Shell processing strings and {} operations

Shell processing strings and {} operations

1. Get the length of a variable

There are about 4 ways to get the length of a string: (in the case of Hello World)

• Passwc -L

   $ echo "hello world" | wc -L    11

• Passexpr length string

  $ expr length "hello world"11

• The length function built through awk

  $ echo "hello world" | awk ‘{print length($0)}‘11

• Passecho ${#string_name}

  $ string=‘hello world‘$ echo ${#string}11

2. Intercept variable by {} action

• #Operator--from the leftmost start to the first occurrence of a substring, delete all

  $echo $stringhello world$echo ${string#*l}lo world$echo ${string#*ll}o world$echo ${string#*llo}world$echo ${string#llo}hello world

Note: this needs to be matched with a *, otherwise it will be as invalid as the last one, and the first space in the penultimate part is removed, this should be a default assignment for the shell, which will remove the space at the beginning of the variable. For example:

$str1=" hello"$echo $str1hello

• ##is from the leftmost match to the last occurrence of the substring.

  $echo ${string##*l}d

• %From the far right to the first occurrence of the substring, the * number is placed to the right of the matched substring

  $echo ${string%l*}hello wor

• %%is from the far right to the place where the substring last appears.

  $echo ${string%%l*}he

• ${string:m:n}Operation

Attention:

The first subscript on the left is denoted by 0.
The first table on the right is denoted by 0-1, the second on the right is 0-2, and so on, n indicates the number of characters, not subscript
${string:3}3 here means starting from the 4th on the left.

$echo $stringhello world$echo ${string:0:2}he$echo ${string:0-2:2}ld$echo ${string:2}llo world

3. Parameter substitution and extension--by {} action

• ${str-value}If STR is not declared, value is assigned

• ${str：-value}If STR is not declared or the value is NULL, assign value

  $var=$echo ${var-helo}   -- 此时变量var为空$echo ${var:-helo}helo

• ${str+value} use value as the default value regardless of str value

• ${str:+value} use value as the default value unless str is empty

• ${str=value} If STR is not declared, use value as the default value, and STR is assigned value

• ${str:=value} If STR is not declared, or is empty, value is used as the default value and STR is assigned as value

• ${str?value} If STR is not declared, output value to stderr

• ${str:?value} If STR is not declared, or is empty, output value to stderr

• ${!var}Indirect variable Reference

  $echo $stringhello world$str="string"$echo ${!str}hello world

• ${!prefix*}Match all previously declared variables with prefix

• ${[email protected]}Match all previously declared variables with prefix

  $cat test.sh c_var1="hello"c_var2="world"for c in ${!c_*}; do    echo "${c}=${!c}"done$sh test.sh c_var1=helloc_var2=world

4. String substitution--by {} action

• ${string/str1/str2}, replace with str2 the first str1 from the left number

• ${string//str1/str2}, replace all str1 with str2

• ${string/#str1/str2}If the string prefix is str1, replace it with STR2

• ${string/%str1/str2}If the suffix of string is str1, replace it with STR2

  $echo ${string/l/s}heslo world$echo ${string//l/s}hesso worsd$echo ${string/#he/h}hllo world$echo ${string/%ld/d}hello word

Shell processing strings and {} actions