Update on 04/24/2016. see a post Shell parameter substitution
| ${parameter:-defaultValue} | Get default shell variables value |
| ${parameter:=defaultValue} | Set default shell variables value |
| ${parameter:?”Error Message”} | Display an error message if parameter is not set |
| ${#var} | Find the length of the string |
| ${var%pattern} | Remove from shortest rear (end) pattern |
| ${var%%pattern} | Remove from longest rear (end) pattern |
| ${var:num1:num2} | Substring |
| ${var#pattern} | Remove from shortest front pattern |
| ${var##pattern} | Remove from longest front pattern |
| ${var/pattern/string} | Find and replace (only replace first occurrence) |
| ${var//pattern/string} | Find and replace all occurrences |
String manipulation is very useful in bash scripts; especially when processing a lot of files (in a for loop or xargs)
with different names, and you want to name your output of each file with part of the input file name.
command file part_file.result
I am going to demonstrate string manipulation below:
Let's create a variable file and print it out:
In [1]:
file=foo.txt
echo "$file"
foo.txt
I want to change the suffix from txt to pdf. one of the commonly known ways is to use the
basename built-in function:
In [2]:
echo "$(basename $file .txt).pdf"
foo.pdf
However, there are other built-in ways to get the same task done
references here
find and replace
Replace only first match:
${string/pattern/replacement}
Replace all the matches:
${string//pattern/replacement}
Following syntax replaces with the replacement string,
only when the pattern matches beginning of the $string.
${string/#pattern/replacement}
Following syntax replaces with the replacement string,
only when the pattern matches at the end of the given $string.
${string/%pattern/replacement}
For more complex replacement, use sed. see my previous blog post here
In [3]:
echo "${file/txt/pdf}"
foo.pdf
In [4]:
# a more complex exmaple
file_1=foo.txt.foo.txt
echo "${file_1//foo/bar}"
bar.txt.bar.txt
In [5]:
echo "${file_1/foo/bar}"
bar.txt.foo.txt
In [6]:
echo "${file_1/#foo/bar}"
bar.txt.foo.txt
In [7]:
echo "${file_1/%txt/pdf}"
foo.txt.foo.pdf
${string%substring} will delete the shortest match of substringfrom back
${string%%substring} will delete the longest match of substring from back
In [8]:
echo "${file_1%txt*}pdf"
foo.txt.foo.pdf
In [9]:
echo "${file_1%%txt*}pdf"
foo.pdf
{string#substring} will delete the shortest match of substring from the begining
{string##substring} will delete the longest match of substring from the begining
In [10]:
echo "bar${file_1#foo*}"
bar.txt.foo.txt
In [11]:
echo "bar${file_1##foo*}.pdf"
bar.pdf
string slicing
${string:position}
Extract substring from $string at $position
${string:position:length}
Extract $length of characters substring from $string starting from $position
In [12]:
echo "${file_1:4}"
txt.foo.txt
In [13]:
echo "${file_1:4:7}"
txt.foo
Finally, the length of the string:
In [14]:
echo "${#file_1}"
15