sed and awk Cheat Sheet

Frequently asked questions

What is the difference between sed and awk?

Both are line-oriented text processors, but they specialize. sed is a stream editor aimed at find-and-replace and simple line edits: deleting, printing, or substituting text on the lines you select. awk is a small programming language built around fields and records, so it excels at columnar data where you print specific columns, filter rows by a condition, and compute sums or counts. A rough rule: reach for sed to transform text and awk to work with columns and numbers.

How do I replace text with sed?

Use the substitute command: sed "s/old/new/" replaces the first match of old with new on each line, and adding the g flag, sed "s/old/new/g", replaces every match on the line. By default sed prints to standard output and leaves your file untouched, so redirect the result to a new file or add -i to edit the file in place. You can also target specific lines, for example sed "2,5s/old/new/g" to substitute only on lines 2 through 5.

How do I print a column with awk?

awk splits each line into fields, so print the column you want by its number: awk "{print $1}" prints the first field, awk "{print $3}" the third, and awk "{print $NF}" the last field no matter how many there are. Fields are separated by whitespace by default; if your data uses another delimiter, set it with -F, as in awk -F"," "{print $2}" to read the second comma-separated value.

What does the g flag do in sed?

In a sed substitution the trailing g means global: replace every match on a line instead of just the first. Without g, sed "s/a/b/" changes only the first a on each line; with g, sed "s/a/b/g" changes them all. You can combine flags, so gi replaces every match case-insensitively, and a number before g, such as 2g, replaces from the second match to the end of the line. The g flag applies per line, so sed still processes every line in the input.

How do I edit a file in place?

Add the -i flag: sed -i "s/old/new/g" file rewrites the file directly instead of printing to the screen. Because this overwrites the original, it is wise to keep a backup: sed -i.bak "s/old/new/g" file saves the untouched original as file.bak first. Note that -i is a GNU extension; BSD and macOS sed require an explicit suffix argument, so use sed -i "" "s/old/new/g" file there, or install GNU sed.

How do I sum a column of numbers with awk?

Accumulate a running total and print it in an END block: awk "{sum+=$1} END{print sum}" file adds up the first field across every line and prints the total once the input is exhausted. To average instead, divide by the record count, as in awk "{sum+=$1} END{print sum/NR}" file. Change $1 to the field you need, and add -F to set the delimiter if the column is not whitespace-separated.

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