
if [[ $Presents == 10 ]]; then
	echo "I have received $Presents presents"
fi
if [[ "$BIRTHDAY" == "Thursday" ]]; then
	echo "I was born on a $BIRTHDAY"
fi

special variables

• $0 - The filename of the current script.
• $n - The Nth argument passed to script was invoked or function was called.
• $# - The number of argument passed to script or function.
• $@ - All arguments passed to script or function.
• $* - All arguments passed to script or function.
• $? - The exit status of the last command executed.
• $$ - The process ID of the current shell. For shell scripts, this is the process ID under which they are executing.
• $! - The process number of the last background command.

2 Passing Arguments

The i-th argument in the command line is denoted as $i， $0 references to the current script, $# holds the number of arguments passed to the script，$@ or $* holds a space delimited string of all arguments passed to the script.

Example

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# function
function File {
	# print the total number of arguments
	echo $#
}

# '-lt': less than 
if [[ ! $# -lt 1 ]]; then
	File $*
fi

3 Array

• An array is initialized by assign space-delimited values enclosed in ().
• The total number of elements in the array is referenced by ${#arrayname[@]}.
• Traverse all the elements in the array is referenced by ${arrayname[@]}.
• The array elements can be accessed with their numeric index. The index of the first element is 0.
• Some members of the array can be left uninitialized.
• The elements in array can be compared with elements in another array by index and loops.

Example

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# arrays
fruits=("apple" "banana" "tomato" "orange")
# The total number of elements in the array is referenced by ${#arrayname[@]}
echo ${#fruits[@]}
fruits[4]="watermelon"
fruits[5]="grape"
echo ${fruits[@]}
echo ${fruits[4]}

4 Basic Operators

• a + b addition (a plus b)
• a - b substraction (a minus b)
• a * b multiplication (a times b)
• a / b division (integer) (a divided by b)
• a % b modulo (the integer remainder of a divided by b)
• a ** b exponentiation (a to the power of b)

Example

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# basic operators
A=3
B=$((12 - 3 + 100 * $A + 6 % 3 + 2 ** 2 + 8 / 2))
echo $B

5 Basic String Operations

• String Length: ${#stringname}

• Find the numerical position in $STRING of any single character in $SUBSTRING that matches.

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    expr index "$STRING" "$SUBSTRING"
    

• Substring Extraction

  • Extract substring of length $LEN from $STRING starting after position $POS. Note that first position is 0.

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      echo ${STRING:$POS:$LEN}
      

  • If $LEN is omitted, extract substring from $POS to end of line.

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      echo ${STRING:2}
      

• Substring Replacement

  • Replace first occurrence of substring with replacement

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      echo ${STRING[@]/substring/newsubstring}
      

  • Replace all occurrences of substring

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      echo ${STRING[@]//substring/newsubstring}
      

  • Delete all occurrences of substring (replace with empty string)

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      echo ${STRING[@]// substring/}
      

  • Replace occurrence of substring if at the beginning of $STRING

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      echo ${STRING[@]/#substring/newsubstring}
      

  • Replace occurrence of substring if at the end of $STRING

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      echo ${STRING[@]/%substring/newsubstring}
      

  • Replace occurrence of substring with shell command output

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      echo ${STRING[@]/%substring/$(shell command)}
      

  Example

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  # string operation
  STRING="this is a string"
  echo "The length of string is ${#STRING}"
  #Find the numerical position in $STRING of any single character in $SUBSTRING that matches
  SUBSTRING="hat"
  expr index "$STRING" "$SUBSTRING" # 1 't'
  # substring extraction
  POS=5
  LEN=2
  echo ${STRING:$POS:$LEN}
  echo ${STRING:10}
  # substring replacement
  STRING="to be or not to be"
  # replace the first occurrence
  echo ${STRING[@]/be/eat} 
  # replace all occurences
  echo ${STRING[@]//be/eat} 
  # the begin
  echo ${STRING[@]/#be/eat now}
  # the end
  echo ${STRING[@]/%be/eat}
  echo ${STRING[@]/%be/be on $(date +%Y-%m-%d)} 
  

6 Decision Making

6.1 If-else

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if [ expression1 ]; then
	#statement1
elif [ expression2 ]; then
	#statement2
else
	#statement3
fi

6.2 Types of numeric comparisons

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$a -lt $b    $a < $b
$a -gt $b    $a > $b
$a -le $b    $a <= $b
$a -ge $b    $a >= $b
$a -eq $b    $a is equal to $b
$a -ne $b    $a is not equal to $b

6.3 Types of string comparisons

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"$a" = "$b"     $a is the same as $b
"$a" == "$b"    $a is the same as $b
"$a" != "$b"    $a is different from $b
-z "$a"         $a is empty

6.4 logical combinations

&&, ||, !

6.5 case structure

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case "$variable" in
    "$condition1" )
        command...
    ;;
    "$condition2" )
        command...
    ;;
esac

7 Loops

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# bash for loop
for arg in [list]
do
 command(s)...
done
# bash while loop
while [ condition ]
do
 command(s)...
done
#bash until loop
# basic construct
until [ condition ]
do
 command(s)...
done

• break and continue can be used to control the loop execution of for, while and until constructs.

8 Shell Functions

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function function_name {
  command...
}

9 Trap Command

It often comes the situations that you want to catch a special signal/interruption/user input in your script to prevent the unpredictables.

Trap is your command to try:

• trap <arg/function> <signal>

Example

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trap "echo Booh!" SIGINT SIGTERM

function booh {
    echo "booh!"
}
trap booh SIGINT SIGTERM

10 File Testing

11 Pipelines

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command1 | command2

12 Input and Output Redirection

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command > file  # Redirect the output to file.
command < file  # Redirect the input to file.
command >> file # Redirect the output to file appends.
n > file  # Redirect the file with descriptor 'n' to file.
n >> file  # Redirect the file with descriptor 'n' to file appends.
n >& m  # Merge the output files m and n.
n <& m	# Merge the input files m and n.

• Details: Shell 输入/输出重定向

13 printf

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printf  format-string  [arguments...]
#example
printf "%-10s %-8s %-4s\n" 姓名 性别 体重kg  

• Details: Shell printf命令

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