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,

#!/bin/bash
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To find out the currently active shell and its path, type the commands followed,

ps | grep $$
which bash
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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

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
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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

# function
function File {
    # print the total number of arguments
    echo $#
}
​
# '-lt': less than 
if [[ ! $# -lt 1 ]]; then
    File $*
fi
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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

# 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]}
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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

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

# 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)} 
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6 Decision Making

6.1 If-else

if [ expression1 ]; then
    #statement1
elif [ expression2 ]; then
    #statement2
else
    #statement3
fi
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6.2 Types of numeric comparisons

$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
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6.3 Types of string comparisons

"$a" = "$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
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6.4 logical combinations

&&, ||, !
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6.5 case structure

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

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

8 Shell Functions

function function_name {
  command...
}
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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

Example

trap "echo Booh!" SIGINT SIGTERM
​
function booh {
    echo "booh!"
}
trap booh SIGINT SIGTERM
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10 File Testing

11 Pipelines

command1 | command2
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12 Input and Output Redirection

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.
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• Details: Shell 输入/输出重定向

13 printf

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