paste

The paste command combines lines as columns. By default, it takes one line from each input file, joins those lines with a tab, and repeats until all inputs reach end-of-file.

Merging Files Side by Side

Create two small files:

$ printf 'alice\nbob\n' > names.txt
$ printf 'admin\nviewer\n' > roles.txt

Pass both files to paste:

$ paste names.txt roles.txt
alice	admin
bob	viewer

The visible spacing between columns is a tab. Unlike cat, which writes one complete file after another, paste combines corresponding input lines.

Checkfirst.txt contains A then B, and second.txt contains 1 then 2. What does paste first.txt second.txt produce by default?

A, B, 1, and 2 on four consecutive lines. A, B, 1, and 2 on one line with no separators. A with 1, then B with 2, separated by tabs.

Choosing a Delimiter

Use -d LIST to replace the default tab separator. For a colon:

$ paste -d ':' names.txt roles.txt
alice:admin
bob:viewer

Quote delimiters that have shell meaning. paste can cycle through multiple delimiter characters when the list contains more than one, but a single character is easiest when building two columns.

CheckWhich command joins corresponding lines from names.txt and roles.txt with a colon?

paste -d ':' names.txt roles.txt paste -s ':' names.txt roles.txt paste names.txt ':' roles.txt

Serializing Lines from One File

The -s option processes each input file serially, joining its lines into one output line. Create a file with one word per line:

$ printf 'The\nquick\nbrown\nfox\n' > words.txt
$ paste -s words.txt
The	quick	brown	fox

Combine -s with -d to choose the separator:

$ paste -s -d ' ' words.txt
The quick brown fox

If several files are supplied with -s, each file becomes its own output line.

CheckWhich command joins every line of words.txt into one space-separated output line?

paste -d ' ' words.txt paste -s words.txt roles.txt paste -s -d ' ' words.txt

Handling Unequal Input Lengths

When parallel input files have different numbers of lines, paste continues until the longest file ends. Missing values from a shorter file become empty fields:

$ printf 'A\nB\nC\n' > letters.txt
$ printf '1\n2\n' > numbers.txt
$ paste -d ':' letters.txt numbers.txt
A:1
B:2
C:

CheckWhat happens when one file passed to parallel paste ends before another?

paste uses empty fields for that file until the longest input ends. paste stops immediately and discards remaining lines. paste repeats the shorter file from its beginning.

Reading One Input from stdin

Use - as a file operand to read that position from stdin:

$ printf 'admin\nviewer\n' | paste -d ':' names.txt -
alice:admin
bob:viewer

CheckIn producer | paste names.txt -, what does the - operand mean?

Write the merged result to stderr. Remove delimiters between the two columns. Read that input column from stdin.

To practice merging line-oriented data, try this hands-on lab:

1. Simple Text Processing - Learn to use powerful commands like tr, col, join, and paste to manipulate and analyze text data efficiently.

Summary

You can now combine line-oriented inputs with predictable alignment and delimiters.

1. Merge corresponding lines from several files.
2. Replace the default tab separator with -d.
3. Serialize one file's lines with -s.
4. Interpret empty fields from shorter inputs.
5. Use - when one input comes from stdin.