What is sed? | DevOps Dictionary

Understanding sed

What Is sed in Simple Terms

sed reads input line by line, applies transformation commands, and writes the result. The most common use is substitution: replace this text with that text. Unlike a text editor, sed works non-interactively — perfect for scripted config file modifications.

sed 's/localhost/prod-db.internal/g' config.template > config.prod — replaces every occurrence of localhost with prod-db.internal and saves to a new file. One command, zero manual editing.

◈ DIAGRAM

+------------------------------------------+
| Input stream (file or stdin)             |
+------------------------------------------+
                    |
              line by line
                    |
                    v
+------------------------------------------+
| sed 's/localhost/10.0.2.100/g'           |
| Pattern space: current line              |
| Apply substitution command               |
| s/old/new/g -- replace all occurrences  |
+------------------------------------------+
                    |
                    v
+------------------------------------------+
| Output stream (modified lines)           |
| localhost -> 10.0.2.100                  |
+------------------------------------------+

Practical Commands

Bash

## Basic substitution: s/old/new/
sed 's/localhost/10.0.2.100/' config.yaml
## Only replaces FIRST occurrence per line
 
## Global substitution (all occurrences per line)
sed 's/localhost/10.0.2.100/g' config.yaml
 
## Case-insensitive substitution
sed 's/error/ERROR/gI' /var/log/app.log
 
## In-place edit (modifies the file directly)
sed -i 's/debug: true/debug: false/' app.yaml
 
## In-place with backup
sed -i.bak 's/debug: true/debug: false/' app.yaml
## Creates app.yaml.bak with original content
 
## Delete lines matching pattern
sed '/^#/d' config.yaml        ## remove comment lines
sed '/^$/d' config.yaml        ## remove blank lines
sed '/DEBUG/d' app.log         ## remove debug log lines
 
## Print specific lines
sed -n '10,20p' /var/log/app.log      ## lines 10-20
sed -n '/ERROR/p' /var/log/app.log    ## only error lines
 
## Insert line before pattern
sed '/^\[Service\]/i User=payment-svc' service.unit
 
## Append line after pattern
sed '/^\[Service\]/a Restart=on-failure' service.unit
 
## Multiple expressions with -e
sed -e 's/localhost/10.0.2.100/g' -e 's/5432/5433/g' config.yaml
 
## Replace using delimiter other than / (useful when pattern contains /)
sed 's|/old/path|/new/path|g' config.yaml
 
## Production patterns:
 
## Remove trailing whitespace
sed -i 's/[[:space:]]*$//' file.txt
 
## Extract lines between two patterns
sed -n '/START/,/END/p' file.txt
 
## Comment out a line matching pattern
sed -i '/PasswordAuthentication yes/s/^/#/' /etc/ssh/sshd_config

Troubleshooting

Symptom	Command	What to Check
Substitution not working	Test without -i first	Preview before in-place edit
Special chars not matching	Escape: `\.` `\*` `\/`	Regex metacharacters
sed -i differs on macOS	Use `sed -i ''` on macOS	macOS sed requires explicit backup suffix

COMMON MISTAKE / WARNING
**Common Mistake:** Using `sed -i` without testing the expression first. Always run `sed 's/old/new/g' file` (without -i) to preview the output before modifying the file in place. One wrong regex can corrupt a config file.

PLACEMENT PRO TIP
**Tip:** When the pattern contains forward slashes (like file paths), use a different delimiter: `sed 's|/etc/nginx|/etc/nginx-new|g'`. sed accepts any character after `s` as the delimiter — `|`, `,`, `@`, or `#` all work.