Usage of shell script
References: Learn Shell, Shell 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
To find out the currently active shell and its path, type the commands followed,
ps | grep $$
which bash
1 Variables
- Variable name can consist of a combination of letters and the underscore “_”.
- Value assignment is done using the “=” sign and there is no space permitted on either side of = sign when initializing variables.
- A backslash “” is used to escape special character meaning. (转义特殊字符含义)
- Encapsulating the variable name with ${} is used to avoid ambiguity.
- Encapsulating the variable name with “” will preserve any white space values.
Example
BIRTHDATE="Dec 13, 2001"
Presents=10
BIRTHDAY='date -d "$BIRTHDATE" +%A'
if [[ "$BIRTHDATE" == "Dec 13, 2001" ]]; then
echo "BIRTHDATE is correct, it is ${BIRTHDATE}"
fi
if [[ $Presents == 10 ]]; then
echo "I have received $Presents presents"
fi
if [[ "$BIRTHDAY" == "Thursday" ]]; then
echo "I was born on a $BIRTHDAY"
fi
special variables
-
$0
- The filename of the current script. -
$n
- The Nth argument passed to script was invoked or function was called. -
$#
- The number of argument passed to script or function. -
$@
- All arguments passed to script or function. -
$*
- All arguments passed to script or function. -
$?
- The exit status of the last command executed. -
$$
- 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
# 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
# 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 ${fruits[4]}
4 Basic Operators
- a + b addition (a plus b)
- a - b substraction (a minus b)
- a * b multiplication (a times b)
- a / b division (integer) (a divided by b)
- a % b modulo (the integer remainder of a divided by b)
- a**b exponentiation (a to the power of b)
Example
# basic operators
A=3
B=$((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}
- Extract substring of length
- 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)}
- Replace first occurrence of substring with replacement
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)}
6 Decision Making
6.1 If-else
if [ expression1 ]; then
#statement1
elif [ expression2 ]; then
#statement2
else
#statement3
fi
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 $b
6.3 Types of string comparisons
"$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" $a is empty
6.4 logical combinations
&&, ||, !
6.5 case structure
case "$variable" in
"$condition1" )
command...
;;
"$condition2" )
command...
;;
esac
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
-
break
andcontinue
can be used to control the loop execution of for, while and until constructs.
8 Shell Functions
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
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
command1 | command2
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.
- 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
- Details: Shell printf命令