Truncate File (>)

Writes stdin to disk - overwriting contents if file already exists

Description

Redirects output to file.

If a file already exists, the contents will be truncated (overwritten). Otherwise a new file is created.

Usage

<stdin> |> [ -i | --ignore-pipeline-check ] filename

<stdin> |> [ -w | --wait-for-eof ] filename

Examples

g * |> files.txt

Flags

• --ignore-pipeline-check Don't check if filename is a parameter for an earlier command in the pipeline
• --wait-for-eof Wait for stdin to return an EOF before opening filename
• -i Alias for --ignore-pipeline
• -w Alias for --wait-for-eof

Detail

Race Condition Detection

If no flags are specified, then |> will check if the filename supplied is used in any parameters for other commands in the pipeline. If it has been, then |> will wait for an EOF (End Of File) from stdin before opening filename.

This is to allow pipelines like the following:

open example.log | regexp m/error/ |> example.log

Under traditional shells and Murex's normal scheduler, all commands in a pipeline would run concurrently. This leads to a race condition where |> opens (and thus truncates) a file before other commands can read from it.

However by default, |> will check the pipeline to look for any other references of filename and if it exists, it will wait for an EOF before |> truncates filename.

This wait for EOF behaviour can be forced with the --wait-for-eof / -w flag.

Alternatively, if you want to force |> to run concurrently then you can disable the pipeline check with the --ignore-pipeline-check / -i flag.

High Memory Usage

WARNING! Waiting for EOF will cause |> to cache the pipeline into RAM. If your pipeline is parsing multi-gigabyte or larger files then you may experience performance issues.

For large datasets, it might be preferable to write to a temporary file first.

open example.log | regexp m/error/ |> example.log.tmp
mv example.log.tmp example.log

The move operation should be instantaneous on most filesystems because your operating system will just alter filesystem metadata rather than move the file contents.

Flag Without A Filename

If you specify a flag without a filename, eg |> --wait-for-eof, then it is assumed that the flag is the filename.

Syntactic Sugar

While |> is referred to as an operator, it's actually a pipe followed by a builtin:

out "foobar" | > example.txt

Thus you can actually use > by itself.

Creating An Empty File

If > is at the start of a pipeline then it is treated as null input. This a convenient shortcut to create an empty file or blank an existing file.

Create a new empty file:

> newfile

Clear a large log file without deleting the file itself:

> large.log

Appending A File

To append a file (ie write at the end of the file without overwriting its contents) use >> instead.

Synonyms

• >
• |>
• fwrite

See Also

• Create Named Pipe (pipe): Manage Murex named pipes
• Create Temporary File (tmp): Create a temporary file and write to it
• Globbing (g): Glob pattern matching for file system objects (eg *.txt)
• Read / Write To A Named Pipe (<pipe>): Reads from a Murex named pipe
• Schedulers: Overview of the different schedulers (or 'run modes') in Murex
• -> Arrow Pipe: Pipes stdout from the left hand command to stdin of the right hand command
• >> Append File: Writes stdin to disk - appending contents if file already exists
• ? stderr Pipe: Pipes stderr from the left hand command to stdin of the right hand command (DEPRECATED)
• | POSIX Pipe: Pipes stdout from the left hand command to stdin of the right hand command

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This document was generated from builtins/core/io/write_doc.yaml.