KodeKloud Notes

Learn how to capture command output in Bash and build powerful pipelines using xargs. This guide covers:

Command substitution with backquotes and $(...)
Storing outputs in variables
Processing file lists with xargs
Handling special characters and custom argument placement

Command Substitution
1.1 Using Backquotes
1.2 Using $(...) Syntax
1.3 Assigning Output to a Variable
Processing Input with xargs
2.1 Basic xargs Example
2.2 Limiting Arguments per Invocation
2.3 Handling Filenames with Spaces
2.4 Placing Arguments at a Specific Position
Common xargs Options
Links and References

Command Substitution

Command substitution allows you to embed the output of one command into another or assign it to a variable. Bash supports two forms:

Backquotes: `…`
Dollar parentheses: $(…)

Using Backquotes

mkdir `date +%Y-%m-%d`
ls
# 2022-12-13

Here, date +%Y-%m-%d generates 2022-12-13, which mkdir uses as the directory name.

Note

Backquotes can be harder to nest and read. Consider using $(...) for complex commands.

Using `$(...)` Syntax

The $(...) form improves readability and nesting:

rmdir 2022-12-13
mkdir $(date +%Y-%m-%d)
ls
# 2022-12-13

Both backquotes and $(...) produce identical results.

Assigning Output to a Variable

Store command output in a variable for reuse:

OS=$(uname -o)
echo "Operating System: $OS"
# Operating System: GNU/Linux

Processing Input with xargs

xargs builds and executes command lines from standard input. It’s perfect for bulk processing of filenames and other arguments.

Basic xargs Example

Find all files in /usr/share/icons starting with debian and report their dimensions via ImageMagick’s identify:

find /usr/share/icons -name 'debian*' \
  | xargs identify -format "%f: %wx%h\n"

Output:

debian-swirl.svg: 48x48
debian-swirl.png: 22x22
...

Steps:

find lists matching files.
Pipe the list into xargs.
xargs runs identify with -format to print filename: width×height.

Limiting Arguments per Invocation

By default, xargs packs as many items as possible per command. Use -n or -L to control batching:

# One file per identify invocation
find /usr/share/icons -name 'debian*' \
  | xargs -n 1 identify -format "%f: %wx%h\n"

# Three files at a time
find /usr/share/icons -name 'debian*' \
  | xargs -L 3 identify -format "%f: %wx%h\n"

Handling Filenames with Spaces

Filenames containing spaces or special characters require a null separator:

find . \
  -name '*.avi' -print0 \
  -o -name '*.mp4' -print0 \
  -o -name '*.mkv' -print0 \
  | xargs -0 du | sort -n

Each match is terminated with \0.
xargs -0 reads these safely.
du shows disk usage, then sort -n orders by size.

Warning

Always use -print0 with find and -0 with xargs when processing arbitrary filenames to avoid word-splitting issues.

Placing Arguments at a Specific Position

Use -I with a placeholder (e.g., {} or PATH) to insert items at a custom position:

find . \
  -mindepth 2 \
  \( -name '*.avi' -o -name '*.mp4' -o -name '*.mkv' \) -print0 \
  | xargs -0 -I {} mv {} ./

This moves each video file from subdirectories into the current directory, replacing {} with the filename.

Common xargs Options

Option	Description	Example
-n N	Use at most N arguments per command	`xargs -n 1 echo`
-L N	Use at most N lines per command	`xargs -L 3 ls -l`
-0	Input items are null-terminated (`\0`)	`find . -print0 \| xargs -0 rm`
-I X	Replace placeholder X in the command line	`xargs -I {} cp {} /backup/`
-P N	Run up to N processes in parallel	`xargs -P 4 -n 1 gzip`

Links and References

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