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Command line interface for CSSO.


npm install -g csso-cli


csso [input] [options]


      --comments <value>           Comments to keep: exclamation (default), first-exclamation or none
      --debug [level]              Output intermediate state of CSS during a compression
  -d, --declaration-list           Treat input as a declaration list
      --force-media-merge          Enable unsafe merge of @media rules
  -h, --help                       Output usage information
  -i, --input <filename>           Input file
      --input-source-map <source>  Input source map: none, auto (default) or <filename>
      --no-restructure             Disable structural optimisations
  -o, --output <filename>          Output file (result outputs to stdout if not set)
  -s, --source-map <destination>   Generate source map: none (default), inline, file or <filename>
      --stat                       Output statistics in stderr
  -u, --usage <filename>           Usage data file
  -v, --version                    Output version
      --watch                      Watch source file for changes

Some examples:

> csso in.css
...output result in stdout...

> csso in.css --output out.css

> echo '.test { color: #ff0000; }' | csso

> cat source1.css source2.css | csso | gzip -9 -c > production.css.gz

Source maps

Source map doesn't generate by default. To generate map use --source-map CLI option, that can be:

  • none (default) – don't generate source map
  • inline – add source map into result CSS (via /*# sourceMappingURL=application/json;base64,... */)
  • file – write source map into file with same name as output file, but with .map extension (in this case --output option is required)
  • any other values treat as filename for generated source map


> csso my.css --source-map inline
> csso my.css --output my.min.css --source-map file
> csso my.css --output my.min.css --source-map maps/

Use --input-source-map option to specify input source map if needed. Possible values for option:

  • auto (default) - attempt to fetch input source map by follow steps:
    • try to fetch inline map from input
    • try to fetch source map filename from input and read its content
    • (when --input is specified) check file with same name as input file but with .map extension exists and read its content
  • none - don't use input source map; actually it's using to disable auto-fetching
  • any other values treat as filename for input source map

Generally you shouldn't care about the input source map since defaults behaviour (auto) covers most use cases.

NOTE: Input source map is using only if output source map is generating.

Usage data

CSSO can use data about how CSS is using for better compression. File with this data (JSON format) can be set using --usage option. Read more about Usage data in CSSO repository.


All debug information outputs to stderr.

To get brief info about compression use --stat option.

> echo '.test { color: #ff0000 }' | csso --stat >/dev/null
Source:     <stdin>
Original:   25 bytes
Compressed: 16 bytes (64.00%)
Saving:     9 bytes (36.00%)
Time:       7 ms
Memory:     0.204 MB

To get details about compression steps use --debug option.

> echo '.test { color: green; color: #ff0000 } .foo { color: red }' | csso --debug
## parsing done in 4 ms

Compress block #1
[0.000s] init
[0.001s] clean
[0.003s] replace
[0.001s] prepare
[0.001s] mergeAtrule
[0.000s] initialMergeRuleset
[0.000s] disjoinRuleset
[0.000s] restructShorthand
[0.001s] restructBlock
[0.000s] mergeRuleset
[0.000s] restructRuleset
## compress done in 9 ms

## generate done in 0 ms


More details are providing when --debug option has a number greater than 1:

> echo '.test { color: green; color: #ff0000 } .foo { color: red }' | csso --debug 2
## parsing done in 4 ms

Compress block #1
[0.001s] init

[0.001s] clean

[0.004s] replace


[0.000s] mergeRuleset

[0.000s] restructRuleset

## compress done in 12 ms

## generate done in 0 ms


Using --debug option adds stack trace to CSS parse error output. That can help to find out problem in parser.

> echo '.a { color }' | csso --debug

Parse error <stdin>: Colon is expected
    1 |.a { color }
    2 |

                throw e;

Error: Colon is expected
    at parseError (/usr/local/lib/node_modules/csso/lib/parser/index.js:54:17)
    at eat (/usr/local/lib/node_modules/csso/lib/parser/index.js:88:5)
    at getDeclaration (/usr/local/lib/node_modules/csso/lib/parser/index.js:394:5)
    at getBlock (/usr/local/lib/node_modules/csso/lib/parser/index.js:380:27)

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