About
sed stands for stream editor.
It is a filter program used for filtering and transforming text
It:
- takes as input a standard stream input
- modifies it based on an expression,
- and returns it as standard output.
In the stream, it can:
- search-and-replace all occurrences of one string to another.
- delete a range of lines.
It's part of the Gnu utility.
Sed is line-based therefore it is hard for it to grasp newlines and to manipulate eol characters.
Use the utility:
- or dos2unix
Syntax
# Default
sed 'expression1;...;expressionN' inputFileName > outputFileName
# In place editing - No outputFileName needed
sed -i 'expression1;...;expressionN' inputFileName
where:
- expression
command/regularExpression/modifier
Script
Using a script file avoids problems with shell escaping or substitutions.
Example script.sed: A sed file script with one command by line and a shebang
#!/bin/sed -f
sedExpression1
sedExpression...
sedExpressionN
Run it:
- with the f option
sed -f script.sed inputFileName > outputFileName
- directly (thanks to the shebang)
chmod u+x subst.sed
script.sed inputFileName > outputFileName
Command
Substitution
The Substitution command replace a string
# First occurence Default
sed 's/searchString/replacementString/' inputFileName > outputFileName
# All Occurences thanks to the g at the end
sed 's/searchString/replacementString/g' inputFileName > outputFileName
# In place editing - No outputFileName needed
sed -i 's/searchString/replacementString/g' inputFileName
# to use backslash characters. tab by arrow and end of line by reverse p
sed 's/\t/￫/g;s/$/¶'
where: in the expression 's/searchString/replacementString/':
- s stands for “substitution”.
- searchString: the search string, the text to find.
- replacementString: the replacement string
- g stands for global (ie replace all occurence)
- i is an option to edit the file directly - no need of outputFileName (a temporary output file is created in the background)
- $ is the single quote format that allows backslash characters
Delete
The d (delete) command delete lines (to delete a word, substitute it with nothing)
# line
sed '/regularExpression/d' inputFileName
# word
Example:
- delete lines that are either blank or only contain spaces
sed '/^ *$/d' inputFileName
- delete word (ie substitute with empty)
s/yourword//g
Others
- N add the next line to the pattern space;
- P print the top line of the pattern space;
- D delete the top line from the pattern space and run the script again.
How to
test the search pattern expression
sed -n "/patternExpression/p" targetFilePath
where p means print
Flow
Flow of control can be managed by:
- the use of a label (a colon followed by a string)
- and the branch instruction b.
An instruction b followed by a valid label name will move processing to the block following that label.