Sed is a powerful stream editor, typically used for editing large amounts of data by providing a simple command. Sed is also used for sophisticated searches, where the Regular Expressions are used.
You can use sed to:
- Automate editing actions to be performed on one or more files
- Simplify the task of performing the same edit on multiple files
- Converting one data format to another
For example the following command:
# sed -i.bup '/^#/d;/^$/d' myfile
Do the following:
- Edit the file in place
- Delete lines started with # (/^#/d)
- Delete blank lines (/^$/d)
- Create a backup file with bup extension
To learn sed , we need to understand the syntax and learn by examples
Print command
Print all file
# sed 'p' [file]
The above displays every line twice because it dump the file content to stdout while read it, to remove the stdout add –n:
# sed –n 'p' [file]
Print lines 1-5
# sed -n '1,5p' [file]
(if we remove the –n flag we get all file output to stdout but only the matched line will displayed twice)
Find line started with ‘root’ and print it
# sed -n '/^root/p' /etc/passwd
Print from line 15 to the end:
# sed -n '15,$p' /etc/passwd
Print all except from line 15 to the end:
# sed -n '15,$!p' /etc/passwd
print lines start with ‘a’ followed by number
# sed -n '/^a[0-9]/p' /etc/passwd
print lines with exactly 3 characters
# sed -n '/^...$/p' filename
Substitute – Search and replace
General syntax
# sed ‘ [range] s/<string>/<replacement>/ ‘ filename
simple example:
search for lines started with root and replace the first occurrence with ‘boot’
# sed ' /^root/ s/root/boot/ '
do it for all occurrences
# sed ' /^root/ s/root/boot/g '
search and replace default shell for user root
# sed ' /^root/ s@/bin/bash@/bin/sh@ ' /etc/passwd
note that we changed the delimiter because we want to use / in the search term
so this will also works:
# sed ' /^root/ s#/bin/bash#/bin/sh# ' /etc/passwd
The range can be regular expression or lines :
# sed ' 4,9 s/nologin/virtual/ ' /etc/passwd
Add -n to suppress the output and p to display the lines affected
# sed -n ' 4,9 s/nologin/virtual/p ' /etc/passwd
Add -i (without p) to make the changes in place (edit the file)
# sed -i ' 4,9 s/nologin/virtual/ ' /etc/passwd
Add -i.bak to make changes and save the original for backup with bak extenstion
# sed -i.bak ' 4,9 s/nologin/virtual/ ' /etc/passwd
Transform Example:
Transform any ‘a’ to ‘_’ and any ‘I’ to ‘*’
# sed 'y/ai/_*/' ./emp
Insert, Append and delete
Add line ‘hello’ before lines 2-4:
# sed '2,4 i hello' ./passwd
Add line ‘hello’ after lines 2-4:
# sed '2,4 a hello' ./passwd
Add line ‘hello’ after each line started with s
# sed '/^s/ a hello' ./passwd
Delete lines 2-4
# sed '2,4 d' ./passwd
Delete lines starting with s
# sed '/^s/ d' ./passwd
more regular expressions:
# sed '/^[sm]/ d' ./passwd # starting with s or m # sed '/^[smp]/ d' ./passwd # starting with s or m or p # sed '/^[^smp]/ d' ./passwd # not starting with m or s or p
Delete blank lines:
# sed '/^$/d' ./emp_s
Multiple sed expressions:
On the command line type:
# sed '{
> 1,3 i hello
> 5,8 d
> 10,12 a bye
> }' ./passwd
write in a file and use -f to run it:
# sed -f ./sedsamp /etc/passwd
in place:
# sed -i.bak -f ./sedsamp ./passwd
Substitution Groups
Example:
# sed 's/\([^,]*\)/\U\1/' ./emp
Lets explain it step by step:
We define groups with ( ) but we need to add escape chars it so it should be written ‘\(\)’
[^,] – anything except comma (,)
[^,]* – zero and more anything except comma
so it should be written like this:
\([^,]*\)
replace string:
/\U\1/
\U – command to uppercase
\1 – the first group
So this example uppercase the first field in CSV file:
It looks better if we use different command separator:
# sed 's@ \([^,]*\) @ \U\1 @' ./emp
using more groups:
# cat ./emp avi,100,haifa dani,200,tel aviv rina,300,aco
Run the substitution
# sed 's/\([^,]*\),\([^,]*\),\([^,]*\)/\U\1#\2#\U\3/' ./emp AVI#100#HAIFA DANI#200#TEL AVIV RINA#300#ACO
Change order
# sed 's/\([^,]*\),\([^,]*\),\([^,]*\)/\U\3#\2#\U\1/' ./emp HAIFA#1200#AVI TEL AVIV#2300#DANI ACO#3100#RINA
Executing commands
# cat ./folders /lib /lib32 /lib64
replace the starting line with ‘ls -l ‘
# sed 's/^/ls -l / ' ./folders ls -l /lib ls -l /lib32 ls -l /lib64
execute the commands:
# sed 's/^/ls -l / e' ./folders
1 thought on “Understanding sed – Practical Guide”
Comments are closed.
Well groups command lists which canbe used frequently. Surprised to see that no one commented yet! Thanks for your effort 🙂