学习笔记丨Shell

Usage of shell script

References: Learn Shell, Shell 教程 | 菜鸟教程

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 $SUBSTRING that matches.
- expr index "$STRING" "$SUBSTRING"
Substring Extraction
- Extract substring of length $LEN from $STRING starting after position $POS. Note that first position is 0.
  - echo ${STRING:$POS:$LEN}
- If $LEN is omitted, extract substring from $POS to end of line.
  - echo ${STRING:2}
Substring Replacement
- Replace first occurrence of substring with replacement
  - echo ${STRING[@]/substring/newsubstring}
- Replace all occurrences of substring
  - echo ${STRING[@]//substring/newsubstring}
- Delete all occurrences of substring (replace with empty string)
  - echo ${STRING[@]// substring/}
- Replace occurrence of substring if at the beginning of $STRING
  - echo ${STRING[@]/#substring/newsubstring}
- Replace occurrence of substring if at the end of $STRING
  - echo ${STRING[@]/%substring/newsubstring}
- Replace occurrence of substring with shell command output
  - 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

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&&, ||, !

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

选项	功能
-e filename	if file exist
-r filename	if file exist and has read permission for the user
-w filename	if file exist and has write permission for the user running the script/test
-x filename	if file exist and is executable
-s filename	if file exist and has at least one character
-d filename	if directory exist
-f filename	if file exist and is normal file

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